ORIENTATION IN THE MOLECULE 155 



We see from these data, first, that in isomorphic compounds 

 an equal difference in composition gives an equal difference in 

 volume, for the molecular volume (molecular weight divided 

 by the density) gives : 



for As P in the potassium salt . . . 4-576 



ammonium ... 4-673 

 and 



for NH 4 K in the phosphate .... 5-924 

 arsenate 6-020. 



Next, however, and this is the chief result, the axial ratio 

 remains almost unaltered on replacement of phosphorus by 

 arsenic, but changes by a considerable amount on replace- 

 ment of potassium by ammonium. Let us calculate, in 

 order to express this behaviour more precisely, the dimen- 

 sions of the fundamental parallelepiped, keeping for 

 potassium phosphate the values 



a = 6 = i, c = 0-664. 

 The volume is then ^ a 2 r, and so 



a-c (= 0-664) : a i 2c i 5^*^46 : 62-822, 



where a t , c l refer to the fundamental parallelepiped of 

 potassium arsenate. But since 



Oj : c t = i : 0-6633, 

 we get 



&j = 1-026, <?! = 0-683. 



The corresponding values for the other salts are given 

 in the fourth and fifth columns of the table. It follows 

 that on replacement of potassium by ammonium, there is 

 produced essentially only an increase in the height c of the 

 fundamental parallelepiped amounting to 0-055 and 0-049 

 respectively; while a is practically unaltered, or only by 

 the amounts 0-009 an( i 0-005 respectively. On the other 

 hand, replacement of phosphorus by arsenic not only 



