406 PROCEEDINGS OP THE AMERICAN ACADEMY. 



If one accepts these guesses, and imagines that the compressibilities 

 decrease with decreasing volume according to the usual approximate law, 

 one arrives at the conclusion that an amount of work equivalent to the 

 heat of combination of sodium and chlorine would correspond to a 

 change of volume in the system not far from the observed change 

 of volume. The outcome is complicated by the fact that even in ele- 

 ments, but especially in compounds, there may be superposed several 

 grades of compressibility. This can be explained hypothetically as fol- 

 lows : When the molecule is composed of two atoms, the highly com- 

 pressed portion of each atom at the point of chemical union should have 

 a much smaller coefficient of compressibility than the slightly compressed 

 remainder of the molecule. If the molecule is polymerized, there will 

 probably be yet other grades of compressibility in the various parts. The 

 only object of a calculation so uncertain as this is to show that the heats 

 of formation are of the same order of magnitude as the work involved in 

 the compression. 



In spite of the inevitable difficulties in the way of interpretation — 

 difficulties which seem to be inherent in the problem — the presumption 

 is strong that the chief source of the heat of chemical combination is the 

 work performed in compressing the material. Since the heat of reaction 

 is known to represent only approximately the free energy of the reaction, 

 while the compression may really represent the affinities at work, one 

 would hardly expect the relation to be exact. The generalization is a 

 question of fact ; it does not necessarily involve any atomic hypothesis, 

 and can be regarded as uncertain only on account of the uncertainty 

 of the data at present accessible. It is my intention to carry out 

 the experimentation necessary to place the law on a more stable 

 basis. 



In the same way any other manifestation of attraction or affinity, such 

 as cohesion or adhesion, should have a compressing effect and therefore 

 evolve heat. The superficial and limited nature of these phenomena 

 would ordinarily prevent any appreciable rise in temperature. In some 

 cases, however, as in the adsorption of liquids and gases by porous ma- 

 terial exposing a large surface, such a heating effect has been actually 

 observed. Thus the essential difference between water of crystallization 

 and adsorbed water is that the former penetrates the mass, while the 

 latter is merely superficial. 



It is obvious, moreover, that the same considerations apply to solidifi- 

 cation and change of allotropic form. For example, liquid phosphorus, 

 yellow phosphorus, and red phosphorus have at 44° the atomic volumes 



