wherry: crystal form and optical properties 



321 



spacing of the cells can be determined by applying the same 

 formula as was used for urea. Here x = 11, W = 135.02, m = 

 1.64 X 10--^ gm., y = 512, p = 1.40, c = 1.024. Solving, da = 

 1.49 X 10-8 cm., and dc = 1.52 X 10-^ cm. The arrangement 

 is too complicated for this to be interpreted in terms of the thick- 



TABLE 4 

 Refraction Relations of Penta-erythrite 



ness of any one kind of atoms, but the values are not far from 

 those of urea in the direction in which hydrogen layers appear, 

 indicating that the spaces occupied by the several kinds of atoms 

 in the two substances are about the same. 



Mellite 



Al2(C.COO)6.18H20 Ditetragonal-bipyramidal; a : c = 1 : 0.746. 

 If the alternate axial ratio of this peculiar mineral is used, the 

 axial and refraction ratios show approximate inverse agreement, 

 as brought out in table 5. 



TABLE 5 

 Refraction Relations of Mellite 



In this substance complete working out of the space-lattice is 

 impracticable, as the dispositions of the atoms in the organic 

 radicals are uncertain. But the partial structure shown in figure 

 3 has several points of interest. In it R stands for (C.COO) and 

 the heavy dots for H2O. The fact that a compound in which 

 certain groups appear in threes or multiples of three should 

 crystallize tetragonal seems at first sight anomalous; but when 



