\92 <M* THE POaMATION OF CRYSTUS. 



richod the collections with cryftals of alum, fea-falt, fulphate 

 of copper, &c. of an extraordinary fize and beauty : he now 

 publifhes the means which he has employed. 

 How made. Flat-bottomed veflels of glafs or porcelain, are the beft for 



obtaining beautiful tingle cryftals. The folutions ought to be 

 brought to the point of cryftallization. They firft yield cry- 

 ftals that are very finall. Amongft thefe fmall cryftals, which 

 Embryo cryftals Cit. Le Blanc calls embryos, a feleclion is made of the ne'at- 

 nurfed or reared, eftj ; n ordcr tQ promote thdr growth> or as C . Le Blanc 



terms it, to nurfe (deter) them. The liquor is decanted in 

 order to purify it, and the fmall cryftals that have been fe- 

 lected are distributed in it, and carefully turned every day. 

 Amongft thefe cryftals a fecoud (election is made, in order 

 feparately to nurfe thofe of which we with either to aug- 

 ment the volume, or change the form, 

 by fele&ingthe In order to make them grow without irregularity, they 

 tht, mother"^!- muft be P laced in the mother-water of a folution that has af- 

 ter, forded a confufed cryftallization Care muft be taken to 

 turn them often, and to give them frelh fupplies of mother- 

 water in proportion as they increate in growth. In this man- 

 ner they may be brought to a conliderable volume. 



If they be left too long in a folution in which they have 

 acquired their full growth, they diminifh inftead of increafing 

 in fize, and it is obferved that this decreafe takes place at the 

 angles and edges, fo as to leave ftrire vifible, which indicate 

 They may be the direction of the ranges of fubjacent molecules. — The po- 

 made to grow fttion of the cryftals in the folution influences their form : this 

 or breadth. * s particularly remarkable in the prifmatic cryftals : they 

 grow in length when they are laid upon one of their fides, and 

 in breadth when they are placed upon their bafe. 9 - 



Secondary forms. Cit. Le Blanc having changed octahedral alum into cubic 

 alum, by plaeing an octahedral cryftal in a folution of alum 

 faturated with its earth, whieh gives the cube, infers from 

 thence that frequently the fecondary forms are owing to dif- 

 ferences in the proportion of the principles *. 



A curious, 



* It appears to us that this fact cannot lead, more than any other, 

 to fuch an inference. According to the experiments of Cit. Vau- 

 quelin, thealumine in excels is mixed with the fulphate of alumine, 

 i>ut it is not combined with it j for fimple folution in water is fulfi- 



cient. 



