170 DISCOURSE ON THE STUDY 



(180.) The surest and best characteristic of a 

 well-founded and extensive induction, however, is 

 when verifications of it spring up, as it were, 

 spontaneously, into notice, from quarters where 

 they might be least expected, or even among 

 instances of that very kind which were at first 

 considered hostile to them. Evidence of this kind 

 is irresistible, and compels assent with a weight 

 which scarcely any other possesses. To give an ex- 

 ample: M. Mitscherlich had announced a law to this 

 effect that the chemical elements of which all 

 bodies consist are susceptible of being classified 

 in distinct groups, which he termed isomorphous 

 groups; and that these groups are so related, 

 that when similar combinations are formed of indi- 

 viduals belonging to two, three, or more of them, 

 such combinations will crystallize in the same geo- 

 metrical forms. To this curious and important 

 law there appeared a remarkable exception. Ac- 

 cording to professor Mitscherlich, the arsenic and 

 phosphoric acids are similar combinations coming 

 under the meaning of his law, and their combin- 

 ations with soda and water, forming the salts 

 known to chemists under the names of arseniate 

 and phosphate of soda, ought, if the law were 

 general, to crystallize in identical shapes. The 

 fact, however, was understood to be otherwise. 

 But lately, Mr. Clarke, a British chemist, having 

 examined the two salts attentively, ascertained 

 the fact that their compositions deviate essentially 

 from that similarity which M. Mitscherlich's law 

 requires; and that, therefore, the exception in 

 question disappears. This was something: but, 



