dalton gay lussac avogadro foundations molecular theory 1893

7/29/2019 Dalton Gay Lussac Avogadro Foundations Molecular Theory 1893

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FOUNDATIONSOF THE

MOLECULAR THEORYCOMPRISING

PAPERS AND EXTRACTSBY

JOHN DALTON,JOSEPH-LOUIS GAY-LUSSAC,

ANDAMEDEO AVOGADRO,(1808-1811.)

OF THEUNIVERSITYOFIFOREDINBURGH :WILLIAM F. CLAY, 18 TEVIOT PLACE.LONDON :

SIMPKIN, MARSHALL, HAMILTON, KENT, & CO. LTD.1893.


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PREFACE.papers here reprinted in chronological order

JL serve to exhibit the historical development of theidea of a connection existing between the number ofparticles in different gases and the volume they occupy.

It will be seen that Dalton from the first entertains thenotion that equal volumes of different gases may containthe same number of ultimate particles at equal tempera-ture and pressure, but that he is legitimately forced toreject this assumption, conceiving no distinction betweenthe atom and the molecule of an element. Gay-Lussac'simportant experimental work on the combining volumesof gases then shows the necessity of a simple relationbetween the ultimate particles of gases and their volumes,although he does not point this out in his paper. Dalton,however, perceives the necessity, and characteristicallyconcludes by doubting the accuracy of Gay-Lussac'sexperiments. Avogadro, finally, accepts both Dalton'stheory and Gay-Lussac's data, and teaches how to recon-cile them by distinguishing between the atom and themolecule of an elementary gas.

It has npt been thought necessary to reprint the letterof Ampere to Berthollet (Annales de Chimie, 90, 43-86,1814), since that paper contains no advance on the viewsof Avogadro published three years earlier, its authorsimply drawing the same conclusions from the samepremises.The English version of the French originals will pro-bably be found more faithful than elegant, especially soin the case of Avogadro's paper, where the French isalways clumsy and occasionally obscure.

J. W.


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FOUNDATIONS OF THEMOLECULAR THEORY.

i. EXTRACTS FROM A NEW SYSTEM OFCHEMICAL PHILOSOPHY. PART I.(1808). BY JOHN DALTON.

I.

(pp. 70-72.)3. The quantity of heat belonging to the ultimate par-

tides of all elasticfluids; must be the same under the samepressure and temperature.

It is evident the number of ultimate particles or mole-cules in a given weight or volume of one gas is not thesame as in another : for, if equal measures of azotic andoxygenous gases were mixed, and could be instantlyunited chemically, they would form nearly two measuresof nitrous gas, having the same weight as the two originalmeasures ; but the number of ultimate particles could atmost be one half of that before the union. No twoelastic fluids, probably, therefore, have the same numberof particles, either in the same volume or the same weight.Suppose, then, a given volume of any elastic fluid tobe constituted of particles, each surrounded with anatmosphere of heat, repelling each other through themedium of those atmospheres, and in a state of equili-


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6 Dalton.brium under the pressure of a constant force, such as theearth's atmosphere, also at the temperature of the sur-rounding bodies ; suppose further, that by some suddenchange each molecule of air was endued with a strongeraffinity for heat ; query the change that would take placein consequence of this last supposition ? The onlyanswer that can be given, as it appears to me, is this.The particles will condense their respective atmospheresof heat, by which their mutual repulsion will be dimin-ished, and the external pressure will therefore effect aproportionate condensation in the volume of air : neitheran increase nor diminution in the quantity of heat aroundeach molecule, or around the whole, will take place.Hence the truth of the supposition, or as it may now becalled, proposition, is demonstrated.

II.

(pp. 187-189.)I shall now proceed to give my present views on the

subject of mixed gases, which are somewhat differentfrom what they were when the theory was announced, inconsequence of the fresh lights which succeeding ex-perience has diffused. In prosecuting my enquiries intothe nature of elastic fluids, I soon perceived it wasnecessary, if possible, to ascertain whether the atoms orultimate particles of the different gases are of the samesize or volume in like circumstances of temperature andpressure. By the size or volume of an ultimate particle,I mean in this place, the space it occupies in the state ofa pure elastic fluid ; in this sense the bulk of the particlesignifies the bulk of the supposed impenetrable nucleus,together with that of its surrounding repulsive atmo-sphere of heat. At the time I formed the theory ofmixed gases, I had a confused idea, as many have, I


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New System of Chemical Philosophy. 7suppose, at this time, that the particles of elastic fluidsare all of the same size ; that a given volume of oxygenousgas contains just as many particles as the same volume ofhydrogenous ; or if not, that we had no data from whichthe question could be solved. But from a train of reason-ing, similar to that exhibited at page 71, I became con-vinced that different gases have not their particles of thesame size : and that the following may be adopted as amaxim, till some reason appears to the contrary : namely,

That every species of pure elastic fluid has its particlesglobular and all of a size ; but that no two species agree inthe size of their particles, the pressure and temperaturebeing the same.


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Gay-Lussac.

2. MEMOIR ON THE COMBINATION OFGASEOUS SUBSTANCES WITH EACHOTHER. BY M. GAY-LUSSAC.*Read before the Philomathic Society', ^\st December 1808,

QUESTANCES, whether in the solid, liquid, or gaseousO state, possess properties which are independent ofthe force of cohesion ; but they also possess others whichappear to be modified by this force (so variable in itsintensity), and which no longer follow any regular law.The same pressure applied to all solid or liquid sub-stances would produce a diminution of volume differingin each case, while it would be equal for all elastic fluids.Similarly, heat expands all substances; but the dilata-tions of liquids and solids have hitherto presented noregularity, and it is only those of elastic fluids which areequal and independent of the nature of each gas. Theattraction of the molecules in solids and liquids is, there-fore, the cause which modifies their special properties;and it appears that it is only when the attraction isentirely destroyed, as in gases, that bodies under similarconditions obey simple and regular laws. At least, it ismy intention to make known some new properties ingases, the effects of which are regular, by showing thatthese substances combine amongst themselves in verysimple proportions, and that the contraction of volumewhich they experience on combination also follows a

* Memoires de la Societe d'Arcueil, II. (1809), pp. 207-234.


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Combination of Gaseous Substances. 9regular law. I hope by this means to give a proof of anidea advanced by several very distinguished chemiststhat we are perhaps not far removed from the time whenwe shall be able to submit the bulk of chemical pheno-mena to calculation.

It is a very important question in itself, and one muchdiscussed amongst chemists, to ascertain if compoundsare formed in all sorts of proportions. M. Proust, whoappears first to have fixed his attention on this subject, isof opinion that the metals are susceptible of only two de-grees of oxidation, a minimum and a maximum ; but ledaway by this seductive theory, he has seen himself forced toentertain principles contrary to physics in order to reduceto two oxides all those which the same metal sometimespresents. M. Berthollet thinks, on the other handreasoning from general considerations and his own ex-periments that compounds are always formed in veryvariable proportions, unless they are determined byspecial causes, such as crystallisation, insolubility, orelasticity. Lastly, Dalton has advanced the idea thatcompounds of two bodies are formed in such a way thatone atom of the one unites with one, two, three, or moreatoms of the other.* It would follow from this mode oflooking at compounds that they are formed in constantproportions, the existence of intermediate bodies beingexcluded, and in this respect Dalton's theory wouldresemble that of M. Proust; but M. Berthollet hasalready strongly opposed it in the Introduction he haswritten to Thomson's Chemistry, and we shall see thatin reality it is not entirely exact. Such is the state of the

* Dalton has been led to this idea by systematic considerations ;and one may see from his work, A New System of ChemicalPhilosophy, p. 213, and from that of Thomson, Vol. 6, that hisresearches have no connection with mine.


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io Gay-Lussac.question now under discussion ; it is still very far fromreceiving its solution, but I hope that the facts which Inow proceed to set forth, facts which had entirely escapedthe notice of chemists, will contribute to its elucidation.

Suspecting, from the exact ratio of TOO of oxygen to200 of hydrogen, which M. Humboldt and I had deter-mined for the proportions of water, that other gases mightalso combine in simple ratios, I have made the followingexperiments. I prepared fluoboric,* muriatic, and carbonicgases, and made them combine successively with ammoniagas. 100 parts of muriatic gas saturate precisely100 parts of ammonia gas, and the salt which is formedfrom them is perfectly neutral, whether one or other ofthe gases is in excess. Fluoboric gas, on the contrary,unites in two proportions with ammonia gas. When theacidt gas is put first into the graduated tube, and theother gas is then passed in, it is found that equal volumesof the two condense, and that the salt formed is neutral.But if we begin by first putting the ammonia gas into thetube, and then admitting the fluoboric gas in singlebubbles, the first gas will then be in excess with regard tothe second, and there will result a salt with excess ofbase, composed of 100 of fluoboric gas and 200 ofammonia gas. If carbonic gas is brought into contactwith ammonia gas, by passing it sometimes first, some-times second into the tube, there is always formed asub-carbonate composed of 100 parts of carbonic gas and200 of ammonia gas. It may, however, be proved thatneutral carbonate of ammonia would be composed ofequal volumes of each of these components. M. Ber-

* M. Thenard and I have given the name of fluoboric gas to thatparticular gas which we obtained by distilling pure fluoride of limewith vitreous boracic acid.

t ["Alkaline" in the original.]


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Combination of Gaseous Substances. 1 1thollet, who has analysed this salt, obtained by passingcarbonic gas into the sub-carbonate, found that it wascomposed of 73.34 parts by weight of carbonic gas and26.66 of ammonia gas. Now, if we suppose it to becomposed of equal volumes of its components, we findfrom their known specific gravity, that it contains byweight

71.81 of carbonic acid,28.19 of ammonia,

100.0a proportion differing only slightly from the preceding.

If the neutral carbonate of ammonia could be formedby the mixture of carbonic gas and ammonia gas, asmuch of one gas as of the other would be absorbed ; andsince we can only obtain it through the intervention ofwater, we must conclude that it is the affinity of thisliquid which competes with that of the ammonia to over-come the elasticity of the carbonic acid, and that theneutral carbonate of ammonia can only exist through themedium of water.Thus we may conclude that muriatic, fluoboric, andcarbonic acids take exactly their own volume of ammonia

gas to form neutral salts, and that the last two take twiceas much to form sub-salts. It is very remarkable to seeacids so different from one another neutralise a volumeof ammonia gas equal to their own ; and from this wemay suspect that if all acids and all alkalis could beobtained in the gaseous state, neutrality would resultfrom the combination of equal volumes of acid andalkali.

It is not less remarkable that, whether we obtain aneutral salt or a sub-salt, their elements combine in simpleratios which may be considered as limits to their propor-tions. Accordingly, if we accept the specific gravity ofX OF THE(UNIV ^


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1 2 Gay-Lussac.muriatic acid determined by M. Biot and myself,* andthose of carbonic gas and ammonia given by MM. Biotand Arago, we find that dry muriate of ammonia iscomposed of

Ammonia, 100.0 38.35Muriatic acid, 160.7 61.65100.oo

a proportion very far from that of M. Berthollet100 of ammonia,2 1 3 of acid.

In the same way, we find that sub-carbonate ofammonia containsAmmonia, 100.0 43.98Carbonic acid, 127.3 56.02

100.00and the neutral carbonate

Ammonia, 100.0 28.19Carbonic acid, 254.6 71.81TOO.OO

It is easy from the preceding results to ascertain theratios of the capacity of fluoboric, muriatic, and carbonicacids; for since these three gases saturate the samevolume of ammonia gas, their relative capacities will beinversely as their densities, allowance having been madefor the water contained in muriatic acid.We might even now conclude that gases combine witheach other in very simple ratios ; but I shall still givesome fresh proofs.* As muriatic gas contains one-fourth its weight of water, we

must only take three-fourths of the density for that of real muriaticacid.


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Combination of Gaseous Substances. 1 3According to the experiments of M. Amedee Berthollet,ammonia is composed of

100 of nitrogen,300 of hydrogen,

by volume.I have found (ist vol. of the Societe d'Arcueil) that

sulphuric acid is composed of100 of sulphurous gas,50 of oxygen gas.When a mixture of 50 parts of oxygen and 100 of

carbonic oxide- (formed by the distillation of oxide ofzinc with strongly calcined charcoal) is inflamed, these twogases are destroyed and their place taken by 100 partsof carbonic acid gas. Consequently carbonic acid maybe considered as being composed of

100 of carbonic oxide gas,50 of oxygen gas.

Davy, from the analysis of various compounds ofnitrogen with oxygen, has found the following propor-tions by weight :

Nitrogen. Oxygen.Nitrous oxide - - 63.30 36.70Nitrous gas - - 44.05 55.95Nitric acid - - 29.50 70.50

Reducing these proportions to volumes we findNitrogen. Oxygen.

Nitrous oxide - - 100 49.5Nitrous gas - 100 108.9Nitric acid - 100 204.7

The first and the last of these proportions differ onlyslightly from 100 to 50, and 100 to 200 ; it is only thesecond which diverges somewhat from 100 to 100. Thedifference, however, is not very great, and is such as wemight expect in experiments of this sort; and I haveassured myself that it is actually nil. On burning the


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14 Gay-Lussac.new combustible substance from potash in 100 parts byvolume of nitrous gas, there remained over exactly 50parts of nitrogen, the weight of which, deducted fromthat of the nitrous gas (determined with great care by M.Berard at Arcueil), yields as result that this gas is com-posed of equal parts by volume of nitrogen and oxygen.We may then admit the following numbers for the pro-portions by volume of the compounds of nitrogen withoxygen :

Nitrogen. Oxygen.Nitrous oxide - 100 50Nitrous gas - - 100 100Nitric acid - 100 200

From my experiments, which differ very little fromthose of M. Chenevix, oxygenated muriatic acid * is com-posed by weight ofOxygen - 22.92

Muriatic acid 77.08Converting these quantities into volumes, we find that

oxygenated muriatic acid is formed ofMuriatic gas - 300.0Oxygen gas - 103.2

a proportion very nearlyMuriatic gas - 300Oxygen gas - ioof

* [Chlorine.]f In the proportion by weight of oxygenated muriatic acid, the

muriatic acid is supposed to be free from water, whilst in the pro-portion by volume it is supposed to be combined with a fourth ofits weight of water, which, since the reading of this paper, M.Thenard and I have proved to be absolutely necessary for its exist-ence in the gaseous state. But since the simple ratio of 300 of acidto loo of oxygen cannot be due to chance, we must conclude thatwater by combining with dry muriatic acid to form ordinary muriaticacid does not sensibly change its specific gravity. We should beled to the same conclusion from the consideration that the specific


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Combination of Gaseous Substances. 1 5Thus it appears evident to me that gases always com-

bine in the simplest proportions when they act on oneanother ; and we have seen in reality in all the precedingexamples that the ratio of combination is i to i, i to 2, ori to 3. It is very important to observe that in consideringweights there is no simple and finite relation between theelements of any one compound ; it is only when there isa second compound between the same elements that thenew proportion of the element that has been added is amultiple of the first quantity. Gases, on the contrary, inwhatever proportions they may combine, always give riseto compounds whose elements by volume are multiplesof each other.Not only, however, do gases combine in very simple

proportions, as we have just seen, but the apparent con-traction of volume which they experience on combinationhas also a simple relation to the volume of the gases, orat least to that of one of them.

I have said, following M. Berthollet, that 100 parts ofcarbonic oxide gas, prepared by distilling oxide of zincand strongly calcined charcoal, produce 100 parts of car-bonic gas on combining with 50 of oxygen. It followsfrom this that the apparent contraction of the two gasesis precisely equal to the volume of oxygen gas added.The density of carbonic gas is thus equal to that of car-bonic oxide gas plus half the density of oxygen gas ; or,conversely, the density of carbonic oxide gas is equal tothat of carbonic gas, minus half that of oxygen gas.Accordingly, taking the density of air as unity, we findgravity of oxygenated muriatic acid, which from our experimentscontains no water, is exactly the same as that obtained by adding thedensity of oxygen gas to three times that of muriatic gas, and takinghalf of this sum. M. Thenard and I have also found that oxygen-ated muriatic gas contains precisely half its volume of oxygen, andthat it can destroy in consequence its own volume of hydrogen.

j OF THE ^w(XJNIVERSITTT)x. OF S


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16 Gay-Lussac.the density of carbonic oxide gas to be 0.9678, instead of0.9569 experimentally determined by Cruickshanks.We know, besides, that a given volume of oxygen pro-duces an equal volume of carbonic acid ; consequentlyoxygen gas doubles its volume on forming carbonic oxidegas with carbon, and so does carbonic gas on beingpassed over red-hot charcoal. Since oxygen produces anequal volume of carbonic gas, and the density of thelatter is well known, it is easy to calculate the proportionof its elements. In this way we find that carbonic gasis composed of

27.38 of carbon,72.62 of oxygen,and carbonic oxide of42.99 of carbon,57.01 of oxygen.

Pursuing a similar course, we find that if sulphur ta\kes100 parts of oxygen to produce sulphurous acid, it takes150 parts to produce sulphuric acid. As a matter offact, we find that sulphuric acid, according to the experi-ments of MM. Klaproth, Bucholz, and Richter, is com-posed of 100 parts by weight of sulphur and 138 ofoxygen.On the other hand sulphuric acid is composed of 2parts by volume of sulphurous gas, and i of oxygen gas.Consequently the weight of a certain quantity of sulphuricacid should be the same as that of 2 parts of sulphurousacid and i of oxygen gas, i.e., 2 x 2.265, P^us I - 10359 =5- 63359 ', seeing that, according to Kirwan, sulphurousgas weighs 2.265, tne density of air being taken as unity.But from the proportion of 100 of sulphur to 138 ofoxygen, this quantity contains 3.26653 of oxygen, and ifwe subtract from it 1.10359 there will remain 2.16294 forthe weight of oxygen in 2 parts of sulphurous acid, or1.08147 for the weight of oxygen contained in i part.


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Combination of Gaseous Substances. 17Now as this last quantity only differs by 2 per cent,

from 1.10359, which represents the weight of i part ofoxygen gas, it must be concluded that oxygen gas, incombining with sulphur to form sulphurous gas, only ex-periences a diminution of a fiftieth of its volume, and thiswould probably be nil if the data I have employed weremore exact. On this last supposition, using Kirwan'svalue for the specific gravity of sulphurous gas, we shouldfind that this acid is composed of100.00 of sulphur,

95.02 of oxygen.But if, adopting the preceding proportions for sulphuric

acid, we allow, as appears probable, that 100 of sulphur-ous gas contain 100 of oxygen gas, and that 50 havestill to be added to convert it into sulphuric acid, weshall obtain for the proportions in sulphurous acid

100.00 of sulphur,92.0 of oxygen.

Its specific gravity calculated on the same suppositions,and referred to that of air, would be 2.30314, instead of2.2650 as Kirwan found directly.*

* In order to remove these differences it would be necessary tomake new experiments on the density of sulphurous gas, on thedirect union of oxygen gas with sulphur to see if there is contraction,and on the union of sulphurous gas with ammonia gas. I havefound, it is true, on heating cinnabar in oxygen gas, that 100 partsof this gas only produce 93 of sulphurous gas. It also appeared asif less sulphurous gas than ammonia gas was necessary to form aneutral salt. But as these experiments were not made under suit-able conditions, especially the last, which could only be made inpresence of water, the sulphurous gas decomposing and precipitatingsulphur immediately on being mixed with the ammonia gas, Iintend to repeat them and determine exactly all the conditionsbefore drawing any conclusion from them. This is all the morenecessary, as sulphurous gas can be used to analyse sulphurettedhydrogen gas, if its proportions are well known.

B


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1 8 Gay-Lussac.Phosphorus is very closely connected with sulphur,

seeing that both have nearly the same specific gravity.Consequently phosphorus should take up twice as muchoxygen to become phosphorous acid, as to pass from thisstate into phosphoric acid. Since the latter is composed,according to Rose, of

100.0 of phosphorus,114.0 of oxygen,

it follows that phosphorous acid should containi oo.o of phosphorus,76.0 of oxygen.

We have seen that 100 parts of nitrogen gas take 50parts of oxygen gas to form nitrous oxide, and 100 ofoxygen gas to form nitrous gas. In the first case, thecontraction is a little greater than the volume of oxygenadded ; for the specific gravity of nitrous oxide, calculatedon this hypothesis, is 1.52092, while that given by Davyis 1.61414. But it is easy to show, from some of Davy'sexperiments, that the apparent contraction is preciselyequal to the volume of oxygen gas added. On passingthe electric spark through a mixture of 100 parts ofhydrogen and 97.5 of nitrous oxide the hydrogen isdestroyed, and 102 parts of nitrogen remain, includingthat quantity which is almost always mixed with thehydrogen, and a little of the latter gas which has escapedcombustion. The residue, after making all corrections,would be very nearly equal in volume to the nitrousoxide employed. Similarly, on passing the electric sparkthrough a mixture of 100 parts of phosphuretted hydrogenand 250 of nitrous oxide, water and phosphoric acid areformed, and exactly 250 parts of nitrogen remain,another evident proof that the apparent contraction ofthe elements of nitrous oxide is equal to the whole volume


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Combination of Gaseous Substances. 19of oxygen added. From this circumstance, its specificgravity referred to that of air should be 1.52092.The apparent contraction of the elements of nitrous

gas appears, on the other hand, to be nil. If we admit,as I have shown, that it is composed of equal parts ofoxygen and nitrogen, we find that its density, calculatedon the assumption that there is no contraction, is 1.036,while that determined directly is 1.038.

Saussure found that the density of water vapour is tothat of air as 10 is to 14. Assuming that the contractionof volume of the two gases is only equal to the wholevolume of oxygen added, we find instead of this a ratioof 10 to 1 6. This difference, and the authority of aphysicist so distinguished as Saussure, would seem to beenough to make us reject the assumption I have justmade; but I shall mention several circumstances thatrender it very probable. Firstly, it has a very stronganalogy in its favour; secondly, M. Trales found bydirect experiment that the ratio of the density of water-vapour to air is 10 to 14.5, instead of 10 to 14 ; thirdly,although we do not know very exactly the volume occu-pied by water on passing into the elastic state, we doknow, from the experiments of Watt, that a cubic inch ofwater produces nearly a cubic foot of steam, i.e., a volume1728 times as great. Now, adopting Saussure's ratio, wefind only 1488 for the volume occupied by water when itis converted into steam ; but adopting the ratio of 10 to1 6, we should have 1700.6. Finally, the refraction ofwater-vapour, calculated on the assumption of the ratio10 to 14, is a little greater than the observed refraction;but that calculated from the ratio 10 to 16 is much morein harmony with the results of experiment. These, then,are the considerations which go to make the ratio 10 to1 6 very probable.Ammonia gas is composed of three parts by volume of


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2O Gay-Lussac.hydrogen and one of nitrogen, and its density comparedto air is 0.596. But if we suppose the apparent contrac-tion to be half of the whole volume, we find 0.594 for thedensity. Thus it is proved, by this almost perfect con-cordance, that the apparent contraction of its elements isprecisely half the total volume, or rather double thevolume of the nitrogen.

I have already proved that oxygenated muriatic gas iscomposed of 300 parts of muriatic gas and 100 of oxygengas. Admitting that the apparent contraction of the twogases is half the whole volume, we find 2.468 for itsdensity, and by experiment 2.470. I have also assuredmyself by several experiments that the proportions of itselements are such that it forms neutral salts with themetals. For example, if we pass oxygenated muriatic gasover copper, there is formed a slightly acid green muriate,and a little oxide of copper is precipitated, because thesalt cannot be obtained perfectly neutral. It follows fromthis that in all the muriates, as in oxygenated muriaticacid, the acid reduced to volume is thrice the oxygen.It would be the same for carbonates and fluorides, theacids of which have for equal volumes the same saturationcapacity as muriatic acid.We see, then, from these various examples, that thecontraction experienced by two gases on combination isin almost exact relation with their volume, or rather withthe volume of one of them. Only very slight differencesexist between the densities of compounds obtained bycalculation and those given by experiment, and it isprobable that, on undertaking new researches, we shallsee them vanish entirely.

Recalling the great law of chemical affinity, that everycombination involves an approximation of the elementarymolecules, it is difficult to conceive why carbonic oxidegas should be lighter than oxygen. Indeed, that is the


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Combination of Gaseous Substances. 2 1principal reason which has led M. Berthollet to assume theexistence of hydrogen in this gas,

and thus explain its lowdensity. But it seems to me that the difficulty arisesfrom supposing that the approximation of the elementarymolecules is represented in gases by the diminution ofvolume which they suffer on combination. This supposi-tion is not always true, and we might cite several gaseouscombinations, the constituent molecules of which wouldbe brought very close together, although there is not onlyno diminution of volume, but even a dilatation. Such,for example, is nitrous gas, whether we consider it asbeing formed directly from nitrogen and oxygen, or fromnitrous oxide and oxygen. In the first case, there is nodiminution of volume; and in the second, there would bedilatation, for 100 parts of nitrous oxide and 50 of oxygenwould produce 200 of nitrous gas. We know too thatcarbonic gas represents an exactly equal volume of oxygen,and that the affinity which unites its elements is verypowerful. Nevertheless, if we admitted an immediaterelation between the condensation of the elements andthe condensation of volume, we should conclude, con-trary to experiment, that there is no condensation.-Otherwise it would be necessary to suppose that ifcarbon were in the gaseous state it would combine inequal volumes (or in any other proportion) with oxygen,and that the apparent condensation would then be equalto the whole volume of the gaseous carbon. But if wemake this supposition for carbonic acid, we may alsomake it for carbonic oxide, by assuming, for instance,that 100 parts of gaseous carbon would produce 100parts of the gas on combining with 50 parts of oxygen.However it may stand with these suppositions, whichonly serve to make it conceivable that oxygen can pro-duce a compound lighter than itself by combining with asolid substance, we must admit, as a truth founded on 'a

OF THEUNIVERsi'i


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2 2 Gay-Lussac.great number of observations, that the condensation ofthe molecules of two combining substances, in particularof two gases, has no immediate relation to the condensa-tion of volume, since we often see that whilst one is verygreat the other is very small or even nil.The observation that the gaseous combustibles com-bine with oxygen in the simple ratios of i to i, i ,to 2,i to J, can lead us to determine the density of thevapours of combustible substances, or at least to approxi-mate closely to that determination. For if we supposeall combustible substances to be in the gaseous state, aspecified volume of each would absorb an equal volumeof oxygen, or twice as much, or else half; and as weknow the proportion of oxygen taken up by each com-bustible substance in the solid or liquid state, it is suffi-cient to convert the oxygen into volumes and also thecombustible, under the condition that its vapour shall beequal to the volume of oxygen, or else double or half thisvalue. For example, mercury is susceptible of two degreesof oxidation, and we may compare the first one to nitrousoxide. Now, according to MM. Fourcroy and Thenard,100 parts of mercury absorb 4.16, which reduced togas would occupy a space of 8.20. These TOO partsof mercury reduced to vapour should therefore occupytwice the space, viz., 16.40. We thence conclude thatthe density of mercury vapour is 12.01 greater than thatof oxygen, and that the metal on passing from the liquidto the gaseous state assumes a volume 961 times as great.

I shall not discuss more of these determinations,because they are only based on analogies, and it isbesides easy to multiply them. I shall conclude thisMemoir by examining if compounds are formed in con-stant or variable proportions, as the experiments of whichI have just given an account lead me to the discussion ofthese two opinions.


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Combination of Gaseous Substances. 23According to Dalton's ingenious idea, that combina-

tions are formed from atom to atom, the variouscompounds which two substances can form would beproduced by the union of one molecule of the one withone molecule of the other, or with two, or with a greaternumber, but always without intermediate compounds.Thomson and Wollaston have indeed described experi-ments which appear to confirm this theory. Thomson *has found that super-oxalate of potash contains twice asmuch acid as is necessary to saturate the alkali; andWollaston, t that the sub-carbonate of potash contains, onthe other hand, twice as much alkali as is necessary tosaturate the acid.The numerous results I have brought forward in thisMemoir are also very favourable to the theory. But M.

Berthollet, who thinks that combinations are made con-tinuously, cites in proof of his opinion the acid sulphates,glass, alloys, mixtures of various liquids, all of whichare compounds with very variable proportions, and heinsists principally on the identity of the force which pro-|duces chemical compounds and solutions.Each of these two opinions has, therefore, a largenumber of facts in its favour; but although they areapparently utterly opposed, it is easy to reconcile them.

. We must first of all admit, with M. Berthollet, thatchemical action is exercised indefinitely in a continuousmanner between the molecules of substances, whatever (their number and ratio may be, and that 'in general wecan obtain compounds with very variable proportions.But then we must admit at the same time that, apartfrom insolubility, cohesion, and elasticity, which tend toproduce compounds in fixed proportions, chemical action

* [See Alembic Club Reprints, No. 2, p. 41.]f [See Alembic Club Reprints, No. 2, p. 35.]


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24 Gay-Lussac.is exerted more powerfully when the elements are insimple ratios or in multiple proportions among them-selves, and that compounds are thus produced whichseparate out more easily. In this way we reconcile thetwo opinions, and maintain the great chemical law, thatwhenever two substances are in presence of each otherthey act in their sphere of activity according to theirmasses, and give rise in general to compounds with veryvariable proportions, unless these proportions are deter-mined by special circumstances.

Conclusion.I have shown in this Memoir that the compounds of

gaseous substances with each other are always formed invery simple ratios, so that representing one of the termsby unity, the other is i, or 2, or at most 3. These ratiosby volume are not observed with solid or liquid sub-stances, nor when we consider weights, and they form anew proof that it is only in the gaseous state that sub-stances are in the same circumstances and obey regularlaws. It is remarkable to see that ammonia gas neutralisesexactly its own volume of gaseous acids; and it is probablethat if all acids and alkalies were in the elastic state, theywould all combine in equal volumes to produce neutralsalts. The capacity of saturation of acids and alkaliesmeasured by volume would then be the same, and thismight perhaps be the true manner of determining it.The apparent contraction of volume suffered by gases oncombination is also very simply related to the volumeof one of them, and this property likewise is peculiar togaseous substances.


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New System of Chemical Philosophy. 25

3. EXTRACT FROM A NEW SYSTEM OFCHEMICAL PHILOSOPHY. PART II.(1810). BY JOHN DALTON.

(Appendix, pp. 555~559-)SOME observations on nitric acid, and the other com-pounds of azote and oxygen, have been made by GayLussac, in the 2d vol. of the Memoires d'Arcueil. Hecontends that one measure of oxygenous gas unites totwo measures of nitrous gas to form nitric acid, and tothree measures to form nitrous acid. Now I have shewnthat i measure of oxygen may be combined with 1.3 ofnitrous gas, or with 3.5, or with any intermediate quantitywhatever, according to circumstances, which he seems toallow : what, then, is the nature of the combinations below2, and above 3, of nitrous gas ? No answer is given tothis ; but the opinion is founded upon an hypothesis thatall elastic fluids combine in equal measures, or in measuresthat have some simple relation one to another, as i to 2,i to 3, 2 to 3, &c. In fact, his notion of measures isanalogous to mine of atoms ; and if it could be proved \that all elastic fluids have the same number of atoms in

jthe same volume, or numbers that are as i, 2, 3, &c. the |two hypotheses would be the same, except that mine isuniversal, and his applies only to elastic fluids. GayLussac could not but see (page 188, Part i. of this work)that a similar hypothesis had been entertained by me,and abandoned as untenable ; however, as he has revivedthe notion, I shall make a few observations upon it,though I do not doubt but he will soon see its inadequacy.

Nitrous gas is, according to Gay Lussac, constituted ofequal measures of azote and oxygen, which, when com-


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26 Dalton.bined, occupy the same volume as when free. He quotesDavy, who found 44.05 azote, and 55.95 oxygen byweight, in nitrous gas. He converts these into volumes,,and finds them after the rate of TOO azote to 108.9 oxygen.There is, however, a mistake in this ; if properly reduced,it gives 100 azote to 112 oxygen, taking the specificgravities according to Biot and Arago. But that Davyhas overrated the oxygen 12 per cent, he shews byburning potassium in nitrous gas, when 100 measures-afforded just 50 of azote. The degree of purity of thenitrous gas, and the particulars of the experiment, are notmentioned. This one result is to stand against the meanof three experiments of Davy, and may or may not bemore correct, as hereafter shall appear. Dr. Henry'sanalysis of ammonia embraces that of nitrous gas also ;he finds TOO measures of ammonia require 120 of nitrousgas for their saturation. Now this will apply to GayLussac's theory in a very direct manner ; for, accordingto him, ammonia is formed of i measure of azote and 3of hydrogen, condensed into a volume of 2 ; it follows,then, that 100 ammonia require 75 oxygen to saturatethe hydrogen; hence, 120 nitrous gas should contain75 oxygen > or I0 should contain 62.5, instead of 50.Here either the theory of Gay Lussac, or the experienceof Dr. Henry, must give results wide of the truth. Inregard to ammonia too, it may farther be added, thatneither is the rate of azote to hydrogen i to 3, nor is thevolume of ammonia doubled by decomposition, accordingto the experiments of Berthollet, Davy, and Henry, madewith the most scrupulous attention to accuracy, to whichmay be added my own. There is another point of viewin which this theory of Gay Lussac is unfortunate, inregard to ammonia and nitrous gas ; i measure of azotewith 3 of hydrogen, forms 2 of ammonia ; and i measureof azote with i of oxygen, forms 2 of nitrous gas : now,


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New System of Chemical Philosophy. 27according to a well established principle in chemistry,i measure of oxygen ought to combine with 3 of hydrogen,or with one half as much, or twice as much ; but no oneof these combinations takes place. If Gay Lussac adoptmy conclusions, namely, that 100 measures of azoterequire about 250 hydrogen to form ammonia, and that100 azote require about 120 oxygen to form nitrous gas,he will perceive that the hydrogen of the former wouldunite to the oxygen of the latter, and form water, leavingno excess of either, further than the unavoidable errors ofexperiments might produce ; and thus the great chemicallaw would be preserved. The truth is, I believe, that'gases do not unite in equal or exact measures in any oneinstance ; when they appear to do so, it is owing to theinaccuracy of our experiments. In no case, perhaps, isthere a nearer approach to mathematical exactness, thanin that of i measure of oxygen to 2 of hydrogen ; buthere, the most exact experiments I have ever made, gave1.97 hydrogen to i oxygen.


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28 Avogadro.

4. ESSAY ON A MANNER OF DETER-MINING THE RELATIVE- MASSES OFTHE ELEMENTARY MOLECULES OFBODIES, AND THE PROPORTIONS INWHICH THEY ENTER INTO THESECOMPOUNDS. BY A. AVOGADRO *

I.

MGAY-LUSSAC has shown in an interesting. Memoir (Memoires de la Societe* d'Arcueil,Tome II.) that gases always unite in a very simple pro-portion by volume, and that when the result of the unionis a gas, its volume also is very simply related to those ofits components. But the quantitative proportions of sub-stances in compounds seem only to depend on the rela-tive number of molecules t which combine, and on thenumber of composite molecules which result. It mustthen be admitted that very simple relations also existbetween the volumes of gaseous substances and thenumbers of simple or compound molecules which form

* Journal de Physique, LXXIII. (1811), pp. 58-76.t [Avogadro has been accused of inconsistency in his use of the

term " molecule," but a careful perusal of his paper will show thathe uses it with its qualifying adjectives quite consistently, asfollows :MoUcule (translated " molecule ") without qualification means inmodern chemical phraseology either atom or molecttle.MoUcule inttgrante (translated "integral molecule") means

molecule in general, but is usually applied only to compounds.MoUcule constitiiante (translated "constituent molecule") is em-ployed to denote the molecule of an elementary substance.

Molecule elementaire (translated "elementary molecule ") standsfor the atom of an elementary substance.]


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Relative Masses of Elementary Molecules. 29them. The first hypothesis to present itself in this con-nection, and apparently even the only admissible one, isthe supposition that the number of integral molecules inany gases is always the same for equal volumes, oralways proportional to the volumes. Indeed, if we wereto suppose that the number of molecules contained in agiven volume were different for different gases, it wouldscarcely be possible to conceive that the law regulatingthe distance of molecules could give in all cases relationsso simple as those which the facts just detailed compelus to acknowledge between the volume and the numberof molecules. On the other hand, it is very well conceiv-able that the molecules of gases being at such a distancethat their mutual attraction cannot be exercised, theirvarying attraction for caloric may be limited to condens-ing a greater or smaller quantity around them, withoutthe atmosphere formed by this fluid having any greaterextent in the one case than in the other, and, conse-quently, without the distance between the molecules vary-ing ; or, in other words, without the number of moleculescontained in a given volume being different. Dalton, itis true, has proposed a hypothesis directly opposed tothis, namely, that the quantity of caloric is always thesame for the molecules of all bodies whatsoever in thegaseous state, and that the greater or less attraction forcaloric only results in producing a greater or less con-densation of this quantity around the molecules, andthus varying the distance between the molecules them-selves. But in our present ignorance of the manner inwhich this attraction of the molecules for caloric isexerted, there is nothing to decide us a priori in favourof the one of these hypotheses rather than the other ;and we should rather be inclined to adopt a neutralhypothesis, which would make the distance between themolecules and the quantities of caloric vary according

(UNIVERSITY)'^S


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3O Avogadro.to unknown laws, were it not that the hypothesis we havejust proposed is based on that simplicity of relationbetween the volumes of gases on combination, whichwould appear to be otherwise inexplicable.

Setting out from this hypothesis, it is apparent that wehave the means of determining very easily the relativemasses of the molecules of substances obtainable in thegaseous state, and the relative number of these moleculesin compounds ; for the ratios of the masses of the mole-cules are then the same as those of the densities of thedifferent gases at equal temperature and pressure, andthe relative number of molecules in a compound is givenat once by the ratio of the volumes of the gases thatform it. For example, since the numbers 1.10359 and0.07321 express the densities of the two gases oxygenand hydrogen compared to that of atmospheric air asunity, and the ratio of the two numbers consequentlyrepresents the ratio between the masses of equal volumesof these two gases, it will also represent on our hypo-thesis the ratio of the masses of their molecules. Thusthe mass of the molecule of oxygen will be about 15times that of the molecule of hydrogen, or, more exactly,as 15.074 to i. In the same way the mass of the mole-cule of nitrogen will be to that of hydrogen as 0.96913 to0.07321, that is, as 13, or more exactly 13.238, to i.On the other hand, since we know that the ratio of thevolumes of hydrogen and oxygen in the formation ofwater is 2 to i, it follows that water results from theunion of each molecule of oxygen with two molecules ofhydrogen. Similarly, according to the proportions byvolume established by M. Gay-Lussac for the elementsof ammonia, nitrous oxide, nitrous gas, and nitric acid,ammonia will result from the union of one molecule ofnitrogen with three of hydrogen, nitrous oxide from onemolecule of oxygen with two of nitrogen, nitrous gas


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Relative Masses of Elementary Molecules. 3 1from one molecule of nitrogen with one of oxygen, andnitric acid from one of nitrogen with two of oxygen.

II.There is a consideration which appears at first sight to

be opposed to the admission of our hypothesis with respectto compound substances. It seems that a moleculecomposed of two or more elementary molecules shouldhave its mass equal to the sum of the masses of thesemolecules ; and that in particular, if in a compound onemolecule of one substance unites with two or moremolecules of another substance, the number of compoundmolecules should remain the same as the number of mole-cules ofthe first substance. Accordingly, on our hypothesiswhen a gas combines with two or more times its volumeof another gas, the resulting compound, if gaseous, musthave a volume equal to that of the first of these gases.Now, in general, this is not actually the case. Forinstance, the volume of water in the gaseous state is, asM. Gay-Lussac has shown, twice as great as the volumeof oxygen which enters into it, or, what comes to thesame thing, equal to that of the hydrogen instead ofbeing equal to that of the oxygen. But a means of explain-ing facts of this type in conformity with our hypothesispresents itself naturally enough : we suppose, namely,that the constituent molecules of any simple gas what-ever (i.e., the molecules which are at such a distancefrom each other that they cannot exercise their mutualaction) are not formed of a solitary elementary molecule,but are made up of a certain number of these moleculesunited by attraction to form a single one ; and further,that when molecules of another substance unite with theformer to form a compound molecule, the integral mole-cule which should result splits up into two or more parts(or integral molecules) composed of half, quarter, &c., the


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32 Avogadro.number of elementary molecules going to form the con-stituent molecule of the first substance, combined withhalf, quarter, &c., the number of constituent moleculesof the second substance that ought to enter into com-bination with one constituent molecule of the first sub-stance (or, what comes to the same thing, combined witha number equal to this last of half-molecules, quarter-molecules, &c., of the second substance) ; so that thenumber of integral molecules of the compound becomesdouble, quadruple, &c., what it would have been if therehad been no splitting-up, and exactly what is necessary tosatisfy the volume of the resulting gas.*On reviewing the various compound gases most gene-rally known, I only find examples of duplication of thevolume relatively to the volume of that one of the consti-tuents which combines with one or more volumes of theother. We have already seen this for water. In thesame way, we know that the volume of ammonia gas istwice that of the nitrogen which enters into it. M. Gay-Lussac has also shown that the volume of nitrous oxideis equal to that of the nitrogen which forms part of it,and consequently is twice that of the oxygen. Finally,nitrous gas, which contains equal volumes of nitrogen andoxygen, has a volume equal to the sum of the two con-stituent gases, that is to say, double that of each of them.Thus in all these cases there must be a division of themolecule into two ; but it is possible that in other casesthe division might be into four, eight, &c. The possi-bility of this division of compound molecules might havebeen conjectured a priori ; for otherwise the integralmolecules of bodies composed of several substances witha relatively large number of molecules, would come to

* Thus, for example, the integral molecule of water will be com-posed of a half-molecule of oxygen with one molecule, or, what isthe same thing, two half-molecules of hydrogen.


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Relative Masses of Elementary Molecules. 33have a mass excessive in comparison with the moleculesof simple substances. We might therefore imagine thatnature had some means of bringing them back to theorder of the latter, and the facts have pointed out to usthe existence of such means. Besides, there is anotherconsideration which would seem to make us admit insome cases the division in question ; for how could oneotherwise conceive a real combination between twogaseous substances uniting in equal volumes without con-densation, such as takes place in the formation of nitrousgas ? Supposing the molecules to remain at such adistance that the mutual attraction of those of each gascould not be exercised, we cannot imagine that a newattraction could take place between the molecules of onegas and those of the other. But on the hypothesis ofdivision of the molecule,* it is easy to see that the com-bination really reduces two different molecules to one,and that there would be contraction by the whole volumeof one of the gases if each compound molecule did notsplit up into two molecules of the same nature. M. Gay-Lussac clearly saw that, according to the facts, thediminution of volume on the combination of gases cannotrepresent the approximation of their elementary mole-cules. The division of molecules on combinationexplains to us how these two things may be made inde-pendent of each other.

III.

Dalton, on arbitrary suppositions as to the mostlikely relative number of molecules in compounds, hasendeavoured to fix ratios between the masses of themolecules of simple substances. Our hypothesis, supposingit well-founded, puts us in a position to confirm or rectifyhis results from precise data, and, above all, to assign themagnitude of compound molecules according to the

c


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34 Avogadro.volumes of the gaseous compounds, which depend partlyon the division of molecules entirely unsuspected by thisphysicist.Thus Dalton supposes* that water is formed by theunion of hydrogen and oxygen, molecule to molecule.From this, and from the ratio by weight of the two com-ponents, it would follow that the mass of the molecule ofoxygen would be to that of hydrogen as 7^ to i nearly,or, according to Dalton's evaluation, as 6 to i. Thisratio on our hypothesis is, as we saw, twice as great,namely, as 15 to i. As for the molecule of water, itsmass ought to be roughly expressed by 15 + 2 = 17 (takingfor unity that of hydrogen), if there were no division ofthe molecule into two ; but on account of this division itis reduced to half, 8|, or more exactly 8.537, as may alsobe found directly by dividing the density of aqueousvapour 0.625 (Gay-Lussac) by the density of hydrogen0.0732. This mass only differs from 7, that assigned toit by Dalton, by the difference in the values for the com-position of water ; so that in this respect Dalton's resultis approximately correct from the combination of twocompensating errors, the error in the mass of the moleculeof oxygen, and his neglect of the division of the molecule.

Dalton supposes that in nitrous gas the combinationof nitrogen and oxygen is molecule to molecule : we haveseen on our hypothesis that this is actually the case.Thus Dalton would have found the same molecularmass for nitrogen as we have, always supposing thatof hydrogen to be unity, if he had not set out from adifferent value for that of oxygen, and if he had takenprecisely the same value for the quantities of the elements

* In what follows I shall make use of the exposition of Dalton'sideas given in Thomson's System of Chemistry. [See Alembic ClubReprints, No. 2, p, 42.]


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Relative Masses of Elementary Molecules. 35in nitrous gas by weight. But by supposing the mole-cule of oxygen to be less than half what we find, he hasbeen obliged to make that of nitrogen also equal to lessthan half the value we have assigned to it, viz., 5 insteadof 13. As regards the molecule of nitrous, gas itself, hisneglect of the division of the molecule again makes hisresult approach ours; he has made it 6 + 5 = 11, whilstaccording to us it is about- = 14, or more exactly15.074+ i3-238 y diyiding21.03636, the density of nitrous gas according to Gay-Lussac, by 0.07321. Dalton has likewise fixed in thesame manner as the facts have given us, the relativenumber of molecules in nitrous oxide and in nitric acid,and in the first case the same circumstance has rectifiedhis result for the magnitude of the molecule. He makesit 6 + 2x5 = 1 6, whilst according to our method it shouldbe- 2obtained by dividing 1.52092, Gay-Lussac's value for thedensity of nitrous oxide, by the density of hydrogen.

In the case of ammonia, Dalton's supposition as to therelative number of molecules in its composition is on ourhypothesis entirely at fault. He supposes nitrogen andhydrogen to'be united in it molecule to molecule, whereaswe have seen that one molecule of nitrogen unites withthree molecules of hydrogen. According to him themolecule of ammonia would be 5 + 1 = 6: according tous it should be ^ ^ = 8, or more exactly 8.119, as maYalso be deduced directly from the density of ammoniagas. The division of the molecule, which does not enterinto Dalton's calculations, partly corrects in this case alsothe error which would result from his other suppositions.

OF THEUNIVERSITY


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36 Avogadro.All the compounds we have just discussed are produced

by the union of one molecule of one of the componentswith one or more molecules of the other. In nitrousacid we have another compound of two of the substancesalready spoken of, in which the terms of the ratiobetween the number of molecules both differ from unity.From Gay-Lussac's experiments (Societe d'Arcueil, samevolume), it appears that this acid is formed from i partby volume of oxygen and 3 of nitrous gas, or, what comesto the same thing, of 3 parts of nitrogen and 5 of oxygen ;whence it would follow, on our hypothesis, that its mole-cule should be composed of 3 molecules of nitrogen and5 of oxygen, leaving the possibility of division out ofaccount. But this mode of combination can be referredto the preceding simpler forms by considering it as theresult of the union of i molecule of oxygen with 3 ofnitrous gas, i.e. with 3 molecules, each composed of ahalf-molecule of oxygen and a half-molecule of nitrogen,,which thus already includes the division of some of themolecules of oxygen which enter into that of nitrousacid. Supposing there to be no other division, themass of this last molecule would be 57.542, that ofhydrogen being taken as unity, and the density of nitrousacid gas would be 4.21267,^6 density of air being takenas unity. But it is probable that there is at least anotherdivision into two, and consequently a reduction of thedensity to half : we must wait until this density has beendetermined by experiment.

IV.We may now look at a few more compounds, which on

our hypothesis can give us at least conjectural informa-tion concerning the relative masses of the molecules andtheir number in these compounds, and compare ourresults with the suppositions of Dalton.


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Relative Masses of Elementary Molecules. 37M. Gay-Lussac has shown that if we assume that dry

sulphuric acid is composed of 100 parts of sulphur and138 of oxygen by weight, as the most recent work ofchemists has established, and that the density of sul-phurous acid gas is 2.265 referred to air as unity(Kirwan's determination), and if we admit, as the resultof Gay-Lussac's experiments, that sulphuric acid is com-posed of two parts by volume of sulphurous acid gasand one of oxygen, then the volume of sulphurous acidis nearly equal to that of the oxygen which entered intoit ; and this equality would be exact if the bases onwhich the calculation rests were the same. If wesuppose Kirwan's determination to be exact, throwingthe whole error on the analysis of sulphuric acid, wefind that in sulphurous acid 100 parts of sulphur take95.02 of oxygen, and consequently in sulphuric acid95.02+^ = 142.53, instead of 138. If, on 'the con-trary, we suppose the analysis of sulphuric acid to beexact, it follows that sulphurous acid contains 92 ofoxygen for 100 of sulphur, and that its specific gravityshould be 2.30314, instead of 2.265.One consideration would appear to weigh in favourof the first assumption, until the density of sulphurousacid gas has been confirmed or rectified by. fresh experi-ments, namely, that there must have been in the deter-mination of the composition of sulphuric acid, a sourceof error tending to increase the quantity of the radical,or, what is the same thing, diminish the quantity ofoxygen. The determination was made from the quantityof dry sulphuric acid produced. Now it seems almostcertain that ordinary sulphur contains hydrogen ; theweight of this hydrogen, which must have been con-verted into water in the operation, has therefore beenadded to the true weight of the radical. I shall therefore


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38 Avogadro.assume sulphurous acid to be composed of 95.02 * ofoxygen to 100 of sulphur, or rather of sulphuric radical,instead of 92.t

In order now to determine the mass of the moleculeof the sulphuric radical, it would be necessary to knowwhat proportion by volume this radical in the gaseousstate would bear to the oxygen in the formation of sul-phurous acid. The analogy with other combinationsalready discussed, where there is in general a doublingof the volume or halving of the molecule, leads us tosuppose that it is the same in this case also, i.e. that thevolume of the sulphur as gas is half that of the sulphurousacid, and consequently also half that of the oxygen withwhich it combines. On this supposition the density ofsulphur gas will be to that of oxygen as 100 to

^^' 2, or2

47.51 ; which gives 2.323 for the density of gaseoussulphur, taking that of air as unity. The masses of themolecules being according to our hypothesis in. the sameratio as the densities of the gases to which they belong,the mass of the molecule of the sulphuric radical will beto that of hydrogen as 2.323 to 0.07321, or as 31.73 to i.One of these molecules combined, as we have said, withtwo of oxygen, will form sulphurous acid (division of themolecule being left out of account), and combined withyet another molecule of oxygen, will form sulphuric acid.Accordingly, sulphurous acid should be analogous to

* [Erroneously 92.02 in the original.]f This was written before I had seen the Memoir of Davy on

oxymuriatic acid, which also contains new experiments on sulphurand phosphorus. In it he determines the density of sulphurousacid gas, and finds it to be only 2.0967, which gives new force tothe above considerations. If we adopt this density, we find that insulphurous acid 100 parts by weight of sulphur take in of oxygen,and in sulphuric acid 167 instead of 138 ; but perhaps this densityof sulphurous acid, according to Davy, is somewhat too low.


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Relative Masses of Elementary Molecules. 39nitric acid, with regard to the relative number ofmolecules of its constituents, sulphuric acid having noanalogue amongst the nitrogen compounds. The mole-cule of sulphurous acid, having regard to division, willbe equal to 31-73 + 2x15.074 Qr ^^ ^ wQuld algobe obtained directly by dividing the density 2.265 fsulphurous acid gas by that of hydrogen gas. As for themolecule of sulphuric acid, it cannot be determined, forwe do not know whether there is further division of themolecule on its formation, or not*

* Davy in the Memoir alluded to, has made the same supposi-tions as to the relative number of molecules of oxygen and radical insulphurous and sulphuric acids. From his determination of thedensity of sulphurous acid gas, the density of the sulphuric radicalwould be 1.9862, and its molecule 27.13, that of hydrogen beingtaken as unity. Davy, by a similar calculation, fixes it at abouthalf, viz., 13.7, because he supposes the molecule of oxygen to beequal to about half our molecule, using Dalton's hypothesis withrespect to water.He finds nearly the same mass, viz., 13.4, by taking as his start-ing-point the density of sulphuretted hydrogen, which his experi-ments make equal to 1.0645, a result only slightly different fromKirwan's, and by assuming that this gas (which contains, as weknow, its own volume of hydrogen combined with sulphur) is com-posed of one molecule of sulphur and one of hydrogen. As wesuppose the molecule of sulphur to be nearly twice as great, wemust assume that this gas is the product of the union of one mole-cule of the radical with two at least of hydrogen, and that itsvolume is twice that of the gaseous radical, as in so many othercases. I say at least with two molecules of hydrogen, for if therewere hydrogen already in ordinary sulphur, as known experimentson this substance indicate, its quantity also must be added. If, forinstance, ordinary sulphur were composed of one molecule of sul-phuric radical and one of hydrogen, sulphuretted hydrogen would becomposed of three molecules of hydrogen and one of radical. Thiscould be decided by the comparison of the specific gravities ofsulphuretted hydrogen and sulphurous acid gas, if both were knownexactly. For example, supposing Davy's determination for sul-


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4O Avogadro.Dalton had supposed that sulphuric acid was com-

posed of two molecules of oxygen to one of radical, andsulphurous acid of one molecule of oxygen to one ofsulphur. These two assumptions are incompatible, foraccording to Gay-Lussac's results the quantities ofoxygen in these two acids for a given quantity of radical,are represented by i and ij. Besides, in his determina-tion of the molecule he set out from a wrong value forthe composition of sulphuric acid, and it is only bychance that the mass 15 which he assigns to it, bears tohis value for the mass of the oxygen molecule a ratio whichapproaches that presented by these two substances onour hypothesis.

Phosphorus has so much analogy with sulphur that wemight apparently assume that phosphoric acid also iscomposed of three molecules of oxygen to one ofradical, and phosphorous acid of only two of oxygen toone of radical. On this assumption we may calculateapproximately the mass of the molecule of the phosphoricradical. Rose found by a method analogous to thatwhich had been employed for sulphuric acid, that phos-phoric acid contains about 115 parts by weight ofoxygen to 100 of phosphorus. There ought to be alittle more oxygen in it if we suppose that phosphorus,like sulphur, contains hydrogen. As an approximationphuretted hydrogen to be exact, the molecule of the sulphuricradical, on the supposition of only two molecules of hydrogen, wouldbe 27.08, that of hydrogen being taken as unity ; but on the sup-position of three molecules of hydrogen, 27.08 would still be thesum of one molecule of radical and one of hydrogen, so that theformer would be reduced to 26.08. If the exact density of sul-phurous acid gas confirmed one or other of these results, it wouldconfirm by that means one or other of these hypotheses : but weare not sufficiently agreed about these densities to be able to drawany conclusion in this respect from the determinations hithertoexisting.


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Relative Masses of Elementary Molecules. 41we can make this increase in the same proportion as wehave seen holds good for sulphuric acid in accordancewith the specific gravity of sulphurous acid, and thusbring the quantity of oxygen up to 120. We then findfrom our hypotheses that the mass of the molecule ofthe phosphoric radical is about 38, that of hydrogenbeing taken as unity. Dalton also has adopted for phos-phorous and phosphoric acids, hypotheses analogous tothose he had made for sulphurous and sulphuric acids ;but since he used different values for the elements ofthese acids by weight, he arrived at a determination ofthe molecule of phosphorus, which does not bear thesame ratio to his determination of the molecule ofsulphur as ought to exist, according to us, between thesemolecules : he has fixed that of phosphorus as 8,hydrogen being unity.*

Let us now see what conjecture we may form as to themass of the molecule of a substance which plays innature a far greater part than sulphur or phosphorus,namely, that of carbon. As it is certain that thevolume of carbonic acid is equal to that of the oxygenwhich enters into it, then, if we admit that the volumeof carbon, supposed gaseous, which forms the otherelement, is doubled by the division of its molecules intotwo, as in several combinations of that sort, it will benecessary to suppose that this volume is the half of thatof the oxygen With which it combines, and that con-

* Davy has adopted the same suppositions as ourselves for thenumber of molecules of oxygen and radical in phosphorous andphosphoric acids ; and by still taking the molecule of oxygen nearlyhalf ours, he finds 16.5 for the molecule of phosphorus, which would.give about 33 on our evaluation of the molecule of oxygen, insteadof 38. The difference arises from Davy having taken from his ownexperiments 34 parts to 25, i.e. 136 to 100 of phosphorus, as thequantity of oxygen in phosphoric acid, instead of 120 as we haveassumed. Further experiments will clear up this point.

/ LIBM^.f P OF THE ^ \UNIVERSITY).. ^


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42 Avogadro.sequently carbonic acid results from the union of onemolecule of carbon and two of oxygen, and is thereforeanalogous to sulphurous and phosphorous acids, accord-ing to the preceding suppositions. In this case we findfrom the proportion by weight between the oxygen andthe carbon, that the density of carbon as gas would be0.832 with respect to that of air as unity, and the massof its molecule 11.36 with respect to hydrogen. Thereis, however, one difficulty in this supposition, for we giveto the molecule of carbon a mass less than that ofnitrogen and oxygen, whereas one would be inclined toattribute the solidity of its aggregation at the highesttemperatures to a higher molecular mass, as is observedin the case of the sulphuric and phosphoric radicals.We might avoid this difficulty by assuming a division ofthe molecule into four, or even into eight, on the forma-tion of carbonic acid ; for in that way we should havethe molecule of carbon twice or four times as great asthat we have just fixed. But such a composition wouldnot be analogous to that of the other acids ; and, be-sides, according to other known examples, the assump-tion or not of the gaseous state does not appear to dependsolely on the magnitude of the molecule, but also onsome other unknown property of substances. Thus wesee sulphurous acid in the form of a gas at the ordinarytemperature and pressure of the atmosphere notwith-standing its large molecule, which is almost equal tothat of the solid sulphuric radical. Oxygenated muriaticacid gas has a density, and consequently a molecularmass, still more considerable. Mercury, which as weshall see further on, should have an extremely largemolecule, is nevertheless gaseous at a temperatureinfinitely lower than would be necessary to vaporise iron,the molecule of which is smaller. Thus there is nothingto prevent us from regarding carbonic acid to be com-


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Relative Masses of Elementary Molecules. 43posed in the manner indicated above, and thereforeanalogous to nitric, sulphuric, and phosphoric acids,and the molecule of carbon to have a mass expressed by11.36.

Dalton has made the same supposition as we havedone regarding the composition of carbonic acid, andhas consequently been led to attribute to carbon a mole-cule equal to 4.4, which is almost in the same ratio to hisvalue for that of oxygen as 11.36 is to 15, the mass ofthe molecule of oxygen according to us.

Assuming the values indicated for the mass of themolecule of carbon and the density of its gas, carbonicoxide will be formed, according to the experiments of M.Gay-Lussac, of equal parts by volume of carbon gas andoxygen gas ; and its volume will be equal to the sum ofthe volumes of its constituents : it will accordingly beformed of carbon and oxygen united molecule to mole-cule, with subsequent halving all in perfect analogy tonitrous gas.The mass of the molecule of carbonic acid will be

ii.36 + 2 x 15.074 1.5*96and that of carbonic oxide will be

11.36+15.074 0.96782- = 1 3.22 =-0.07321V.

Amongst the simple non-metallic substances there isstill one of which we have to speak. This substance,being naturally gaseous, can leave no doubt, on ourprinciples, as to the mass of its molecule ; but the latestexperiments of Davy upon it, and even the earlier experi-ments of MM. Gay-Lussac and Thenard, force us todepart from ideas hitherto received, although the last-


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44 A vogadro.named chemists have attempted to explain them in accord-ance with these ideas. This is the substance hithertoknown by the name of oxygenated muriatic acid, oroxymuriatic acid. In the present state of our knowledgewe must now regard this substance as still undecom-posed, and muriatic acid as a compound of it withhydrogen. It is in accordance with this theory, there-fore, that we shall apply our principles regarding com-binations to these two substances.The density of oxymuriatic acid according to MM.

Gay-Lussac and Thenard is 2.470 referred to atmosphericair as unity ; this gives for its molecule referred to that ofhydrogen as unity, 33.74, adopting the density of hydrogendetermined by MM. Biot and Arago. According toDavy 100 English cubic inches of oxymuriatic gasweigh 74.5 grains, and an equal volume of hydrogen gas2.27. This would give for the molecule of the former74-5 =32.82. These two estimates differ very little fromthe mass that Davy himself, from other considerations,assigns to this substance, viz., 32.9. It follows from theexperiments both of Gay-Lussac and Thenard, and ofDavy, that muriatic acid gas is formed by the combina-tion of equal volumes of oxymuriatic gas and hydrogen,and that its volume is equal to their sum. This means,according to our hypothesis, that muriatic acid is formedof these two substances united molecule to molecule,with halving of the molecule, of which we have alreadyhad so many examples. Accordingly the density ofmuriatic acid gas, calculating from that given above foroxymuriatic gas, should be 1.272; it is 1.278 accordingto the experiments of MM. Biot and Gay-Lussac. If wesuppose this last determination to be exact, the densityof oxymuriatic gas should be 2.483, and the mass of itsmolecule 33.91. Should we prefer to adopt this value,


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Relative Masses of Elementary Molecules. 4 5the mass of the molecule of muriatic acid will be

I I>27 The determination of the2 0.07321specific gravity of the muriatic acid gas by Davy, accord-ing to which 100 cubic inches of that gas weigh 39grains, would give numbers only slightly different, viz.,33.36 for the mass of the molecule of oxymuriatic acid,and 17.18 for that of muriatic acid.

VI.Let us now apply our hypothesis to some metallic sub-

stances. M. Gay-Lussac assumes that mercurous oxide,in the formation of which 100 parts by weight of mercuryabsorb 4.16 of oxygen, according to Fourcroy andThenard, is analogous to nitrous oxide, /.*., that themercury, supposed gaseous, is combined in it with half itsvolume of oxygen gas, which on our hypothesis is tosay that one molecule of oxygen combines with twomolecules of mercury. Supposing this to be the case,the density of mercury gas ought to be to that of oxygenas 100 to 8.32, which would give 13.25 as its densitytaking that of air as unity, and for the mass of themolecule of mercury 1 8 1 taking as unity that of hydrogen.On this supposition mercuric oxide, which contains twiceas much oxygen, should be formed of mercury andoxygen united molecule to molecule ; but some reasonslead me to think that it is mercurous oxide whichrepresents this last case, and that in mercuric oxide onemolecule of mercury combines with two of oxygen.Then the density of mercury gas, and the mass of itsmolecule, would be double what they are on the preced-ing hypothesis, viz., 26\ for the first, and 362 for thesecond. In this assumption I am supported by analogiesdrawn from other metals, and particularly from iron. Itfollows from the experiments of different chemists, care-


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46 A vogadro.fully discussed by Hassenfratz, that the two best knownoxides of iron, the black oxide and the red oxide, arecomposed respectively of 31.8 and 45 parts by weight ofoxygen to 100 of iron. We see that the second of thesetwo quantities of oxygen is nearly half as great again asthe first, so that we are naturally led to suppose that inthe first oxide one molecule of iron combines with twomolecules of oxygen, and in the second with three. Ifthat is so, and if we admit the proportion for the blackoxide to be the more exact, the proportion for the redoxide would be 47.7 for 100 of iron, which comesvery near the proportion found directly by Proust, viz.,48. The mass of a molecule of iron will therefore be tothe mass of a molecule of oxygen as TOO to 15.9, whichgives about 94 with regard to hydrogen as unity. Itwould appear from this that there should be anotheroxide of iron which would contain 15.9 of oxygen to 100of iron, and this is perhaps the white oxide, although theexperiments hitherto performed point to this substancecontaining a greater proportion of oxygen. Now the twooxides of mercury of which we have spoken, one of whichcontains twice as much oxygen as the other, shouldapparently be analogous to this last oxide of iron and tothe black oxide, the red oxide having no analogue in thecase of mercury.

In the same way the other metals present for the mostpart two oxides in which the quantities of oxygen are asi to 2, so that from the proportions of their elements byweight, we may determine in the same manner the massof their molecules. I find for example, 206 for themolecule of lead, 198 for that of silver, 123 for copper,etc.*

* I shall here add a few words regarding the molecule ofpotassium. Davy, assuming that potash is formed from potassiumand oxygen united molecule to molecule, has fixed the mass of the


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Relative Masses of Elementary Molecules. 47VII.

We shall now make a few applications of our principlesto saline compounds, which will furnish us with theopportunity of examining an important point in thetheory of these compounds. M. Gay-Lussac has shownthat the neutral carbonate, fluoborate, and muriate ofammonia are composed of equal volumes of ammoniagas and of the respective acids. Let us pause to considerthe carbonate. On our hypothesis, this salt is composedof one molecule of carbonic acid with one molecule ofammonia i.e. (according to the values previously givenand independently of any division), of one molecule ofmolecule of potassium at 40.5, in accordance with the quantity ofoxygen by weight in the substance, and has taken the molecule ofoxygen to be 7.5. Assuming, as we have done, this last moleculeto be nearly twice as great, the molecule of potassium will also bedoubled, viz., about 81, if we adopt the other assumptions of Davy.But it may be that in potash one molecule of potassium takes two ofoxygen, in which case we should again have to double the lastvalue and make it 162. It might also be (for the analogy drawnfrom other metals is not in this case a very safe guide) that twomolecules of potassium combine with one of oxygen, which wouldbring back the molecule of potassium to 40.5.

It is on the assumption of this last value for the molecule ofpotassium that Davy finds 32.9 for that of oxymuriatic acid, calculat-ing from the composition of muriate of potash, and assuming thatthis salt is formed of one molecule of potassium with one of acid. Ifwe suppose the molecule of potassium to have a different mass, wemust admit another relative number of molecules in the muriate,since both from our hypothesis and from the density of the gas,32.9 is very nearly the molecule of oxymuriatic acid. On the sup-position that the molecule of potassium is 81, and that in sulphideof potassium the combination is molecule to molecule, the moleculeof sulphur will be about 27 instead of 13^ as found by Davy on thelatter assumption, which will bring about between this result andthat derived from sulphurous acid according to our calculations theagreement which exists between Davy's

OF THEUNIVERSITY


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48 A vogadro.carbon, two of oxygen, one of nitrogen, and three ofhydrogen, which would give 57.75 for the mass of itsmolecule ; but admitting the division into two which hadalready taken place in the components, this molecule isreduced to 28.87. It would be brought down again tohalf this number, if there were another division on theunion of the acid with the alkali.M. Gay-Lussac has suspected that the equality ofvolume between a gaseous alkali and acid, which by theirunion form a neutral salt, may be general. That is asmuch as to say, on our hypothesis, that neutral salts are

composed of acid and alkali united molecule to mole-cule ; but certain considerations appear to be opposed tothe admission of this principle in all its generality. Theidea of acidity, alkalinity, and neutrality, which still seemsto me the most conformable to the phenomena, is thatwhich I have given in my Memoir on this subject(Journal de Physique, tome Ixix.). According to it, allsubstances form amongst themselves a series, in whichthey play the part of acid or alkali with respect to oneanother; and this series is the same as that on whichdepends the positive or negative electricity they developon mutual contact. I express by the term oxygenicity theproperty in virtue of which substances are ranked in thisscale, placing first those which play the part of an acidwith respect to the others. In this scale there is a pointabout which are placed the substances we term neutral,above it are those which are absolutely acid, below it arethose which are alkaline, when their state of aggregationpermits them to exhibit these qualities. Lastly, com-posite substances occupy in this scale a place intermediatebetween those of which they are composed, having regardto the degree of oxygenicity and to the proportion byweight of these constituent substances ; so that a neutralsubstance results from the combination of two substances,


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Relative Masses of Elementary Molecules. 49one acid, the other alkaline, in a certain proportion (seethe Memoir referred to).* The recognition of the simpleratios observed on combination, and in particular incases where neutral substances are the result, leads usnow to a more exact manner of conceiving the state ofneutrality. The oxygenicity in two bodies which com-bine, cannot be supposed to have such a relation to themasses of their molecules, that from the union of certaindefinite numbers of these molecules there should result acertain definite degree of oxygenicity which would bethat of neutrality, and would only depend, as we havealready assumed for oxygenicity in general, on the pro-portion by weight and the degree of oxygenicity of thecomponents. It appears, then, that we must admit thatthe degree of oxygenicity which corresponds to neutralityis not quite fixed, although approximating more or less toa fixed limit, and that this state depends on the excess ofmass of one of the components (from which the acid oralkaline quality might result) being prevented from exer-cising these qualities by the simple combination with thecontrary principle which retains it by its attraction,although the compound otherwise might have a state ofaggregation permitting it to act as an acid or an alkali, ifit were endowed with these qualities. The excess ofmass thus held back is that which is necessary to com-plete a certain simple relation between the number ofcombining molecules. Thus amongst the different simpleratios in which molecules can combine, there is onewhich gives neutrality ; that, namely, which gives the

* The properties of oxymuriatic acid, as Davy conceives them,being analogous to those of oxygen, are not at all extraordinary fromthis point of view ; they simply show that this substance is veryoxygenic. I had already remarked in my Memoir that the pro-perties of the alkalies, supposed to be oxides, are easily explainedaccording to these ideas. D


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50 Avogadro.compound approximating most closely to the definitepoint of oxygenicity mentioned above, so that if in thecompound formed according to this ratio, one of thecomponent principles let one molecule of the otherescape, or took up one in addition, the compound woulddiverge further from this precise point, about which thereoscillate, as it were, the oxygenicities of the variousneutral compounds ; and it is this point which wouldgive the neutral state in the combination of two sub-stances which could combine in all proportions, or inratios expressible by any number of molecules whatever.It is evident that this way of regarding the neutrality ofcompound substances reconciles the theory given in theMemoir quoted with the ideas put forward by M. deLaplace on this point, and expounded by M. Haiiy inhis Traite* de Physique.

According to this theory it is evident that if theoxygenicity of two acids and two alkalies which combinerespectively in pairs, is not extremely different, and if atthe same time the mass of the molecule of one of theacids is not in a different ratio to its alkali from that ofthe other acid with regard to its own alkali, the ratiobetween the numbers of molecules which gives neutralitymay be the same in both compounds ; but in the contrarycase, the ratio may vary in such a way that instead of theequality of volumes, or of combination molecule to mole-cule which we see between carbonic and a few other acidson the one hand and ammonia on the other, there maybe other simple ratios such as i to 2, &c., which give theneutral state. Nevertheless, the simplicity which willalways exist amongst these ratios, in conjunction with theinformation we may obtain from other sources as to themass of the molecules and the degree of oxygenicity ofthe components, will sometimes put us in a position todetermine, or at least conjecture, what are the simple


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Relative Masses of Elementary Molecules. 5 1ratios which may occur in a given case ; but it is the taskof experiment to confirm or correct these theoreticalestimates.

VIII.It will have been in general remarked on reading thisMemoir that there are many points of agreement between

our special results and those of Dalton, although we setout from a general principle, and Dalton has only beenguided by considerations of detail. This agreement is anargument in favour of our hypothesis, which is at bottommerely Dalton's system furnished with a new means ofprecision from the connection we have found between itand the general fact established by M. Gay-Lussac.Dalton's system supposes that compounds are made ingeneral in fixed proportions, and this is what experimentshows with regard to the more stable compounds andthose most interesting to the chemist. It would appearthat it is only combinations of this sort that can take placeamongst gases, on account of the enormous size of themolecules which would result from ratios expressed bylarger numbers, in spite of the division of the molecules,which is in all probability confined within narrow limits.We perceive that the close packing of the molecules insolids and liquids, which only leaves between the integralmolecules distances of the same order as

dalton gay lussac avogadro foundations molecular theory 1893

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