r 78 ] 



Xf, Curfory View of fome »ij the late Difcoverles In Sciense, 

 ^Continued from Page ^14 of t!:e lall Volume.} 



i^ ^ CRYSTALLOGRAFHy, 



JP OURCROY, m treating urine, obferv-ecl a phenomenon 

 ef sreat importance to cryliallography. It is known that 

 cabes offal-ammoniac and oi::.laedra of marine fait have been 

 found in urine. But whence happens it that marine fait 

 CTvBallifes in oftaedra, and not in cubes, and that fal-aramo- 

 fiiac cryftallifes in cubes, and not in nftaedra ? FourcToy is 

 of opinion that thefe forms are owing to the prefence o^ ur.e. 

 TlTis he proves by the following experiment : 



Marine fait, or muriat of foda, exceedingly pure and in 

 perfetSt cubes, having nothirrg in common with human urine, 

 to which it never belonged, fince it arofe from fpring- water 

 evaporated, was diflblved, with an equal part of cryftallifed 

 »r.V, in five times its weiglit of di (tilled water. The fokition 

 was put into a porcelain eapfule, which was covered with paper 

 to prevent the accefs of foreign bodies^ and was left to fpon- 

 taneous evaporation. In a few weeks there were formed oc- 

 laetlral cryRals exceedingly regular, and of a reddifli brown 

 colour. 



Sal-ammoniac, or muriat of ammonia, treated in the fame 

 manner, cryftallifed in cubes, though it cryllallifes in general 

 in 0(3aedra, 



'* It is a h8t then, fully proved," fays he, '* that ur'-Cy 

 difiToTved in the fame water as the two falts above mentioned, 

 modifies and reverfcs their natural form by combining with 

 each of them, and by penetrating the laminae of their cryf- 

 tals. To it, therefore, is due the oftaedral form aflTumed by 

 marine fait, with which human urine his been faturated." 



Haiiy has defcribedf various crvftals; fuch as thofc of cin- 

 nabar or fulphurated mercury, 01 fulphat of (Irontian, and of 

 the arrsgonite or calcareous fpar cryftallifed in hexagonal 

 prifms, which is found in Arragon. 



VOLCANOES. 



Kirwan has publifhed an excellent memoir on the primi- 

 tive ftate of the globe, and the cataftrophe which fucceeded it, 

 9 «e 



