410 Prof. Johnston on apparent exceptions to tJie law that 



Iftliereforc we can depend on the chemical analyses and the 

 crystalline vieasurements of the above substances^ it is not uni- 

 versally true that like crystalline forms indicate like chemical 

 formula. 



Does this conclusion invalidate the general doctrine of iso* 

 morphism, or present a stumbling-block in the way of re- 

 ceived opinions on that subject ? By no means. It forms, if 

 •correct, a distinct and independent as it is an important ac- 

 cession to our knowledge. 



To place this statement in its true light, let us consider 

 what has been the general progress of the doctrine of isomor- 

 phism, and in what position it really stands. 



1. It was observed that the sulphates, the carbonates, &c. 

 of entire classes of bases crystallized in forms which were 

 specific not to each base, as magnesia and baryta, but to each 

 class of bases, two of which were represented by magnesia 

 and baryta as their types. 



2. A similar observation was made in regard to certain 

 salts containing different acids, as the arsenic and phosphoric ; 

 the same form being observed not merely in the arseniate and 

 phosphate of the same base, but in such salts also of each 

 class of bases. 



3. It was therefore inferred, abstraction being made of 

 what is common to the chemical constitution of such as have 

 like forms, that the remainders have the same form also ; that 

 the whole class of bases, for example, lime, magnesia, &c., had 

 the same form, and that the class of acids, the arsenic, phos- 

 phoric, and antimonic, had also the same form, or were isomor-^ 

 phous. 



4'. This inference was confirmed by the subsequent obser- 

 vation that the several members of these and other classes of 

 bases and acids might take the place of (replace) each other 

 in quantities which might vary, but which were always pro- 

 portional to their equivalents, without affecting the form of the 

 resulting crystal. 



5. But it was further observed that certain other sub- 

 stances appeared to replace each other, the forms of which 

 had not been determined; that oxide of chromium, for ex- 

 ample, might replace alumina; and hence it was inferred 

 that such compounds must also be isomorphous; an inference 

 which in the case of the oxide of chromium was confirmed by 

 the examination of the crystals of this substance subsequently 

 obtained by Wcihler. 



6. Now in all these cases, substances having the same form 

 or replacing each other, were found, or inferred to be, ana- 

 logous in chemical constitution ; to be oxides, sulphurets, &c. 



