XXX.] CLASSIFICATION. 689 



and hardly yet reduced to law. Some crystals, called 

 pyro-electric, manifest vitreous electricity at some points 

 of their surface, and resinous electricity at other points 

 wlien rising in temperature, the character of the electricity 

 being changed when the temperature sinks again. This 

 production of electricity is believed to be connected with 

 the hemihedral character of the crystals exhibiting it. 

 The crystalline structure of a substance again influences 

 its magnetic behaviour, the general law being that the 

 direction in which the molecules of a crystal are most 

 approximated tends to place itself axialiy or equatorially 

 between the poles of a magnet, respectively as the body is 

 magnetic or diamagnetic. Further questions arise if we 

 apply pressure to crystals. Thus doubly refracting crystals 

 with one principal axis acquire two axes when the pressure 

 is perpendicular in direction to the principal axis. 



All the phenomena peculiar to ciystalliue bodies are 

 thus closely correlated with the f(»rmation of the crystal, or 

 will almost certainly be found to be so as investigation 

 proreeds. It is upon empirical observation indeed that 

 tlie laws of connexion are in the first place founded, but 

 tlie simple hypothesis that the elasticity and approximation 

 of the particles vary in the directions of the crystalline 

 axes allows of the application of deductive reasoning. 

 I'he whole of the phenomena are gradually being proved 

 to be consistent with tliis hypothesis, so that we have in 

 this subject of crystallography a beautiful instance of 

 successful classification, connected with a nearly perfect 

 ]>liysical hypothesis. Moreover this hypothesis was verified 

 experimentally as regards the mechanical vibrations of 

 sound by Savart, who found that the vibrations in a plate 

 of biaxial crystal indicated the existence of varying 

 elasticity in varying directions. 



Classification an Inverse and Tentative Operation. 



If attempts at so-called natural classification are really 

 attempts at perfect induction, it follows that they are 

 suljject to the remarks which were made upon the inverse 

 character of the inductive process, and upon the difiiculty 

 of every inverse operation (pp. ii, 12, 122, &c.). There 

 will be no royal road to the discovery of the best system, 



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