242 COSMIC PHILOSOPHY. [pt. i. 



hand, in inquiring into the cause of crystallizatkn, we 

 employ the method of agreement as follows. "We compare 

 instances in which bodies are known to assume crystalline 

 structure, but which have no other point of agreement; and 

 we find them to have one, and as far as we can observe, 

 only one, antecedent in common, — the deposition of a solid 

 matter from a liquid state, either a state of fusion or of 

 solution. We conclude, therefore, that the solidification of a 

 substance from a liquid state is an invariable antecedent of 

 its crystallization." In this particular case we may say that 

 it is not only the invariable antecedent, but the unconditional 

 invariable antecedent, or cause ; since, having detected the 

 antecedent, we may produce it artificially, and find that the 

 effect follows it. It was thus in Sir James Hall's splendid 

 experiment, in which " he produced artificial marble by the 

 cooling of its materials from fusion under immense pressure." 

 And it was thus when Dr. Wollaston, " by keeping a vial of 

 water charged with siliceous particles undisturbed for years, 

 succeeded in obtaining crystals of quartz." 



Manifestly, however, unless we can artificially produce the"' 

 antecedent, and so reason back from cause to effect, our 

 method of agreement is not exhaustively conclusive. Unless 

 we can be sure that the observed antecedent is the only one 

 common to all the instances, the sequence may turn out to 

 be only a derivative sequence, like that of day and night. 

 A.nd unless the phenomena are very simple, we cannot be 

 sure that the observed common antecedent is the only one. 

 It is otherwise with the method of difference. Whenever we 

 can bring that method to bear upon the phenomena, its results 

 are finally conclusive ; since it is the very essence of that 

 method to compare two instances which are exactly alike in 

 every respect save in the presence or absence of the given 

 antecedent. Unfortunately, in the operations of nature these 

 requisites are seldom fulfilled. So that the method of 

 difference " is more particularly a method of artificial experi- 



