CRY

CRY

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nearly equally foluble in water of any tempe-rature, thele therefore can only be obtained ina cryftalline Hate by evaporation, and to fucceedcompletely requires confiderable management.If the folution is made to boil the evaporationit is true goes on rapidly, but the commotionthus occafioned in the liquor prevents that or-derly depofition of the faline particles which isabfolutely rcquifite in order to obtain large andwell formed cryftals: neither mull; the evapo-ration be very flow otherwife the particles asthey feparate from the liquors are principallyattracted to the fides of the veflel up which theyafcend, thus lining the upper part of it with anamorphous cruft, while hardly any cryftals aredeposited at the bottom of the veflel.

All cryftals that are formed in water returna portion of this fluid, which is called the waterof cryftallization. Its proportion varies accord-ing to the nature of the fait that is cryftallized,and appears to be in an inverfe ratio to the forceof its cryftailine polarity. Thus fulphat ofpotalh, which requires a large quantity of waterto counterbalance the cohefive force of its par-ticles contains but little water of cryftallization,whereas fulphat of foda which is readily folublein water, holds more than half its weight ofwater. This water appears to be in a ftate ofchemical combination with the fait, and notAmply interpofed between its cryftalline laminae;the affinity however which it exerts is but feeble,at leaft in thofe falts into the compofition ofwhich it enters largely, fince a confiderable pro-portion of it is driven off by fimple expofure tothe air. In thefe cafes the cryftal mouldersaway to powder, and entirely lofes its peculiarfigure and texture; hence it has been arguedthat this water is effential to the figure of allthe falts in which it exifts: in fome degree nodoubt it is, for if the water is haftily driven off,the texture of the cryftal is entirely deftroyed,and there are no faffs as yet obferved to provethe poflibiiity of the earthy fulphats affirming acryftalline form by any other medium than wa-ter, yet the following fa£t feems to {how thatthe forrp of the cryftal is not liable to be greatlymodified by the water of cryftallization. If anumber of cubes of common fait, produced byevaporation in the ufual manner, are placed ina covered veffel over the fire, as foon as theybecome heated they decrepitate, or fly to pieceswith a crackling noife on account of the eva-poration of their water of cryftallization : whenthis noife ceafes the water is driven ofi, andthe fait has loft its cubic fhape and cryftallinetexture, and is reduced to a fine white powder:

by augmenting the heat this powder entersinto fulion, and when cooled flowly exhibitsagain its cryftalline texture, and breaks intocubical fragments in the fame manner as beforeit was heated.

If a cryftallizable fait is perfectly pure, itsfolution will continue to afford cryftals by thecommon treatment to the very laft drop : butas all falts have a greater or lefs chemical actionon each other, it ufually happens when two ormore coexift in the fame folution, that aftercryftals have been obtained by fucceffive evapo-rations and coolings, the remaining portion offluid, though charged to faturation with falinematter, refufes to yield any more cryftals. Thisliquor, whatever may be the nature of its con-tents, is known by the general appellation ofMother water. Thofe falts, the cryftals of whichare permanent in the air, have the ftrongeftdegree of cryftalline polarity; in thofe whichare efflorefcent this force is confiderably lels,but it is the weakeft of all in thofe that deli-quefce on expofure to the air. Now if twofalts of the firft clafs are diffolved together inthe fame quantity of water, provided they donot decompofe each other, and efpecially iftheir ratios of folubility are different, althoughthey are rendered more foluble by their mutualaffinity, yet they may be obtained again in acryftalline ftate without leaving any motherwater. Thus equal parts of nitrat of potalhand fulphat of potalh, though foluble whenmixed together in lefs water than would havebeen neceffary for both feparate, afford by eva-poration fucceffively and in proportion to theirfolubility, firft, fulphat of potalh, and thennitrat of potalh, without leaving any uncryftal-lizable liquid. But on the other hand if nitratof foda and fulphat of foda are fubjccted to thefame experiment, both of which have only aflight tendency to cryftallize and are of nearlyequal folubility, only a fmall quantity of fulphatof foda will feparate by cryftallization, all thenitrat and the remainder of the fulphat remain-ing liquid and uncryftallizable. When themutual a£tion of the two falts is fuflicient toeffect a double decompofition of them, it isneceffary to take into confideration the folubilityof the new falts, in order to make a correcteftimate of the quantity of uncryftallizable reli-

due. Thus if equal parts of fulphat of magnefiaand muriat of foda are mixed together, althoughthe ratio of folubility of the two is different,and their force of cryftallization is alfo con-fiderable, yet the liquor can hardly be made toyield any cryftals at all; for in confequence of