THE PATH OF EVOLUTION 



of foreign salts. On the other hand, instances of 

 isomorphism are not rare where different crystals, 

 having their sides and angles nearly or quite equal 

 one with another, yet differ in their chemical consti- 

 tution. Generally, however, the crystaline formation 

 is a true index to the chemical composition. The 

 same characteristics apply to many substances, de- 

 rivates of organic structure. The deviation pro- 

 duced by certain crystals Iceland Spar, Tourma- 

 line, etc. upon a ray of light has already been 

 noticed. The closest relations exist between the main 

 geometrical axis of many crystals, the optic axis of 

 refraction and the transmission of heat, light and mag- 

 netism. Whether these relations depend upon the 

 properties of the Ether, upon the chemical affinities 

 of the atoms therein, or solely upon the geometrical 

 form and construction of the crystal, are as yet un- 

 solved problems. 



178 



