78 HISTORY OF MIKERALOGY. 



ments and calculations of Haiiy of comparatively little 

 value. 



It may here be expedient to offer a brief definition of the 

 recent discoveries of those laws of chemistry, which have so in- 

 timate a relation with the nature and composition of minerals. 



Isomerism, discovered by Berzelius, is a principle which 

 is somewhat vague and doubtful in its application ; it may be 

 defined as that law by virtue of which bodies having the 

 same molecular composition, and the same atomic weight, 

 have different physical properties. 



Isomorphism is the law by which an equal number of 

 atoms, combining in the same manner, may give birth to 

 similar crystalline forms, although the constituent elements 

 are of a different nature. 



Dimorphism is a law which, though previously known, has 

 been confirmed by the discoveries of Mitscherlich. It is 

 considered to be only a peculiar kind of isomerism. 



The law of equivalents is that by which bodies combine 

 with each other in constant and invariable quantities. 



The law of substitutions shows that constituent elements 

 may be substituted for each other, without producing any 

 change in the nature of the compounds. This law, which is 

 perhaps a peculiar case of the law of equivalents, was fatal to 

 the electro-chemical theory of Berzelius, since it proves that 

 electro-positive bodies may be substituted for electro-nega- 

 tive, and vice versa. 



The most important additions, since the period of Haiiy, 

 have been made by the endeavours of Weiss and Mohs to 

 establish distinct systems of crystallisation, founded on 

 essential distinctions of crystalline form. Professor Weiss 

 showed the importance of considering the axes in crystals, 

 and established on these the distinction and classification of 

 crystalline systems; he also published a theory of zones, 

 calculated to facilitate the development of compound forms, 

 which has served as the basis of the representatives of 

 crystalline forms to two of his pupils, Neumann and 

 Quenstedt. Professor Mohs gave a new exposition of the 

 principles of crystallography, and published a remarkable 

 classification of minerals, founded solely on their physical 

 characters. He was followed by Bre'ithaupt, Haidinger, 

 and Zippe. Neumann proposed a new notation of crystal- 



