142 SCIENCE PROGRESS. 



mental work ; namely, the independence of the parts in the 

 crystal structure as shown by the additive nature of the 

 crystal characters. There is nothing resembling the com- 

 position of forces or of motions, each part contributes its 

 own effect independently of the others ; morphologically, in 

 the axial ratios ; physically, in the refractive equivalents 

 and other properties ; this appears to be equally true, 

 whether our attention is directed to the molecule or to the 

 atom. Further, the effects of different atoms or radicles 

 are most marked in certain directions within the crystal, 

 but this may be simply the result of the rigidity of the 

 structure, and does not necessarily indicate the relative 

 positions of the atoms. 



All the observed effects are, however, in strict harmony 

 with the nature of a homogeneous rigid structure as de- 

 fined above. Whatever the parts may be of which a crystal 

 consists they must be uniformly repeated ; and whatever 

 motions they possess must be confined in such a manner 

 that each may be regarded as simply juxtaposed beside its 

 neighbours without encroaching upon their domain. 



H. A. Miers. 



