116 



HABIT AND INTELLIGENCE. 



[chap. 



Sciences 

 of resem- 

 blance, or 



classi- 



ficatory 



sciences. 



Crystallo- 

 graphy. 



The general problem of this whole group of sciences is to 

 infer causes from effects and effects from causes. 



But in the higher or more complex chemistry, especially 

 in the chemistry of organic compounds, the known and 

 possible combinations are so many, and the variety of 

 interactions is so great, that in the present state of science 

 it is impossible to do for chemistry what we have been 

 doing since Newton's time, and are still doing, for the dyna- 

 mical sciences ; namely, to refer the multitude of pheno- 

 mena to a few causes, of which the phenomena are effects. 



Every science, however, must have some principle, or 

 principles, of logical relation; were it without any, it 

 would not be a science at all, but only a mass of uncon- 

 nected facts. In chemistry, this very multiplicity, variety, 

 and complexity of the facts, which places any knowledge 

 of their causes at present beyond our reach, supplies us 

 with a new principle of scientific relation : that of re- 

 semblance. Compounds are not formed without relation 

 to each other, or as it were at random ; they are formed in 

 series, and in series of series ; and this relation is most 

 decided in the higher or more complex chemistry. 

 Chemistry, in its higher branches, has thus become a 

 classificatory science ; and the most important result 

 which has been yet achieved in the higher chemistry 

 consists in the establishment of the theory of types and 

 substitutions, by which a rational classification of the vast 

 multiplicity of compounds has been made possible. 



Crystallography also is a science in which causes are as 



yet almost totally unknown. "VVe know little or nothing 



of the causes that determine crystalline formation ; we 



can only describe and classify crystals by their chemical, 



geometrical, and optical properties. Crystallography, or 



the science of crystallization, is thus a classificatory science. 



"We have seen that chemical compounds are formed in 



series, and in series of series. CrystaUine species exist 



in groups, and in groups of groups ; or, to use technical 



language, in genera and classes.^ 



''■ The difference between a series and a group is, that the names of the 

 members of a series can be properly enumerated in only one order, those 



