258 SENARMONT ON THE OPTICAL CHARACTERS OF 



portions ; for it was only necessary to alter the limits of genus 

 and species, properly so called, in order to explain how different 

 compounds might present one and the same geometrical type ; 

 by this means it even became possible to reproduce, at least 

 mentally, the relatively simple compounds, nearly all the true 

 species which occur naturally only as complex mixtures. 



But although the truth of these beautiful laws, for which 

 mineralogy is indebted to M. Mitscherlich, is proved even by 

 the comprehensiveness and simplicity of their results, as far as 

 regards their application to the limited class of facts which are 

 capable of direct observation, and under general conditions which 

 do not widely differ from those in which isomorphism is obvious 

 at first sight, it is very difficult to advance beyond those limits ; 

 and on attempting to abstract from the phaenomena a precise 

 idea of isomorphism, of its actual principles and its essential 

 conditions, it soon becomes evident that our knowledge of the 

 subject is not profound, but at best merely superficial. 



We are, in the first place, acquainted with elements isomor- 

 phous themselves and in many of their compounds, but which 

 cease to be so in nearly all the others. Thus, for example, pot- 

 ash and soda, manifestly isomorphous both geometrically and 

 chemically when in the state of chlorides, bromides, nitrates, and 

 alums, replacing each other moreover in variable proportions in 

 many minerals, present a constant antagonism in the chemical 

 and geometrical character of almost all their other homologous 

 compounds. What then has destroyed in these compounds the 

 chemical and geometrical isomorphism which ought invariably 

 to accompany the elements of which they are composed ? 



We find, on the other hand, substances chemically very dif- 

 ferent possessing almost the same crystalline form ; for example, 

 carbonate of lime and nitrate of potash, which are not only iso- 

 morphous but isodimorphous ; arragonite and bournonite, cal- 

 cite and proustite, together with many other salts and minerals, 

 which it would be useless to mention in this place. The resem- 

 blance of form in the above instances can scarcely be referred to 

 a chemical isomorphism, which ought to be the cause or the 

 consequence of the geometrical isomorphism, and that remark- 

 able capability of associating in crystallization, which appears to 

 be one of its essential characters. 



