1859.] as investigated by Light. 103 



of ammonia, brookite [Grailich], and other crystals, sudn as at a 

 high temperature, glauberite [Brewster, Descloiseaux], and gypsum 

 [Mitscherlich]. It is strange to think that thus in one and the same 

 substance there should co-exist a principal direction of vibration of a 

 different order {e.g. the a axis for the red, with the b axis for the 

 violet), for different colours in one and the same direction. 



3rdly. Again, the prolific labours of Grailich, and his pupils, have 

 established a most interesting and important law regarding isomorphous 

 substances, viz. this : that the optical similarity between any series of 

 isomorphous bodies frequently diminishes in proportion as the isomor- 

 phous chemical substances, not common to the 

 tL _ different bodies, preponderate over those which are 

 / ^ common to them. 

 Y-^ A 6 V The crystallographic orientation of the form com- 



mon to the isomorphous group of the following sub- 

 stances, is expressed by the parameters in the order 

 of their magnitude, a b c. 

 In sulphate of potash the optical elasticity is expressed by the 

 symbol a c b ; in sulphate of ammonia by b a c ; in chromate of potash, 



+ + 



by a c b ; the order of the letters indicating in each case the orientation 



of the different magnitudes, as expressed by the letter (a being greater 

 than b and b than c), and as compared with the directions in space of 

 the original crystallographic parameters a b c. 



Thus the chemical substance entirely overrules the crystalline 

 arrangement in impressing on the crystallized body an optical elasticity ; 

 so that we are driven to seek the fundamental cause of optical cha- 

 racter, not in the arrangement of molecules, but in the inner nature, 

 constitution, structure of these. 



How, then, are these results, discordant as they are with crys- 

 tallographic facts, to be reconciled with that general harmony between 

 morphological symmetry and crystallo-optic^ phenomena, which we 

 have already in part accepted ? 



To this question, the answer is not readily found. It is, however, 

 the more probable view, that the chemical units of matter (of the nature 

 of which we are profoundly ignorant, and are without even analogies 

 to guide us ; but which the atomic theorists, assume to be compounded 

 of atoms in some form of geometrical arrangement, and clustered to- 

 gether in chemical groups), form the basis of the crystalline system : that 

 these are ordinated (with the ether of the mathematical theory) in higher 

 groups which form the ultimate crystal-units-of-mass, or crystal mole- 

 cules — but that the form and internal arrangement of the molecule 

 bears a close analogy, and a general identity of symmetry, with the 

 form of the crystal of which it is the constituent mass-unit. 



The chemical units of mass we can at any rate treat as distinct 

 mechanical units with centres of gravity, and centres of volume 

 (probably distinct), and endowed with powers that cannot be the same 

 in different directions. These then would thus upbuild the molecular 



