On the Intimate Structure of Crystals. 283 



small. It is only when the ions possess a volume identical with that 

 of the molecules of water that exact agreement between theory and 

 observation can result. The molecular volume of water at 15 is 

 0'9987/18'006 = 18'083, and if its molecules are closely packed, the 

 true molecular volume will be obtained by dividing this number by 

 1-35; this gives 13'395 ; the diamete? of the molecular volume is 

 2*9465. Comparing the diameter of the ions with this, taken as 

 unity, we have : 



Water* TO 



Li 0-6289 



Na 07711 



K 0-9609 



01 0-8259 



Br 0-9472 



1 1-0828 



It will be seen that the ions of potassium bromide and iodide 

 make the nearest approach to equality with the atomic volume of 

 water, and should consequently give the most accordant results with 

 theory ; this will be found to be the case on reference to the tables, 

 the differences which appear are well within the limits of experi- 

 mental error. The chlorides are all characterised by giving to 

 theory a density which is in excess. 



Alternative explanations may be offered ; on the one hand, the 

 molecular volume of water has been taken as spherical, because its 

 molecules are most probably in rotation, but it is quite within the 

 bounds of possibility that the ions in their migrations may roll upon 

 the constituent atoms of the water molecules, and thus to some 

 extent invade the boundary of the molecular volume. 



In the case of the smaller ions with relatively small volume this 

 would lead to a considerable increase in the calculated density, while 

 in the case of the larger ions it would be scarcely affected. It is 

 also possible when the commingled spheres of the ions and water 

 molecules are of a diameter bearing a certain rabio to each other 

 that closer packing may be brought about, so that the volume of 

 interstices we have allotted to the smaller ions would be reduced ; 

 this is a question for geometers. 



The crystallographic study of these salts remains for consideration. 

 There are two ways in which the primitive cubelets may be arranged 

 in the construction of crystals : they are shown in plan in figs. 2 and 

 3; both give homogeneous assemblages in the cubic system, but 

 that shown in fig. 3 is holohedral, that in fig. 2 is hemihedral. 



* March 4. But, as Eamsay and Shields have shown, the molecule of liquid water 

 includes four molecules of the formula H 2 O ; the difference between its dimensions 

 and those of the ions is therefore greater than is given here. 



