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known as the Cavendish experiment, where he reporteda result that has been unsurpassed by any of the later

investigations on this subject. Terrestrial magnetism, asubject on which he first began to make observa-tions with his father, continued to occupy his at-tention at intervals throughout his life. Much ofCavendishs work remained in the form of unpub-lished papers; eventually these were edited in twovolumes, the first one containing the electricalresearches and the second the chemical and dy-namical ones (Maxwell et al., 1921). Had he publish-ed during his lifetime all the researches he com-pleted, his reputation would have been much widerand more varied even than it was.

Cavendish was not only a gifted experimentalist,but he was also a highly competent mathematician,skilled in the use of infinitesimal calculus (Berry,1960) He became extremely rich after one of hisuncles bequeathed him all his fortune; at the time ofhis death he was the largest holder of Bank stock inEngland.

Cavendish died on February 24, 1810, at the ageof 79, after a few days illness, and was buried in AllSaints Church, Derby, now Derby Cathedral.

The many eulogies pronounced in his honorreflect the high esteem in which his English andFrench colleagues held him. In a lecture delivered in

1810 by Sir Humphry Davy (1778-1829) he said: Itmay be said of him, what can, perhaps, hardly besaid of any other person, that whatever he has donehas been perfect at the moment of its production.Since the death of Newton, if I may be permitted togive an opinion, England has sustained no scientificloss as great as that of Cavendish. Like his greatpredecessor, he died full of years and glory. His namewill be an object of more veneration and of glory.

Jean-Baptiste Biot (1774-1862) felt that he could trulysay that Cavendish was le plus riche de tous lessavants et probablement aussi, le plus savant de tousles riches (the wealthiest of all learned men and the

most learned of wealthy men). Georges Cuvier (1769-1832) pronounced the loge Funbrein his honor atthe Acadmie des Sciences (Cuvier, 1811).

Scientific contributionSome of Cavendishs most important contributions tochemistry and physics will be described in detail now.

1. Arsenic (Maxwell et al., 1921)Research in this topic was done about 1764, but neverpublished. Its purpose was to determine the differ-

ences between regulus of arsenic (metallic arsenic),white arsenic (arsenious acid, AsO3), and arsenical

acid (AsO5). This work is important because it con-tains a method for preparing arsenic acid, which isthe one now in use, and because it was done beforeCarl Wilhelm Scheeles (1742-1786) published hisfindings on the subject (Scheele, 1775). It also con-tains a very detailed description of arsenic acids andsome of its salts.

Cavendish first proceeded to repeat some of theexperiments that Pierre-Joseph Macquer (1718-1784)(Macquer, 1746, 1748) had done to prepare what wascalled neutral arsenical salt, by heating a mixture ofarsenious oxide and potassium nitrate and then crys-tallizing the product from hot water. His resultsindicated that this salt was not strictly neutral, asMacquer supposed, since it dissolved alkaline car-bonates and showed a weak acid reaction towardssyrup of violets. Cavendish then studied the effect ofheating arsenious oxide directly with nitric acid. Hetook notice of the red fumes released (nitrous acidproduced by the action of air on nitric acid) anddiscussed this phenomenon on the basis of the reduc-tion of the acid by heat and inflammable matter. Theproduct of the reaction was evaporated to completedryness and the residue heated to almost as great aheat as the furnace would admit of. Cavendish

found that the product weighted about 1/6th

partmore than arsen, from which it was made. Theactual result was, of course, the oxidation of arseni-ous oxide to arsenic pentoxide. Although he did notrealize the real meaning of the reaction (oxygen wasyet to be discovered) he remarked: I think theseexperiments shew pretty plainly that the only differ-ence between plain arsenic and the arsenical acid isthat the latter is more thoroughly deprived of itsPhlogiston that the former. The nitrous acid is knownto have a great disposition to lay hold of Phlogiston,and there are strong reasons for thinking that thedissolving of metallic substances in that acid is a very

powerful method of depriving them of it (Maxwellet al., 1921).

Cavendish did also a number of qualitativeexperiments on his arsenic pentoxide and showedthat the pentoxide was soluble in water with pro-nounced acidic properties, and that it not containnitric acid.

2. GasesCavendishs first memoir on gases was entitled ThreePapers Containing Experiments on Factitious Airand was

PARA QUITARLE EL POLVO

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published in 17661 (Cavendish, 1766). As the titleindicates, this paper is divided into three sections;

the first one deals with inflammable gas, the secondwith carbonic acid (CO2), and the third with the gasesevolved during fermentation and putrefaction. Allthree sections contain the method of preparationof the gases and a study of their reactions. The firstsection, entitled Containing Experiments on Inflamma-ble Air, deals with the action of zinc, iron, and tin ondilute sulfuric acid and hydrogen chloride.Cavendish found that although zinc dissolved inboth acids faster than iron or tin, it yielded the sameamount of gas, independently of the acid employed.The attack of iron by solutions of sulphuric acid ofdifferent concentrations resulted in the same quan-tity of inflammable air. Tin dissolved best in warmmuriatic acid. The three metals dissolved readily innitrous (nitric) acid and generated air (nitric oxide),which was not inflammable. They also dissol-ved with effervescence in hot oil of vitriol and dis-charged plenty of vapors which smell strongly of thevolatile sulphurous acid, and which are not at allinflammable. Being was a supporter of the phlogis-ton theory, he interpreted these results correspond-ingly: Their phlogiston flies off without having itsnature changed by the acid, and forms inflammableair but when they are dissolved in nitrous acid or

strong oil of vitriol, the phlogiston of the metalsunites to the acid used for their solution and fliesaway with it in fumes, and the phlogiston loses itsinflammability. The sulphurous acid, which isevolved when oil of vitriol is employed, is thusrepresented as being phlogisticated sulphuric acid. Itappears that Cavendish believed that the hydrogenthat evolved did not originate from the dilution waterbut from the metal itself as it underwent solution.Initially he assumed that the heating principle ofbodies was phlogiston, which was supposed to fly offduring the phenomena of heat and light, and thatwhen metals were plunged in either of these dilute

acids, it escaped in the form of an aeriform body,which he called inflammable air. Later experimentsled him to assume that inflammable air was a com-bination of the mysterious phlogiston and water(Cavendish, 1778).

Cavendish observed that the volumes of inflam-mable air generated depended on the nature of the

metal and not upon the acid, and he quoted thevolumes of gas obtained from one ounce of each

metal. In addition, he pointed out that sulfuric acidhad very little action on tin, but that the metaldissolves slowly in strong spirit of salt when cold;while the assistance of heat it dissolves moderatelyfast. He then showed that all three metallic sub-stances dissolve readily in the nitrous acid and gen-erate air; but the air is not at all inflammable. Theyalso unite readily, with the assistance of heat, to theundiluted acid of vitriol; but very little of the salt,formed by their union with the acid, dissolves in thefluid. They all unite to the acid with a considerableeffervescence, and discharge plenty of vapors, whichsmell strongly of the volatile of the volatile sulfurousacid, and which are not at all inflammable.

As mentioned before, analysis of Cavendishsreasoning shows that his initial belief was that thehydrogen originated from the metals and not fromthe acids. Afterwards, he changed his opinion statingthat the gas was actually given off by the water thataccompanied the acids.

Cavendish subsequently investigated some ofthe properties of his inflammable air. He performedmany experiments using an electrical spark to firethe gas mixed with ordinary air in varying propor-tions and noting the loudness of the resulting explo-

sion. Thus with 7 parts of inflammable to 3 ofcommon air, there was a very gentle bounce orrather puff; it continued burning for some secondsin the belly of the bottle. His conclusion was thatfrom these experiments this air, like other inflam-mable substances, cannot burn without the assis-tance of common air. It seems, too, that unless themixture contains more common than inflammableair, the common air, therein, is not sufficient toconsume the whole of the inflammable air,whereby part of the inflammable air remains, andburns by means of the common air, which rushesinto the bottle after the explosion. Inflammable gas

did not lose its elasticity during storage and sparinglysoluble in water and in fixed or volatile alkalis. Thenext experiments were concerned with attempts todetermine the relative density of the gas comparedwith that of air. Two methods were employed, thefirst consisted in weighing a bladder when filled withthe gas and when empty, and the second was that ofloss of weight. The gas was generated by the actionof zinc or iron on dilute hydrogen chloride or sul-phuric acid within an apparatus from which it es-caped after being dried by potassium carbonate. The


1According to Cavendish factitious air is any kind of air, whichis contained in other bodies in an unelastic state, and isproduced from thence by art.

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whole apparatus was weighed before and after thereaction had taken place. Cavendish concluded that

inflammable gas was about 8,760 times lighter thanwater, or eleven times more than air.

Cavendish also tried to dissolve copper withhydrogen chloride and found that although the reac-tion did not generate inflammable air it produced agas which immediately loses its elasticity, as soon asit comes in contact with water. He did not examinethe characteristics of this elastic fluid (gaseous hydro-gen chloride).

The second section of Cavendishs paper dealswith Experiments on Fixed Air, or that Species of Facti-tious Air, Which is Produced from Alkaline Substances, by

Solution in Acids or by Calcination.In this study on

carbon dioxide Cavendish made several experi-ments on the weight of the gas, which was evolvedfrom different carbonates (marble, ammonium, andpotassium carbonates) by the action of acids; deter-mined some of the properties of the gas, such as itsdensity and solubility in water and other liquids.

Cavendish also tested the elastic fluid extractedfrom chalk and alkalis and compared it with the airproduced by fermentation and putrefaction and withthat found at the bottom of wells caves, and mines.He found that all these gases had the same propertiesand were, in fact, identical. He called this gas fixed

air. He determined the density of fixed air and foundit to be a third higher than that of ordinary air, a resultthat explained why fixed air was found in caves andlow places. Furthermore, he discovered that fixed aircombined with water and the solution was capableof dissolving limestone and iron, a property thataccounted for the formation of stalactites and thepresence of iron in mineral waters (Cuvier, 1811).

The concluding section of Cavendishs paper isentitled Containing Experiments on the Air, Produced byFermentation or Putrefaction. It was already known thatcarbon dioxide was produced during fermentationand Cavendish tried to determine whether any other

gas was evolved simultaneously when fermentingbrown sugar and apple juice with yeast. With brownsugar he found that there is not the least of any kindof air discharged from the sugar and water by fer-mentation, but what is absorbed by the sope lees (asolution of potassium hydroxide), and which maytherefore be reasonable supposed to be fixed air.

Cavendish found that the putrefaction of gravybroth generated a mixture of gases, as only partly thegas produced was absorbed by sope leys. Thesoluble part thus consisted of fixed air, while the insol-

uble residue when mixed with common air took fireon applying a piece of lighted paper, and went off

with a gentle bounce. Based on the loudness of theexplosions Cavendish concluded that this sort ofinflammable gas was nearly of the same kind as thatproduced by the action of acids on metals.

From what we know today about fermentationit can be assumed that the inflammable gas thatCavendish obtained after removing the carbon diox-ide was a variable mixture of hydrogen, methane,and carbon monoxide.

In two additional papers (Cavendish, 1784, 1785)Cavendish described his findings about the forma-tion of nitric acid when mixtures of nitrogen andoxygen were sparked in the presence of moisture.Most of his experiments were performed in a simpleapparatus consisting of a narrow glass tube, bentnearly at a right angle, and containing mercury withthe open ends dipping into vessels containing thesame liquid. The bend of the tube was uppermost,and the gases, which were to be subjected to theaction of the spark, were admitted over the mercuryso as to occupy a space within the bend. Liquid suchas alkaline solutions could be admitted above thesurface of the mercury so as to absorb the productsof the reaction. Cavendish made a series of experi-ments using limewater, which demonstrated conclu-

sively that carbon dioxide was not produced whenair was sparked. He also showed that sparking eitherpure oxygen or pure nitrogen produced no diminu-tion in volume, but a contraction always occurredwhen a mixture of the two gases was subjected to theaction of the spark. In the latter case his experimentsyielded an unexpected result: When the electricspark was made to pass through common air, in-cluded between short columns of a solution of litmus,the solution acquired a red colour, and the air asdiminished, conformably to what was observed byDr. Priestley. When lime water was used, instead ofthe solution of litmus, and the spark continued till the

air could be no further diminished, not the leastcloud could be perceived in the lime water, but theair was reduced to 2/3 of its original bulk, which is agreater diminution than it could have suffered bymere phlogistication, as that is very little more that1/6 of the whole. Neither was any cloud producedwhen fixed air was let up to it; but on the furtheraddition of a little caustic volatile alkali, a brownsediment was immediately perceived. Hence we canconclude that the limewater was saturated by someacid formed during the operationthe brown col-



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our of the sediment, it most likely proceeded fromsome of the quicksilver having been dissolved. Wemay safely conclude, that in the present experimentsthe phlogisticated air was enabled, by means of theelectrical spark, to unit to, or form a chemical com-position with the dephlogisticated air, and was thusreduced to nitrous acid, which united to the soap-leesand formed a solution of nitrethe sope-lees being thenpoured out of the tube, and separated from thequicksilver, seemed to be perfectly neutralized asthey did not at all discolour paper tinged with the

juice of blue flowers. Being evaporated to dryness,they left a small quantity of salt, which was evidentlynitre, as appeared by the manner in which paper,

impregnated with a solution of it burned.Cavendish reported that when five parts of

pure dephlogisticated air were mixed with threeparts of common air almost the whole of the air wasmade to disappear when sparked over sope-lees.This conclusion is of interest and significant becauseCavendish was stating definitely that he regarded theatmosphere as consisting of a mixture of nitrogenand oxygen: It must be considered that common airconsists of one part of dephlogisticated air, mixedwith four of phlogisticated, so that a mixture of fiveparts of pure dephlogisticated air, and three of com-

mon air, is the same thing as a mixture of seven partsof dephlogisticated air with three of phlogisticated.During these experiments Cavendish made an-

other discovery of great importance, which he didnot pursue further and remained completely ignoredfor almost one century, until John William Strutt(Lord Raleygh, 1842-1919) recognized its signifi-cance in connexion with some puzzling results whichhe had obtained in his determinations of the densityof nitrogen. Raleygh had observed that the density ofatmospheric nitrogen was greater than that of nitro-gen prepared from its compounds. Although the

difference was very small, it was large enough tounderstand that it could not be due to experimentalerror. Having abandoned ideas of light impurities inchemical nitrogen, Raleygh concluded that atmos-pheric nitrogen must contain a small proportion of aheavy gas, and having recalled Cavendishs forgot-ten experiment, continued the work, which led to thediscovery of argon and the other inert gases.

In 1778 Cavendish published another paper inwhich he reported corrected results of his first experi-ments (Cavendish, 1778).

3. The water controversySeveral chemists had noted that the burning of in-

flammable air deposited dew on the walls of thevessel. In 1766, Pierre-Joseph Macquer (1718-1784)while studying the possibility that a flame of inflam-mable gas evolved smoke or soot, observed thatwhenever a glass vessel was held over the burning

jet, moisture formed inside the vessel, which heassumed to be water (Macquer, 1766). In the sameyear, and independently of Macquer, John Warltireobserved that when a jet of inflammable gas wasallowed to burn under a bell jar, closed below andcontaining air, until the flame went out, immedi-ately after the flame was extinguished there ap-peared through almost the whole of the received afine powdery substance like a whitish cloud, and thatthe air left in the glass was perfectly noxious. Warl-tire explained this result by the supposition that thegases contained some water diffused through them,and that that this, being condensed, appeared indrops on the sides of the containing vessel. Warltirepaid little attention to this result because he wasinterested in determining whether heat has weight ornot. In further experimentation he used a closedcopper vessel, holding about three pints, to avoid therisk of injury from explosions. He weighted the ves-sel before and after the explosion and to his surprise

he found that a loss of weight of about two grainsalways occurred, although the vessel was hermeti-cally closed so that no air could escape by theexplosion. He repeated his experiments using glassvessels and again he found the same loss in weightbut now he also noticed that the inside of the glass,though clean and dry before, immediately becamedewy. To Priestley, these results confirmed his opin-ion that common air deposits its moisture by phlo-gistication (Priestley, 1781).

The formation of dew was neglected as an irrele-vant effect but all except Cavendish who thought thatperhaps the dew was what was left behind as phlo-

giston was released from inflammable air. Caven-dish, intrigued by the loss in weight, decided torepeat Warlfires experiments using a larger vesselto be sure that the weight loss was not caused byerrors in weighing. His results contradicted those ofWarlfire: The experiment did not succeed with me;for though the vessel I used held more than Mr.Warltires, namely 24,000 grains of water (1 lb = 7,000grains), and though the experiment was repeatedseveral times with different proportions of commonand inflammable air, I could never perceive a loss of

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weight of more than one-fifth of grain, and com-monly none at all. In all the experiments the inside

of the glass globe became dewy, as observed by Mr.Warltire. Cavendish analysed his results and con-cluded that 423 measures of inflammable air arenearly sufficient to completely phlogisticate 1,000 ofcommon air; and that the bulk of the air remainingafter the explosion is then very little more thanfour-fifths of the common air employed, so that wecan safely conclude that when they are mixed in thisproportion and exploded, almost all of the inflam-mable air and about one-fifth of the common air losetheir elasticity, and are condensed into the dewwhich lines the glass.

Additional experiments to identify the nature ofthe liquid phase led him to conclude that: by thisexperiment it appears that this dew is plain water,and consequently that almost all the inflammable air,and about one-fifth of the common air are turned intopure water. The airs has disappeared in the ratiotwo to one (Cavendish, 1784, 1785).

Shortly after Cavendish made public his find-ings about the burning of inflammable air and theformation of water, James Watt came forward withthe claim that he had announced the same result ona previous date, and afterwards, Antoine-LaurentLavoisier (1743-1794) declared that he had discov-

ered the compound nature of water before, andindependently of either. A controversy accordinglyarose in which Cavendish and Watt disputed witheach other the priority of the discovery while at thesame time they rejected Lavoisiers arguments re-garding the identification of the composition ofwater. This dispute not only was not settled duringthe lifetime of the claimants, every so often it continuesto flare again in the literature (Schofield, 1963;Miller, 2002).

This paternity controversy was caused a seriesof fortuitous events. Cavendish reported his findingsto Joseph Priestley (1733-1804), who repeated the

experiments and reported them to James Watt (1736-1819). In a letter that circulated among members ofthe Royal Society, Watt gave his interpretation of theexperiments. Hearing about Cavendishs experi-ments and Watts conclusions from Charles Blagden(1748-1820) on a trip to Paris in 1783, Antoine-Laurent Lavoisier (1743-1794) did experiments of hisown and wrote an account of them (Lavoisier, 1781).As noted by Wilson (Wilson, 1851) although theoriginal claims were founded on similar experi-ments, Cavendish, Watt, and Lavoisier arrived at

their conclusions while performing them for differ-ent purposes. Cavendish was investigating the prod-

ucts of combustion; Watt was studying the changesthat a vapor may undergo if all its latent heat becamesensible; and Lavoisier was seeking in the combus-tion of inflammable gases for additional proofs of thetruth of his view that oxygen is the great acidifyingagent.

In June 1783 Lavoisier, after learning thatCavendish had obtained water by burning a mixtureof inflammable air and dephlogisticated air under-stood immediately the remarkable significance ofthis result and explained it by saying that water wasa compound of both gases (Lavoisier, 1781). With thisexplanation he had the key for the difference be-tween the reaction of a metal or its calx with an acid,by admitting that water participated in the reaction.In the first reaction water decomposed and releasedits inflammable gas and dephlogisticated air (oxy-gen). The latter combined with the metal to yield themetallic calx (the term oxide had not been definedyet). The second reaction was simply the combinationof the acid with the calx to give the pertinent salt.

Lavoisiers hypothesis about the composition ofwater was further demonstrated by its decomposi-tion (Lavoisier, 1786). The experiments were wit-nessed and controlled by a commission appointed

by the Acadmie de Sciences, which included,among others, Claude-Louis Berthollet (1748-1822)and Gaspard Monge (1746-1818). Water was con-tacted with hot iron filings, which rusted, giving offinflammable air. The weight of the inflammable airplus the weight gain of the rusted filings was shownto be equal to the weight of the water consumed. Thecommissions report included the following state-ments: One of the parts of the modern doctrine themost solidly established, is the formation, decompo-sition, and recomposition of water. And how can wedoubt it, when we see that in burning together fifteengrains of inflammable air and eighty-five of vital air,

we obtain exactly one hundred grains of water, inwhich, by decomposition, we find again the sameprinciples and in the same proportions. If we doubtof a truth established by experiments so simple andpalpable, there would be nothing certain in naturalphilosophy.

This interpretation was accepted gradually butit was, however, powerless to convince Priestley,who remained faithful to the phlogiston theory untilhis death. The phlogiston theory was now on its wayout to be replaced by Lavoisiers new chemistry.



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4. HeatCavendish did research on different aspects of the

thermodynamics of sensible and latent heat, at inter-vals over a period of some thirty years of his scientificactivities. The range of his studies included ther-mometry, dilation of gases, specific heats, latentheats of fusion and vaporization, and the propertiesof freezing mixtures. He published only his findingson thermometry and freezing mixtures; the rest re-mained recorded in his laboratory notes (Maxwell etal., 1921; Cavendish, 1783, 1786, 1788).

Measurements of temperature by observationsof the thermal expansion of liquids beforeCavendishs time had been widely made, but hadscarcely attained any degree of precision (Wisniak,2000). Various liquids were used as thermometricsubstances, such as alcohol (spirit of wine), linseedoil, and mercury. The use of fixed points, particularlythose of the melting point of ice and the boiling pointof water, in the construction of thermometric scalesappear to have originated before 1701, when Newtonpublished a short note on the subject, entitled scala

graduum caloris.In November 1773 the Council of the Royal

Society decided that daily observations should bemade at their house with the barometer, thermome-ter, rain gage, wind gage, and hygrometer. Caven-

dish was appointed to direct and supervise the ob-servational work, and this led to the publication ofhis paper entitled An Account of the MeteorologicalInstruments Used at the Royal Societys House (Caven-dish, 1776b). This paper was divided into four sec-tions, the first of which dealt with the thermometers,the second with the barometer, rain gage, wind gage,and hygrometer, the third with the variation com-pass, and the fourth with the dipping needle.

Cavendish realized immediately that when thebulb of a thermometer was submerged in a hot liquidthe difference in temperatures between the bulband the stem of the instrument could lead to a sig-

nificant error in the true temperature. He stated thatthe only accurate method was to take care that allparts of the thermometric fluid be heated equally:for a thermometer dipped into a liquor of the heatof boiling water will stand at least 2 higher, if it isimmersed to such a depth that the quicksilver in thetube is heated to the same degree as that in the ball,than if it is immersed no lower than the freezingpoint, and the rest of the tube is not much warmerthan the air. The only accurate method is to take carethat all parts of the quicksilver should be heated

equally. For this reason, in trying the heat of liquorsmuch hotter than or colder than the air, the ther-

mometer ought if possible to be immersed almost asfar as the top of the column of the quicksilver in thetube (Cavendish, 1788). He understood that his wasnot an easy operational feature so he drew a table ofcorrections to be applied to the observed readings.His table was based on the coefficient of expansionof mercury, which he expressed by stating: quick-silver expands 11500th part of its bulk for eachdegree of heat.

Cavendish also drew attention to the precau-tions that had to be taken when fixing the boilingpoint of water in mercury thermometers; at a meet-ing of the Royal Society on April 18, 1766 he used anumber of thermometers to give a practical demon-stration on the best conditions for fixing the boilingpoint of water.

Cavendish measured the vapor pressure ofwater between 52 and 172 F using the constantvolume method and obtained results very close tothose that Victor Regnault (1810-1878) would obtainalmost 50 years later (Regnault, 1847). Afterwards,he proceeded to measure the coefficient of expan-sion of atmospheric air, nitrous air (nitric oxide),fixed air (carbon dioxide), heavy inflammable air(presumably a mixture derived from the destructive

distillation of wood, or from heated coal), dephlogis-ticated air (oxygen), phlogisticated air (nitrogen), andinflammable air (hydrogen). It was well known longtime before the time of Cavendish that air expandsto a much greater extent than solids and liquids dounder the influence of heat; some quantitative deter-minations had been made by various experimental-ists but there was no agreement between their results.Cavendishs method was based on varying the pres-sure of the gas so as to maintain the volume constant,a procedure that in 1847 Regnault would claim to bethe most accurate (Wisniak, 2001). The resultingvalues of the thermal coefficient of expansion were

mostly about 1/370 for one degree Fahrenheit (1/205in the centigrade scale). From this Cavendish con-cluded that different gases had the same thermalcoefficient of expansion.

The experiments by which Cavendish was ledto the discovery of specific and latent heat appear tohave originated in connection with the question asto whether the mixing of equal quantities of water atdifferent temperatures would result in the final tem-perature being the arithmetical mean of the two. Itwas widely held that such would not be the case. For


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example, according to Jean Andr De Luc (1727-1817)when equal weights of water at 212F and 32F

respectively were mixed, the final temperature wasonly slightly lower (119F !) that the hotter one.Cavendish was able to show that when knownweights of water at different temperature are mixedtogether, the heat lost by the hotter liquid is strictlyequal to the heat gained by the colder one. Here,without realizing it, he was anticipating the conceptof enthalpy.

In the course of this work Cavendish discoveredthat when different substances at different tempera-tures were mixed, the above simple rule no longerheld, that is, the final temperature was not deter-mined in the same way as when quantities of thesame substance at different temperatures weremixed. Substances accordingly were found to differas regards their thermal capacity. Cavendish de-scribed this property as the effect of substance inheating or cooling others, which is the same thingas specific heat, or the thermal capacity per unitmass.

By these trials Cavendish justified the truth of hisproposition that on mixing hot and cold water thequantity of heat in the liquors taken together shouldbe the same after the mixing as before, or that thehot water should communicate as much heat to

the cold water as it lost itself. He then proceededto try the effect of mixing unlike liquids initially atdifferent temperatures. He described these experi-ments with the following significant note: Onewould naturally imagine that if cold mercury or anyother substance is added to hot water, the heat of themixture would be the same as if an equal amount ofwater of the same degree of heat had been added, orin other words that all bodies heat and cool eachother when mixed together equally in proportion totheir weights. The following experiments will showthat this is far from being the case. His first experi-ments led to the general result that hot water is

cooled near as much by the addition of one part ofcold water as by that of 30 parts of mercury of thesame heat. Repetition of the experiments with othersubstances led him to the general conclusion that itwould seem, therefore, to be a constant rule thatwhen the effects of any two bodies in cooling onesubstance are found to bear a certain proportion toeach other, that their effects in heating or cooling anyother substance will bear the same proportion toeach other. The true explanation of these phenom-ena seems to be that it requires a greater quantity of

heat to raise the heat of some bodies a given numberof degrees by the thermometer, than it does to

raise other bodies the same number of degrees.Between 1783 and 1786 Cavendish published

three papers related to the freezing of pure sub-stances and mixtures, in particular mercury, nitricacid, sulphuric acid, and spirit of wine (Maxwellet al., 1921, Cavendish, 1786, 1788). These paperswere commentaries about observations made inNorth America by officers of the Hudson Bay Com-pany on the effect of strong natural cold on the abovementioned substances. The observations were madeunder Cavendishs guidance and financed by him.In a way, these experiments were a sequel ofCavendishs previous work on the correct calibrationof the mercury thermometer. From the times of thealchemists it was universally believed that mercurycould not be frozen at the lowest temperature so farattainable. It was believed that mercury owed itsapparent permanent fluidity to some anomalous pe-culiarity. Then, on December 1759, Braun of St.Petersburg, when using a mixture of snow and nitricacid observed that the mercury in the thermometersank to such a depth that by extrapolation from 0Fthe observed depths represented temperatures ofseveral hundred degrees below 0; some measure-ments corresponding even to ----556F (!). In this

situation Braun found that the mercury was com-pletely solidified. His measurement of the tempera-ture was, however, completely erroneous because hedid not take into account the contraction of mercuryduring cooling and, also, that mercury, contrary towater, contracts during solidification; he simply as-sumed that the descent of the mercury in the stemtook place while the mercury was in the liquid state.Thomas Hutchins, who was to become Governor ofAlbany Fort at Hudsons Bay, performed additionalexperiments during the winter there and also foundthat mercury could be frozen. Cavendish understoodBrauns errors and requested from Hutchins to per-

form additional experiments in which mercury wasfrozen not in the tube of the thermometer, but in aseparate vessel, in which the thermometer was im-mersed, and taking adequate measurements to avoidthe freezing of the whole mass of mercury. From hisearly experiments on latent heats Cavendish wasalready aware that the temperature remains constantduring a change in phase. In this manner it was foundthat the freezing temperature of mercury was some-where between 39 to 40 F below zero (Cavendish,1783). In the latter publication Cavendish remarked



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that the cause of the rise in temperature that takesplace during freezing is to be thought in the general

principle that all, or almost all, bodies by changingfrom a fluid to a solid state, or from the state of anelastic to that of an inelastic fluid, generate heat; andthat cold is produced by the contrary process (Max-well et al., 1921).

5. Density of the EarthNewtons theory of gravitation explained the motionof terrestrial objects and celestial bodies by postulat-ing that pairs of massive objects attracted each otherwith a force proportional to the product of the twomasses and inversely proportional to the square ofthe distance between them. In his book

Principia(Newton, 1687) Newton reported the relative densi-ties of the Sun, Jupiter, Saturn, and the Earth as 100,94.5, 67, and 400, respectively, so that if the densityof the Earth were known the densities of the Sun andthe two planets could be calculated. On the basisof the known density of rocks at the surface of theEarth and rocks from mines, Newton guessed thatthe average density of the Earth is 5 to 6 times thedensity of water (Newton, 1687).After Newton, andat different times, British and French scientists at-tempted to weight the Earth by observing the gravi-tational force on a test mass from a nearly large

mountain. The experimental procedure involvedmeasuring the line of a pendulum hung near a moun-tain and comparing it to the line expected if themountain was not there. These effects were ham-pered, however, by a very imperfect knowledge ofthe composition and average density of the rockcomposing the mountain. The interference of nearbymasses on the free oscillation of a pendulum was wellknown from Newtons time; he was aware that thetime of vibration of a pendulum varied according tothe latitude, and indeed observations on the lengthof the seconds pendulum in different parts of theworld had been made in his time. The length of

the seconds pendulum was also shown to decreasewith an increase in height, but this was not in agree-ment with the inverse square law of the distance asreckoned from the center of the Earth.

The determination of the structure and compo-sition of the interior of the Earth was one of the manysubjects that interested Cavendish. In an exchangeof letters with his friend John Mitchell (1724-1793)they discussed the possibility of devising an experi-ment to weigh the Earth. Mitchell suggested anarrangement similar to the one used by Charles

Coulomb (1736-1806) for measuring the electricalforce between small charged metal spheres, that is,

using a torsion balance to detect the very smallgravitational attraction between metal spheres. Mit-chell built a first prototype of the equipment but died(1793) before he was able to conduct the experiments.

The apparatus eventually was moved toCavendishs laboratory where he rebuilt most of it.His balance was constructed from a 6-foot woodenrod suspended at his middle point by a long metalfiber and carrying two 2-inch diameter lead spheresat each extremity; this dumbbell had a period ofabout 7 minutes when it was disturbed. The rod wasthus capable of vibrating in a horizontal plane. Two350-pound lead spheres, brought close to the enclo-sure housing the rod provided the attraction effect.In this way vibrations were set up between theattractive force and the torsion of the wire and equi-librium was determined between the turning mo-ment and the moment of torsion.

Cavendish was aware that the force with whichthe balls would be attracted by the weights was verysmall (not more than one fifty-millionth part of theirweight) so that air currents set up by changes oftemperature could easily vitiate the measurements.Accordingly, he mounted a finely ruled scale nearthe end of the dumbbell, which could be read to

one-hundredth of an inch resolution. He enclosedthe apparatus in a mahogany case, placed the wholearrangement in a shut room, and observed the mo-tion of the arm from the outside, by means of atelescope. Observations were made of the naturalperiod of the pendulum and its displacement fromrest by the attraction of the external lead spheres,twisting the pendulum first in one direction and thenin the other. In addition, the observations were per-formed with the attractive spheres in three positions,which Cavendish designated as positive, negative,and midway position, respectively.

Cavendishs paper contains a detailed account

of the calculation method, as well as of the necessarycorrections; he made seventeen experiments andarrived at the result that the density of the Earthshould be 5.48 greater than that of water (Cavendish,1798). In 1841 Francis Baily (1774-1844) reviewedCavendishs calculations and found a small mistakeof simple arithmetic that decreased the expecteddensity of the Earth from 5.48 to 5.45 (Berry, 1960).The present accepted value of the average meandensity of the Earth is 5.52 times the density of water.

Cuvier was so impressed by the elegance of


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Cavendishs method that he wrote: Archimedesneeded a fulcrum in order to move the Earth, but

Cavendish needed none to weight it. Cavendishequipment and experiment are so clever and simpleto build and perform that it is usually part of thestandard experiments performed in many teachingphysics laboratories; it is known under the name theCavendish experiment.

6. ElectricityUp to the seventeenth century all the knowledgeabout electricity was basically what the Greeks knewat the time of Tales of Miletus (625-547 BCE), thatwhen amber was rubbed it acquired the ability topick up light bodies. There was also no clear distinc-tion between electricity and magnetism. Althoughthe electrostatic machine developed by Otto vonGuericke (1602-1686) in 1660 was well known, itsworkings were far from being understood. A crucialstep took place in 1745-1746 when Ewald Georg vonKleist (1700-1748) and Pieter van Musschenbroek(1692-1761) discovered the Leyden jar. This discov-ery led to a flurry of experimentation during whichmany more facts were added about electrical phe-nomena. The fundamental differences between con-ductors and insulators became better known, as wellas the phenomena of electrification by induction and

by friction. At the timewhenCavendish began hiswork on electrostatics there was a fairly considerableknowledge of the qualitative features of the subject.As stated in the Encyclopedie(Diderot, 1751) of DenisDiderot (1713-1784), although physicists did not agreeon the cause of electricity, they agreed on the exist-ence of electrical matter, concentrated more or lessin electrified bodies. Movement of this particularmatter produced the effects observed, but the expla-nations of the same disagreed. Two rival theories hadbeen put forward for explaining the familiar phe-nomena of electricity, the two-fluid and the one-fluidtheories.

The two-fluid theory, associated with the namesof Charles Franois de Cisternay Du Fay (1698-1739;a great lieutenant of Louis XV and superintendentof the royal gardens at Versailles) and Robert Sym-mer (1707-1763), arose as a result of the differenceswhich had been observed when materials such asglass on the one hand, and amber and other resinousbodies on the other, were electrified by friction.According to Du Fay: Thus there exists two elec-tricities of distinct nature, to wit that of transparentand solid bodies like glass, crystal, etc., and that of

bituminous or resinous bodies like amber, copal,sealing wax, etc. Both kinds repel all such as are of

the same electricity as their own, but, on the con-trary, attract all such as are of different electricityIshall call the one resinous electricity and the othervitreous electricity.

The one-fluid theory appears to have originatedwith Benjamin Franklin (1706-1790), who regardedall electrical phenomena in terms of a body beingeitherover or undercharged. For Franklin, electricalmatter consisted of particles extremely subtle, sinceit could permeate common matter with no difficulty.Common matter was a kind of sponge of the electricfluid. In common matter there was as much of theelectrical as it would contain within its substance. Ifmore was added then it would lie on the surface andform what Franklin called an electrical atmosphere.A body having an electrical atmosphere was said tobe electrified. According to Franklins theory, a posi-tively charged body was regarded as having a redun-dancy of electricity, while a deficiency of electricitywas synonymous with its being negatively charged.The state of an uncharged body was accordinglyregarded in terms of there being no excess or defectof the electric fluid (Franklin, 1751). In terms of thetwo-fluid theory an uncharged body was supposedto contain both fluids in a condition comparable with

that of a product resulting from their mutual neutrali-zation, and electrification was regarded as a conse-quence of the separation of the two fundamentalfluids. In summary, according to Franklin, Du Faystwo fluids were actually two aspects of the sameforce: excess and a deficiency of the electric fluid,which he named positive and negative electricity.

Franklins theory of the existence of an electricatmosphere was unable to provide a satisfactoryexplanation to the experimental fact that bodiescharged negatively (having less than the commonquantity of electricity) repelled as well as those thathave more. This difficulty was removed when in 1759

Franz Maria Ulrich Theodosius Aepinus (1724-1802)proposed the hypothesis that when molecules ofcommon matter were deprived of their normal elec-tricity they repelled each other exactly like the par-ticles of electrical matter. Aepinus was led to thisexplanation by his discovery of piezoelectricity(when tourmaline crystals are heated they developopposite electric charges at the ends of their polaraxes). The discovery of electric poles also convincedAepinus that there must be a close connection be-tween electric and magnetic induction.



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The one and two-fluid theories were capable ofproviding some understanding of the fundamen-

tal principles of electrostatics, and particularly of thephenomena of attraction and repulsion. Aepinus andCavendish supported the one fluid theory, whileCharles Coulomb (1736-1806) seemed to have pre-ferred the two-fluid theory, which was later devel-oped mathematically by Simon-Denis Poisson(1781-1840).

Franklin had defined the concept of electricalcharge or amount of electricity, but neither him norhis successors had been able to measure it. Most ofthe concepts were still qualitative. It was left toCavendish and Coulomb to transform electricityinto a quantitative science.

Cavendish published only two papers on thesubject of electricity (Cavendish, 1771, 1776a), the restof his many findings remained as unpublished ma-nuscripts. The first paper (Cavendish, 1771) was al-most completely theoretical. Cavendish started fromthe theory of Aepinus assuming that electricity wascaused by an elastic fluid (electric fluid) interspersedbetween the particles of bodies and surrounding thebodies themselves in the form of an atmosphere.Although this atmosphere was located at a very smalldistance from the body, its attractive and repulsivepower extended to very considerable distances. The

particles of the electric fluid repelled each other andattracted particles of all other matter with a forceinversely proportional as some less power of thedistance than the cube. The particles of all matteralso repelled each other, and attracted those of theelectric fluid with a force varying according tothe same power of the distances. Cavendish gavetwo fundamental definitions regarding bodies beingpositively or negatively electrified: (1) the notion ofdegree of electrification of a conductor, which helater denominated the compression of electricity(today: electric potential); when the fluid within anybody was more compressed than in its natural state

he called the body positively electrified; when it wasless compressed he called it negatively electrified,and (2) when any body contained more of the electricfluid that it did in its natural state, he called itovercharged, and vice versa.

Cavendish put forward the following argumentsto demonstrate that the law of the inverse square ofthe distance governed electrostatic attraction andrepulsion: if electric attraction and repulsion wereinversely proportional to some higher power of thedistance than the cube, a particle could not have

been sensible affected by the repulsion of any fluid,except what was placed close to it. If the repulsion

was inversely as the cube of the distance, a particlecould not be sensibly affected by the repulsion of anyfinite quantity of fluid, except what was close to it.But as the repulsion was supposed to be inversely assome power of the distance less than the cube, aparticle could be sensibly affected by the repulsionof a finite quantity of fluid placed at a finite distancefrom it. His reasoning that there could not be elec-trostatic force within a closed spherical containedwas based on Newtons gravitational theory. Hethen showed that it is likely that it is inversely as thesquare, for only on that assumption one could ex-plain the absence of electrical force inside a hollowsphere, or the surface charges of sphere and planeparallel plates, and the phenomenon of electricinduction.

Further unpublished work (1772-1773) describedCavendishs ingenious experiment to demonstratethat all points inside an electrified hollow sphere areelectrically neutral so that the redundant fluid(electrical charge) is lodged entirely on its surface.He also provided the definition of electrostatic capa-city: the charges of two conductors that are electrifiedto the same degree are proportional to the capaci-ties. The capacity could be measured directly by

discharging the conductors by means of a test plane.Cavendish also discovered that the capacity of con-densers of equal dimensions varies according to thesubstance employed to separate the plates. The latterresult, which would later be rediscovered by MichaelFaraday (1791-1867), would lead to the definition andmeasurement of the dielectric constant.

A large part of Cavendishs experimental workon electricity was concerned with measurements ofthe capacity of conductors and a comparison of theelectrical conductivities of various bodies (Maxwellet al., 1921). According to his results iron wireconducts roughly 400 million better than rain or

dishwater; that is, the electricity meets no moreresistance in passing through a piece of iron wire400,000,000 inches long, than through a column ofwater of the same diameter only one inch long.Seawaterconducts 100 times, and a saturated solutionof sea salt about 720 times, better than rain water.Much of his work was concerned with finding theratios of the charges of various bodies of differentshapes and sizes to that of a sphere 12.1 inches indiameter, all bodies being equally electrified (char-ged to the same potential). These findings carried


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with them the concept, which would later be knownas Ohms law.

ReferencesBerry, A. J., Henry Cavendish----His Life and Scientific

Work, Hutchinson, London, 1960.Cavendish, H., Three Papers Containing Experi-

ments of Factitious Air, Part I, Containing Expe-riments on Inflammable Air, Phil, Trans., 56,141-184 (1766).

Cavendish, H., An Attempt to Explain Some of thePrincipal Phnomena of Electricity by Meansof an Elastic Fluid, Phil. Trans., 61, 584-677(1771).

Cavendish, H., An Account of Some Attempts toImitate the Effects the Torpedo by Electricity,Part I., Phil. Trans., 66, 192-225 (1776a).

Cavendish, H., An Account of the MeteorologicalInstruments Used at the Royal Societys House,Phil. Trans.,66, 375-389 (1776b).

Cavendish, H., On the Conversion of a Mixture ofDephlogisticated and Phlogisticated Air Into Ni-trous Acid by the Electric Spark, Phil. Trans., 78,261-274 (1778).

Cavendish, H., Observations on Mr. Hutchinss Ex-periments for Determining the Degree of Coldat Which Quicksilver Freezes, Phil. Trans., 73,

303-328 (1783).Cavendish, H., Experiments on Air, Phil. Trans., 74,119-173 (1784).

Cavendish, H., Experiments on Air, Phil. Trans., 75,372-384 (1785).

Cavendish, H., An Account of Experiments madeby Mr. John McNab, at Albany Fort, HudsonsNay, Relative to Freezing Mixtures, Phil. Trans.,76, 241-258 (1786).

Cavendish, H., An Account of Experiments madeby Mr. John McNab, at Albany Fort, HudsonsNay, Relative to Freezing of Nitrous and Vitrio-lic Acids, Phil. Trans., 78, 166-175 (1788).

Cavendish, H., Experiments to Determine the Den-sity of the Earth, Phil. Trans., 88, 469-526 (1798).

Cuvier, G., loge Historique de M. Cavendish, His-toire Class. Sci. Math. Phys. Inst. Royal de France,12, cxxvi-cxliv (1811).

Diderot, D., Encyclopdieou Dictionnaire Raisonn desSciences, des Arts, et Mtiers, le Breton, Paris, 1751-1766.

Franklin, B., Opinions and Conjectures Concerning theProperties and Effects of the Electrical Matter, Arising

from Experiments and Observations at Philadelphia,Fothergill, London, 1751. Reprinted by Har-

vard University Press, Cambridge, 1941.Lavoisier A., Mmoire Dans Lequel on a Pour Ob-

ject de Prouver que lEau nest Point une Subs-tance Simple, un lement Proprement dit, maisquelle est Susceptible de Dcomposition & deRecomposition, Mm. Acad. Royale Sci., 468-494,1781 (Published in 1784).

Lavoisier, A., Meusner, J. P. M. C., Dveloppementdes Dernires Expriences de la Dcomposition& Recomposition de lEau, Journal Polytype, 1,21-44 (1786).

Macquer, P. J., Recherches sur lArsenic. PremierMmoire,

Mm. Acad. R. Sci., 233-236, 1746 (Pub-

lished in 1751).Macquer, P. J., Second Mmoire Sur lArsenic, Mm.

Acad. R. Sci.,35-50, 1748 (Published in 1752).Macquer, P. J., Dictionnaire de Chymie, Lacombe,

Paris 1766; vol. II, page 314.Maxwell, J. C., Larmor, J., Thorpe, T. E., editors, The

Scientific Papers of the Honourable Henry Cavendish,The University Press, Cambridge, 1921; in twovolumes.

Miller, D. P., Distributing Discovery BetweenWatt and Cavendish: A Reassessment of theNineteenth Century Water Controversy, Ann.

Sci., 59, 149-178 (2002).Newton, I., Philosophiae Naturalis Principia Mathema-tica, (London) 1687, Book III, Proposition X,Theorem X.

Priestley, J., Experiments and Observations on Air,in Experiments and Observations Relating to Va-rious Branches of Natural Philosophy,vol. 5 (1781),Birmingham, 395-398.

Regnault, V., Des Forces lastiques de la VapeurdEau aux Diffrents Tmperatures, Mm. Acad.Sci., 21, 465-476 (1847).

Scheele, C. W., Om Arsenik Och Dess Syra, Kongl.Vetenskaps Acad. Handlingar, 36, 265-267 (1775).

Schofield, R. E., Still More on the Water Contro-versy, Chymia, 9, 71-76 (1963).

Wilson, G., The Life of the Honourable Henry Cavendish,Cavendish Society, London, 1851.

Wisniak, J., The Thermometer----From the Feeling tothe Instrument, Chem. Educator [Online], 5(2000); S1430-4171 (00) 02371X.

Wisniak, J., Henri-Victor Regnault----The Heat Man,Educ. qum., 12, 163-174 (2001).



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