CRY

CRY

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but they occur with all the geometrical charac-ters of calcareous fpar, even when mixed witha very large portion of foreign ingredients. Thusthe acute rhomboids of calcareous fpar whichare found at Fontainbleau, contain more thantwo-thirds of their weight of granular quartz: theprimitive figure and cryftalline varieties of pearlfpar are precifely thofe of calcareous fpar, al-though, in many inftances, the carbonat of limein this mineral amounts to no more than abouta third of its weight, the remainder being oxydof iron and manganefe ; again in the bitterfpaththe carbonat of lime preferves its eflential cryf-talline character though mixed with nearly halfits weight of carbonated magnefia. On theother hand, the cryftalline polarity of fulphuretof copper is probably but fmall, on account ofthe minutenefs of its cryftals, and their compa-rative rarity.

Hardly any of the cryftallizations that areperformed by nature have been imitated by art;in return, however, chemiftry has been able toeffect the cryftallization of a variety of fub-ftances, chiefly falts, which' are not found cryf-tallized naturally; and by taking proper advan-tage of this circumftance has fucceeded inobtaining them in a ftate of greater purity thanany other method could afford them.

All fubftances in order to be cryftallizedrequire that their integrant molecules fhouldbe feparated from each other by the interven-tion of a medium in which they may movefreely according to the attraction of their cryf-talline polarity. But this neceffarily implies twodiitinct operations; for the fame attractionexerted by the medium or folvent to overcomethe aggregation'of a body, and reduce it to itsintegrant molecules will alfo prevent the effi-cacy of its cryftalline polarity. It is neceflarytherefore after having annulled the aggregationof a folid by the requifite quantity of a folvent,to abftratt by degrees fuch a portion of it thatthe attra£fion of the remainder fhall be inferiorto the cryftalline polarity of the fubftance dif-folved. The fimpleft folvent that can be em-ployed is caloric, and many great advantagesattend its exclufive ufe; a number of cafeshowever occur, in which its application is im-practicable, except in combination with fomeliquid menltruum, as water or alcohol.

Solids that are eafily volatilized without de-compofition at a moderate temperature, may beobtained in a cryftalline ftate by gradual fubli-mation in clofe velTels. Thus when grey oxydof arfenic is expofed to a low red heat the ag-gregation of its integrant molecules is deftroyed,

and it is converted into a vapour which at. thefame or any higher temperature would continuein a permanently elaftic ftate. This is the firftpart of the procefs, namely the feparation fromeach other of the integrant particles of thearfenic, in confequence of the addition ofcaloric, which interpofes itfelf between them:the fecond part of the procefs, or the formationof cryftals depends on the tendency to an equi-librium by which it is characterized, and itsbeing able to pafs with readinefs through mediathat are impermeable to other bodies. When,therefore, the vapour, confiding of caloric andoxyd of arfenic, arrives in the upper part ofthe veflel, the caloric pafles through, like waterthrough a filter, while the oxyd of arfenic isleft behind; this latter, therefore, is continu-ally increafing in proportion to the remainingcaloric, till the cryftalline polarity of the par-ticles of arfenic becomes fuperior to the attrac-tion of the caloric. As foon as this takes place,a ftratum of particles begins to be depofited onthe inner furface of the containing veflel, andthefe ffirve as a bafis to which all the fucceed-ing ones attach themfelves according to theirpeculiar mode of arrangement, in proportion astheir cryftalline attra&ion becomes fuperior tothe affinity exerted by the caloric. In this waynot only are volatilizable fubftances when un-mixed with others obtainable in a pure cryftal-line ftate, but they may often be feparated here-by from intimate mixture or combination withbodies of greater fixity. Thus benzoic acid isfeparable in a cryftalline form from the refin withwhich it is naturally combined by taking advan-tage of its ready volatility at a moderate heat:thus alfo the manufacturers of fal-ammoniacfeparate this fait from fulphat of foda, and pro-cure it of a cryftalline texture by the applicationof a degree of heat fufficient to fublime theformer, while it has no fuch action in the latter.

Bodies may be made to aflume the cryftallineftate not only by the procefs of fublimationbut in many cafes by a temperature only fuffi-cient to fufe them. If a nrafs of bifmuth ismelted in a crucible, and allowed to cool gra-dually, although its external figure is cafual oramorphous, yet its internal texture will be foundto be completely lamellar in two or three direc-tions, or cryftalline ; and if the external lamellaeare fucceflively removed from the furface of afragment, of the fize of a hazle nut for example,a regular octohedron will at length be obtainedwhich is the primitive form of this metal. Notonly the texture but even the cryftalline figureof bifmuth, may be obtained by fufion with a