257 



Article IX. 



Researches upon the Optical Characters of Double Refraction in 

 Isomorphous Substances, By M. H. de Senarmont. 



[From the Annales de Chimie et de Physique, vol. xxxiii. Dec. 1851.] 



The laws of isomorphism, in the form in which they were 

 made known by M. Mitscherlich, and in the sense in which he 

 appears to have conceived them at the time of that great dis- 

 covery, have a signification which is perfectly inteUigible and 

 clearly defined. 



Elements which possess the same chemical characters ought, 

 according to those laws, when entering into combination in the 

 same equivalent proportions, to assume geometrical forms simi- 

 lar in their principal features, almost identical in their angles, 

 and ought likewise to be capable of associating together in cry- 

 stallization in indefinite proportions. They would thus form a 

 kind of type at once chemical and geometrical, a structure almost 

 independent of the particular nature of its constituent materials, 

 and in which one substance may be replaced by another without 

 modifying any of its essential properties, provided that they are 

 capable of occupying the same place in the structure. 



Isomorphism appears, moreover, according to the views of 

 M. Mitscherlich, to be connected with a certain identity in the 

 molecular constitutions of particular elements, and ought, ex- 

 cept in cases of dimorphism, to attend them in all their com- 

 binations in such sort, that by an almost constant reciprocity, 

 the isomorphism of compounds may be inferred from the iso- 

 morphism of elements, and the isomorphism of the elements 

 themselves, from the isomorphism of their compounds. 



This doctrine, supported by numerous examples in which its 

 truth amounts almost to certainty, in which there is no circum- 

 stance which leaves room for doubt or objection, has moreover 

 thrown an entirely new light upon the chemical constitution of 

 what have hitherto been called mineral species ; it has brought 

 that constitution into accordance with the law of multiple pro- 



