326 wherry: crystal form ^nd optical properties 



carbon atoms, which are no doubt anisotropic ; a deviation of the 

 two ratios is thus not unexpected. 



In the case of /3-methyl-ghicoside the situation is similar to the 

 preceding. Here, however, the change needed in the standard 

 axial ratio is merely the use of the alternate value for c, and the 

 tabular habit and basal cleavage point to the fundamental cor- 

 rectness of this value, for in accordance with Fedorov's principle 

 c should exceed a in such circumstances. The ratios are then: 

 refraction, 1.035; crystallographic, 1.137. The complication of 

 the structure, and the presence of several asymmetric carbons, 

 are probably sufficient explanation of the divergence. 



In guanidine carbonate the refraction ratio is 1.020, while 

 the standard axial is 0.991. The numerical deviation is not 

 very great, but is remarkable because the two lie on opposite sides 

 of unity. However, crystals of this substance exhibit rotation 

 of the plane of polarized light and abnormal dispersion of double 

 refraction — showing a minimum around 589 and increasing 

 both directions therefrom — , thus offering evidence of possible 

 reasons for the above unusual relation. 



The exceptions to the inverse relation of refraction and crystal- 

 lographic axial ratios are thus readily explainable, although no 

 data are as yet available that would permit the evaluation of the 

 disturbance caused by each of the several factors which may be 

 represented. 



Preliminary observations have also been made on several other 

 organic compounds that possess simple formulas and crystallize 

 in fairly symmetrical systems and classes, comprising oxalic acid, 

 iodoform, acetamide, both stable and unstable forms, and 

 aldehyde-ammonia. For various reasons the measurements of 

 the indices which it has thus far been possible to make are not 

 particularly accurate, and although simple numerical relations 

 appear to exist between the crystallographic and refraction ratios 

 in many instances, the details of the structures are so compli- 

 cated that no satisfactory space-lattices have yet been worked 

 out. Further study of these and other compounds is planned, 

 however, and will be reported from time to time as definite results 

 are obtained. 



