CRY

CRY

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little management: the metal being melted inan iron ladle held in a flanting direction, is tobe cooled very gradually, and when it begins toconfolidate, the ladle being gently and graduallyinclined in the oppofite direction, the fluid partof the metal will flow off from that which hasbecome fixed, and thus a group of cubicalcryftals will be obtained. A better way of ob-taining cryltals of bifmuth, as well as of anti-mony, fulphur, the fulphurets of bifmuth andantimony, and fome other fubftances, is to melta confiderable quantity in a crucible, and after-wards to cool it as flowly as poffible, till a thincruft has formed upon the furface, two fmalloppofite apertures being then made through thecruft with a pointed iron, the fluid portion is tobe quickly poured out by one opening while theair enters by the other: when the crucible withits contents is grown cold, the cruft is to becarefully removed by a chizel or other fuitableinftrument, and a cup-fhaped hollow will pre-fent itfelf ftudded with cryftals more or lefsperfect according to the magnitude of the ori-ginal mafs, the flownefs with which it has cooled,and the dexterity with which the fluid part hasbeen evacuated.

There are, however, but few chemical fub-ftances, comparatively, that can be obtained in acryftalline ftate by the agency of heat alone ;for fome are decompofable by fire, and othersare either infufible or require fuch a high tem-perature for this purpofe as to make its appli-cation exceedingly troublefome and inconveni-ent. In efl'eCHng the cryftallization of nearlythe whole of the compound falts and a fewother bodies, recourfe has been had to theagency principally of water and alcohol, as thefetwo fluids exert a confiderable folvent poweron the fubftances expofed to their adlion, with-out however decompoiing them; and beingthemfelves readily volatile at a moderate tem-perature, they may be abftracted with fufficientflownefs to allow the fubftances held by themin folution to be depofited in a cryftalline form.

When a mafs of fait, nitre for example, isimmerfed in water of the common temperature,the cohefive attraction of its particles is oppofedby the folvent power or chemical attraction ofthe water. At firft, the nitre, being furroundedon all fides by particles of uncombined water,begins to be rapidly diflolved ; but as the folu-tion proceeds, this efFeCt becomes more andmore languid, till at length it entirely ceafes,when the number of uncombined particles of■water is fo far reduced that the fum of thefolvent forces of thofe which are in contact

with the nitre, exaCtly counterbalances the at-traction of cohefion by which the undiflolvedrefidue of the nitre is held together. Whenthis ftate of equilibrium takes place the folutionis faid to be faturated; but although this is thecafe as long as it undergoes no increafe of tem-perature, yet in proportion as the heat of thewater is raifed the folvent power of the liquidis greatly augmented. The fait therefore thatis contained in boiling hot water may be con-fidered as rendered fluid by the combined ac-tion of the heat and water, and if this men-ftruum is completely faturated, it is manifeft thatin proportion as it cools the cohefive attractionbetween the particles of nitre will gain the al-cendency, and continue to aCh till the oppofingforces of the ingredients of the folution findthemfelves again in equilibrium. All faltstherefore which are more foluble in hot thanin cold water, are in part depofited from a fatu-rated folution by mere cooling, and if this isdone gradually, and without the interference ofany materially difturbing caufe, the particles asthey confolidate will arrange themfelves in re-gular cryftals. When a folution of this kindhas depofited by cooling, all the faline particleswhich it was enabled to hold by its increafedtemperature, in addition to thofe which wereretained by the mere aCtion of the water, it isobvious that recourfe mull be had to the ab-ftraCtion of the liquid menftruum itfelf if wewifti to procure from it any additional cryftals.We therefore fubjetft the whole to a boilingheat which has the double advantage of drivingoff part of the water, and enabling the remain-der in confequence of the increafed tempe-rature ftill to hold the fait in folution. Whenthe evaporation has proceeded for fome time, a'faline pellicle begins to appear on the furfaceof the liquor, which is a proof that the cohefiveattraction of the particles of the fait is obtain-ing a fuperiority over the folvent power of thehot water, and by continuing the evaporationthe whole of the water will be driven off, andthe fait depofited in an amorphous uncryftallizedmafs: but if when the pellicle appears the fo-lution is flowly cooled, it will depofit, as at firft,in a cryftalline ftate all that portion of faitwhich in the cold faturated folution was com-bined with the water that has been evaporated.Thus by fucceflive evaporation and gradualcooling all fubftances capable of cryftallizing,and which are more foluble in hot than in coldwater, may be procured in their proper cryf-talline forms.

A few falts, efpecially muriat of foda, are