RICHARDS. — SIGNIFICANCE OF CHANGING ATOMIC VOLUME. 597 



than the evenly compressed white tin, because tenacity must depend 

 upon the weakest cohesive attraction existing in a substance, just as the 

 strength of a chain is the strength of its weakest link. As a matter of 

 fact, gray tin seems to possess far less tenacity than white tin, for the 

 former is always described as crumbling to powder ; hence the fact is 

 consistent with this interpretation. The situation is far too complicated, 

 involving the simultaneous action of too many intensities of pressure to 

 attempt at present a precise mathematical solution, or to afford a basis 

 for the discovery of the reason why tin in particular acts in this peculiar 

 way. The explanation above is given only to show that this kind of 

 case, instead of offering insuperable objection to the theory of com- 

 pressible atoms, accords with it and affords a means of predicting an 

 entirely different property of the anomalous form — namely, its slight 

 tenacity. 



Besides the changes of volume which may thus be effected by the poly- 

 merization of a solid or liquid, changes of volume are undoubtedly occa- 

 sioned by the exigencies of crystal form. These will be considered in a 

 subsequent paper. Taking into account all these possible irregularities, 

 it is not at all surprising that the changes of volume do not exactly corre- 

 spond to the heats of formation in chemical reactions; indeed, it would 

 be incredible that the correspondence should be exact. Nevertheless, in 

 spite of these expected and reasonable irregularities, enough obvious 

 regularity still persists to show the general tendency, and to make highly 

 probable the fundamental statement that the atomic volume is not a con- 

 stant, but is a function of the total pressure (internal and external) and 

 the temperature. 



It has been suggested above that in general the lower the boiling point, 

 the greater should be the compressibility, — because a low boiling point 

 indicates slight molecular cohesion ; and when the pressure upon the sur- 

 face of the molecules is small, these molecules should be in an expanded 

 condition, and therefore easily compressible. This postulate is supported 

 not only by the facts given above, but also by many other data of the 

 same kind. Of course the relation is sometimes partly hidden by the 

 specific nature of the elements concerned ; hence it is shown most clearly 

 by the comparison of isomeric organic substances. In any complex 

 molecule of this kind, it is true that the compressibility must be an 

 average value ; for the application of an outside mechanical pressure must 

 add not only to the pressure of cohesion between the molecules, but also 

 to the various affinity-pressures which exist between the atoms. Never- 

 theless, the change of volume produced by the former of these effects 



