Be marls on Mineralogical Classification. 31 



chemical perturbations that have eliminated our mountain 

 masses — as fossils have told the story of past life on the 

 globe, — our nomenclature must be enlarged to suit their 

 new application. Without attempting to show in what 

 way this revolution might be effected, I shall merely indi- 

 cate how recent researches are exhibiting the defects of the 

 present system. 



Certain recent investigations of Professor J. P. Cooke of 

 America,* on two new compounds of zinc and antimony, go 

 to show that the harmony of chemical composition and 

 form, as laid down in mineralogical books, is subject to de- 

 viations, which are apparently undefined laws. The two 

 compounds experimented on are named respectively stibio- 

 bizincyle and stibiotrizincyle ; and their chemical symbols 

 are Sb,Zn 2 , and Sb,Zn 3 . The two crystalline forms which 

 they assumed, were found to be constant under very wide 

 variations of the per-centage of the chemical constituents 

 of the crystals. Crystallizations were made, or attempted, 

 of alloys differing in composition by one-half to 5 per cent, 

 according to circumstances ; from the alloy containing 95 

 per cent, of zinc to that containing 95 per cent, of antimony ; 

 but only the two crystalline forms were obtained. 



If, thus, two beautifully crystallised products differing so 

 very widely that any single analysis might lead to an en- 

 tirely erroneous conclusion as to the general formula of the 

 substance, the question arises, may not such variations in 

 composition be quite compatible with the persistency of the 

 crystalline forms found in nature ? From a variety of ex- 

 amples, Professor Cooke shows that this actually prevails. 

 As a consequence, the general chemical formulse of some of 

 the best known mineral species, such as mica and tourma- 

 line, are still uncertain. The results of these investigations 

 must greatly modify our notion of a mineral species. The 

 idea of this has hitherto been, — first, a definite crystalline 

 form ; second, a constant general formula ; and any im- 

 portant variation in either of these characters has been re- 

 garded as equivalent to a change of species. 



* Silliinan's American Journal of Science, vol. Ixxix. 1860. 



