Mr. Levy on thf Forms of Artificial Salts. 283 



not a sufficient chemical analogy. On the contrary, the com- 

 position of the substances crystallized artificially is better 

 known than that of minerals, or at least more easily ascer- 

 tained ; and, perhaps, a sufficient number among them, having 

 a certain desired relation of composition, may be examined, 

 to lead to some important result. It is iu this point of view 

 that the determination of their forms appears to me to deserve 

 attention. 



This subject has acquired, lately, a new degree of interest, 

 from the two papers of Mr. Mitscherlich. He has himself 

 examined a great many artificial crystals, and has given, in 

 the last of his two papers, it seems, with great accuracy, the 

 forms and complete determination of many salts produced by 

 the combination of the phosphoric and arsenic acid with several 

 bases. His object is to establish, that the same number of 

 atoms, combined in the same manner, produce the same crys- 

 talline form ; and that the same crystaUine form is independent 

 of the chemical nature of the atoms, and is only determined by 

 their number and relative position. In both his papers, and 

 especially in the last, will be found the facts and reasons he 

 adduces in support of this opinion ; and, I think, that after 

 their perusal, even those who are most adverse to generaliza- 

 tion, must, at least, admit that the analogies and identities of 

 forms, which he has noticed in several compounds, are ex- 

 tremely interesting. Another proposition he advances is, that 

 the same substance may crystallize under two different and 

 incompatible forms ; and mentions, as examples, carbonate of 

 lime and arragonite, and the two forms he has obtained for the 

 bi-phosphate of soda*. 



The preceding considerations, and the results obtained by 

 Mr. Mitscherlich, made me very desirous to begin an examina- 

 tion of artificial crystals ; and having mentioned my intention to 

 Mr. Children, he very kindly offered to take his share of the 



• There is not, however, the same degree of incompatibility between tte 

 two forms of the bi-phosphate of soda, as between those «/ arj-agomte and 

 earbonite of lime, the one being a right rhombic prism, and tl^e other a r«c- 

 tangnlar octahedron ; butl suppose, Mr. M. has satisfied himself that the 

 uu« could not simply be deduced from the other. 



