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On the MANUFACTURE of LARGE AVAILABLE CRYSTALS ofSULPHATE of IODO-QUININE (Herapathite), for OPTICALPURPOSES as ARTIFICIAL TOURMALINES. By WILLIAM BIRDHERAPATH, M.D., Bristol.

HAVING been repeatedly applied to by various parties for thedetails of my process for the manufacture of these useful crys-tals, I have been induced to enter into numerous experimentsto obtain greater certainty in the results, and to study the con-ditions necessary for the production of broad foliaceous plates.Permit me to make the formula known to science, together withthe precautions necessary for adoption to secure the crystalswhen obtained, and to mount them so as to be available aspolarisers or analysers for the microscope, or even to enableus to perform all the experiments in the polariscope.

The success which 1 have obtained is so great, that there isno doubt tourmalines and Nichol's prisms will be soon com-pletely superseded by these new crystals, since the scarcity ofthe one and the difficulty in manufacturing the others renderthem very costly apparatus. But a little practice in the follow-ing process will soon enable any one to make them largeenough for every purpose; and so superior are they in powerto the best tourmaline, that two plates scarcely thicker than gold-leaf may (by a slight modification of my formerly publishedmethod) be rendered totally impervious to light when they arecrossed at right angles.

[Herewith are enclosed two marvellously thin plates of con-siderable size ; one being six-tenths of an inch long and three-tenths of an inch broad, the other the same length butone-tenth of an inch broader. Upon crossing them you willperceive that they are optically perfect as polarisers, and asuseful as plates of tourmaline for which you would be chargedfour guineas each plate. I have succeeded in getting muchlarger ones by the same process and equally good.*]

The materials employed are the same as before, the chiefmodification being in the proportions of the ingredients, andthe care taken in the method of crystallization.

It is necessary to procure pure disulphate of quinine, and forthis purpose none approaches so thoroughly to the standard ofabsolute purity as that manufactured by Messrs. Howard andKent.

I dissolve it in pyroligneous acid, having a specific gravityof 1*042, and dilute the solution with an equal quantity of

* We have examined Dr. Herapath's crystals, and have no hesitationin pronouncing them equal to any polarising arrangement we have everemployed.—EDS., MIC. JOUBN.

G 2

84 ON THE MANUFACTURE OF CRYSTALS OF SULPHATE

proof-spirit, made by adding rectified spirit of wine, spec,grav. 0837, to equal bulks of distilled water.

The spirituous solution of iodine is made by dissolving40 grains of iodine in 1 fluid ounce of rectified spirit of wine.

I can, after these explanations, give the formula.Take of disulphate of quinine 50 grains,

,, „ pyroligneous acid, 2 fluid ounces,„ „ proof-spirit, 2 fluid ounces,„ „ spirituous solution of iodine, 50 drops ;

dissolve the disulphate of quinine in the pyroligneous acidmixed with the spirit; warm the solution to 130° Fahr., anddirectly add the solution of iodine by drops, agitating themixture from time to time.

This formula gives to the mother-liquid, after crystallizationat 52° Fahr., a specific gravity of 0-986, which appears highlyfavourable to the deposition of the majority of the crystallineproduction, and yet allows only the very broad and thinnerplates to float—thus getting them perfectly free from all inter-fering and adhering plates.

It is necessary to perform this operation in a wide-mouthedFlorence flask or matrass, and to take care that the temperatureis maintained for a little time after the addition of the iodine,so that the solution should become perfectly clear, dark,sherry-wine colour ; then set it aside to crystallize, under thefollowing conditions:—

1st. It is essential that the apartment should be tolerablyequable in temperature, about 45° or 50° Fahr., as a slightvariation in the temperature produces currents in the crystal-lizing fluid which destroy the parallelism of the crystals, andof course negative all the efficiency of the manufacture ; and agreater rise, if only to 60° Fahr., redissolves the thinner plates.

2nd. It is equally necessary that the liquid should be keptin a perfect state of repose during the whole act of crystalliza-tion—even the common vibration of the apartment must becounteracted, for the same important reason as the last.

The best method to adopt is one which my friend Mr.John Thwaites employs, namely, to suspend the flask by theneck with strong twine, and attach this to a similar stringstretching across from one wall of the apartment to the other.This certainly gives the most uniform results, and offers otheradvantages.

The plan I had usually employed was to set aside the flaskon the steadiest support to be found, a wall, pillar, or table;and imbed it on a feather, cotton, or tow pillow, to act asa non-conductor, and at the same time destroy vibration.

3rd. It is also necessary that the surface of the fluid should

OF IODO-QUININE FOR OPTICAL PURPOSES. 85

not be exposed to too rapid evaporation, as the temperaturewould fall too quickly, and various currents and intestinal mo-tions would result; therefore the flask or matrass answersbetter than the evaporating dish.

4th. It is also decidedly an advantage to have a broadsurface in proportion to the depth of liquid; the reasonbeing, that the thinner and most easily reached plates form onthe surface, and float there until the time arrives to removethem; and the greater the surface, the more numerous are theplates.

5th. These broad plates are not always formed; but ifafter six hours none make their appearance, it is merely neces-sary to apply a spirit-lamp to the bottom of the flask andwarm the liquid to dissolve all the deposited crystals, then adda little spirit and a few more drops of iodine solution, andagain wait for crystallization.

6th. Supposing that we obtain a crop of these broad floatingplates, which generally occurs under the aforementioned con-ditions, we permit them to remain from twelve to twenty-fourhours to complete their disc and fill up all crevices, &c, andto attain a sufficient degree of thickness ; for if too thin, theydo not stop the red or purple-violet rays, as Haidinger hasbeautifully shown and admirably explained (vide Phil. Mag.,Oct. 1853, and Poggendorff's Annalen, for June last). If thecrystals are allowed to remain too long in their mother-liquid,we run the risk of loss and injury ; for after some time a dis-solving or disintegrating action appears to occur, and con-siderable disappointment is occasioned. I have lost severalbatches of beautiful and magnificent plates from inabilityto secure them at the nick of time.

Having by these means obtained the object of our bestwishes, it now remains to secure the prize. This requiresa little patience and a tolerable amount of care; the followingplan is the most ready, and requires but little practice anda steady hand to insure success.

The first stage of the process is to procure a table as near aspossible to the crystallizing spot, furnished with the followingapparatus :—

1. A gallipot or small mortar, to hold the flask on as a support.2. A supply of perfectly clean circular glass discs, small

enough to pass down the neck of the flask with ease.3. A glass rod of sufficient length to descend to the bottom

of the flask, if necessary.4. A little marine glue or sealing-wax.5. A spirit-lamp and matches.6. A quantity of blotting-paper cut in strips about an inch

86 ON THE MANUFACTURE OF CRYSTALS OF SULPHATE

wide and two inches long, and also a folded sheet of the sameto act as a pad or support.

Now remove the flask with the greatest amount of care fromits attachment to the horizontal string; this is best done byholding the perpendicular twine in the left finger and thumb,at the same time cutting the upper end of it with a pairof scissors to avoid all disturbance. It will now swing easilyand steadily, and may be carried and gently deposited upon itsgallipot support. Then attach the edge of one of the circularglass discs to the end of the glass rod by a little of the wax ormarine glue, and let it, when cold, be carried flatly down theneck of the flask, which should be very gently inclined, asnearly horizontally as possible, to admit of this being easilyaccomplished. Having selected the largest crystalline plate,pass the glass circle gently beneath it, raise the plate bydepressing the hand, and the little crystalline gem is at oncecaught on its surface.

If this operation be neatly accomplished, it appears spreadout as a thin uniformly-coloured film upon the glass; if anyblack patches appear, they are occasioned by the accidentalcrossing of some interposed crystals, or from some on theunder surface of the glass disc. These last must be at oncewiped off by the blotting-paper, the others will sometimes floatout upon raising the edge of the glass disc to a perpendicularposition; if they are near the edge, they may sometimesbe gently drawn out from under the large crystal by alittle dexterity on the part of the operator; frequently thereare no such precautions necessary.

Now rapidly dry the plate by imbibing all the fluid mostscrupulously by blotting-paper. This must be done withouttouching the crystalline surface, for the least contact destroysits beauty, symmetrical arrangement, and optical usefulness.Having done so, let it dry by exposure to the air in a coolroom, say at 40° to 50° ; this is to prevent resolution anddisintegration of the crystal in its own mother-water, a little ofwhich must remain attached after all our care.

It is sometimes necessary to float them on, or dip them_/bran instant only in a little cold distilled water, somewhat imbuedwith iodine. This serves two purposes ; it removes allmother-liquid, and prevents crystals of sulphate of quininesubsequently forming and interfering with the perfectpolarization of the new tourmaline, as every crystal of thissubstance interposed between the plates would, of course,rotate the polarized beam as far as its influence extended, anddepolarize it. The iodine acts also in preventing the solutionof the new crystals in the water. They must again be dried

OP IODO-QUININE FOR OPTICAL PURPOSES. 87

by imbibition and by exposure to air, as before, and thenplaced under a cupping-glass, having a watch-glass, with a fewdrops of tincture of iodine in it. This gives a decidedly blacktone to the field ; and, if the crystal were before too thin toobstruct all the light, and thus give a red or purplish violet-tint, its power of polarization will be very materially improvedby following the above simple directions.

It is essential in iodizing the plate that the exposure to thevapour be not too long continued; the time necessary will, ofcourse, depend on the temperature of the apartment; aboutthree hours at 50° Fahr., being generally necessary. Thereason of this precaution will be at once evident upon makingthe experiment, for the crystals assume a rich golden yellowcolour, both by reflected and transmitted light; the field will,therefore, when the two plates are parallel, be intensely yellow,a most objectionable colour for the examination of objects.The crystals have lost the power of stopping the yellow rays,and the complementary relation of the body to the superficialcolours appears to be lost also—a very remarkable fact.

The rationale of the periodizing process appears to bethe addition of as much iodine to the crystal as will be suf-ficient to communicate the exact complement of yellow toneutralize the red and blue rays of the purple " body-colour."These rays are now absorbed by the plates as they would beby yellow glass.* If too much iodine be added, an intenseyellow light becomes transmissible when the crystals areparallel, and the plate becomes rotten and brittle, and will bealmost certainly destroyed in mounting, even if it be exposedto the air for some time before attempting to do so; by whichprocess the superadded iodine again volatilizes, clearly showingthat no chemical union could have existed.

Having so far prepared the " artificial tourmalines," itmerely remains to cover them by another plate of thin micro-scopic glass, interposing some highly refractive cement orvarnish between the two plates.

Several cements offer themselves to our notice, but someselection is necessary. Canada balsam is one of the best:however, in using this it is necessary to have it very fluid,and not to employ much heat in the process ; in fact, I believeit best to use it so as to be fluid at the ordinary temperature.I have found, however, that it appears to attack the crystalsand dissolve out the iodine. In order to correct this destruc-

* I have since found that yellow glass has no effect in absorbing thesered or violet-red rays—the only absorptive media which I have foundpossess any power in stopping them, are, copper solutions, glass chargedwith copper, or a thin crystal of sulphate of copper.

88 ON THE MANUFACTURE OF CRYSTALS OF SULPHATE

tive tendency, it is essential to saturate the fluid Canadawith iodine, at the ordinary temperature. This is best doneby warming some small quantity of the balsam in a test-tubeor thin bottle, and dropping into it some crystals of iodine,agitating them well together by a glass rod : giving time tocool, and the excess of iodine to subside, it is fit for use. It ismerely necessary to take a small drop of this fluid on the endof a glass rod, place it on the larger and clean glass circulardisc, then invert the disc, carrying the crystal upon it, pressthe two together gently and steadily with the finger or a glassrod, or piece of stick, taking great care not to use much force,in case the circles or crystal may sustain injury. Now removeall the extra Canada balsam from around the edge, and exposethe little apparatus to the air, so that the balsam may becomedry ; it is then fit to mount in the brass work of the micro-scope in the same manner as a tourmaline.

I have found it best to employ an ethereal solution ofCanada balsam in this process, made by dissolving the hardold balsam in washed pure sulphuric aether, afterwards addinga little iodine to it as before. This dries more rapidly,hardens quicker, and more perfectly than the usual fluidCanada, and it does not attack the crystals—a very great ad-vantage.

These directions may appear very prolix, tedious, andexcessively troublesome; but, however, when set in practice,the whole operation resolves itself into the utmost simplicity ;habit soon reconciling oneself to the routine, and the differentprecautions appear to offer themselves unconsciously to us aswe proceed. I have frequently prepared a dozen good tour-malines in an hour, as far as the catching and drying partof the operation ; the others, of course, require longer time,but for these we must wait, and occupy ourselves with someother stages of the same process.

When it is absolutely necessary to obtain a perfectly blackfield with a total stoppage of all the incident rays (upon" crossing" the two crystals), it is much the better plan toemploy a thicker plate of this substance : such a crystal willbe generally found in the flask at the bottom of the mother-fluid. There is more trouble requisite in obtaining perfectplates, free from all intervening crystals, but the experi-menter is generally repaid in the end by the perfection of thepolarizing medium.

When the selenite stage is employed, the thinner and violet-coloured crystals are far preferable to those which give a blacktone to the field ; as the colours are more brilliant and theflood of transmitted light much greater, so that we are enabled

OF IODO-QUININE FOR OPTICAL PURPOSES. 89

to use a less illuminating power. I am not in the habit ofusing an achromatic condenser with my polarizing apparatus,which probably accounts for some discrepancies in the resultsof observations made by different experimenters upon thesame crystalline plates: those crystals which will transmitthe violet rays when strongly illuminated will not do so whenthe instrument is used in daylight, or with a plane instead of aconcave mirror, and without the achromatic condenser.

If it be necessary to obtain a most decidedly black field, theviolet rays may be readily absorbed by interposing a thinplate of sulphate of copper beneath the polarizing plate ofHerapathite and the source of illuminating power.

The author has recently employed a plate of this substance,l-20th of an inch thick, cut on a hone, polished and mountedbetween two plates of thin glass in Canada balsam, as ameans of correcting the defects of the thinner plates of hisnew tourmalines*—this substance possessing the power ofabsorbing the violet rays of the spectrum in a pre-eminentdegree. In order to succeed in this experiment it is neces-sary that the sulphate of copper should be inclined at a certainangle to the plane of primitive polarization, as it is a sub-stance possessing two neutral axes or planes of no-depolariz-ing power ; the position of which may be easily found, andtheir direction marked upon the support, so that the interven-ing plate may be always inserted at the angle of its greatestactivity.

Professor Stokes has lately, in a letter to me, suggested theemployment of a glass laden with the oxide of copper as ameans of attaining the same end : having, therefore, prepareda boracic glass, coloured by the black oxide of copper, I haveused it effectually as an absorbent medium for counteractingthe violet-red colour of the polarized beam. But although itoffers great and manifest advantages when the new tourmalinesare crossed at right angles, yet, upon revolving the superiorcrystal, and therefore bringing the two plates into a parallelposition, we have a blue colour in the field, which mustassuredly alter the colours of depolarizing media: it is, how-ever, a very agreeable light to work by, as the intense yellowof gas-light is much mellowed down and counteracted by it.This corrective medium would be inadmissible when theselenite stage is employed, as the tints would be materiallychanged by its absorptive agency.

The mode of making this glass is simply to dry powderedbiborate of soda in a crucible by the heat of an ordinary fire ;again reduce the effloresced mass to powder, and mix it with

* A solution of the sulphate or nitrate of copper iu water will equallysucceed in producing a black field.

90 GREGORY ON THE NEW FORMS, &c,

a small quantity of the oxide of copper, such as is generallyused in organic analysis, then introduce the mixture into aplatinum crucible, and with a steady, long-continued heat,thoroughly vitrify it, pour it out upon a flat slate, cleanmetallic, or Wedgwood-ware surface, and press it while stillsoft into a flattened plate. Upon cooling, a portion must bequickly ground down on a hone, polished, and then mountedin Canada balsam between glass : the unmounted boracic glassmay be kept for any length of time in turpentine without change,but in the air it effloresces, and becomes opaque and useless.

There is not the least doubt that, before long, these splendidand useful crystals will be offered for sale by opticians atas many shillings as tourmalines now cost pounds, and cer-tainly of equal value and practical utility—in my own opinion,of even greater, for less light is lost by these than by any of ourpolarizing apparatus at present in use.

I have invariably used, in this description, the originalterms employed by me, namely, " artificial tourmalines," and" crystals of sulphate of iodo-quinine." Professor Haidinger'sterm of " Herapathite " is certainly a highly complimentaryone to myself; but as it does not give either an idea as to theoptical properties or chemical characters of the substance inquestion, it does not appear to me so suitable as those I origi-nally attached to them.

Notice of the New Forms and Varieties of Known Forms occur-ring in the DIATOMACEOUS EARTH of MULL ; with Remarkson tlie CLASSIFICATION of the DIATOMACE^;. By WILLIAMGREGORY, M.D., F.R.S.E., Professor of Chemistry in theUniversity of Edinburgh.

THE two notices which have already appeared in the Journalhave made known the occurrences, in this deposit, of aboutninety distinct forms, of which two were noted as being newto science. But the continued and diligent examination of it,which I have carried on during my residence abroad last sum-mer, has led to results so much more remarkable, that I haveto beg of the readers of the • Journal' to regard those papersas merely introductory to a more satisfactory and completeaccount of this interesting deposit. This I shall now attemptto give; but the limits of this paper, and of the illustrativeplate, will not allow me to complete it at this time, and alarge portion of my materials must therefore be reserved for asubsequent number of the ' Journal.'

My observations have been made on a very large number of