688 



SCIENCE PROGRESS 



its volume on AgBr and AgCl. The considerable differences 

 met with at o° are thus easily understood. 



The half of 20*36 (io'iS) is very similar to the volume of Ag 

 in the solid state, so that we must attribute to CI, Br, or I this 

 volume or one twice as great. Since Ag and the Halogens are 

 typically monovalent, we have perhaps here an example of 

 the operation of Barlow and Pope's Valency Law. 



The Effect of Physical Modification in Solid State on the A.V. 



In the solid state we find that differences in physical modi- 

 fication affect the atomic volumes. This is well illustrated by a 

 number of Complex Felspars, studied by Day and Allen of the 

 U.S. Geological Survey {The Isomorphism and Thermal Proper- 

 ties of Felspars, Part I.). 



Volumes of Certain Complex Felspars 



In the case of both crystalline and glassy varieties the additive 

 law strictly holds — the volumes of the Simple Felspars being 

 preserved in the Complex Felspars. In the glassy state we may 

 suppose that amorphous structure is the true one, and in the 

 crystalline varieties we have orientation of the molecules. This 

 arrangement of the molecules, although it involves a certain 

 amount of compression as compared with those which show 

 want of arrangement, yet does not prejudice the principle of 

 additivity. If molecular interspaces existed this would not be 

 so. We must suppose that in the amorphous condition the 

 structure is compact in the sense already stated, because the 

 vibrating atoms, held together by the forces of affinity, fully 

 occupy the space. When arrangement of the molecules takes 

 place, owing to the action of intermolecular forces, it may well 

 be that the molecules are in the most favourable position for 



