RICHARDS. — SOURCE OF CHEMICAL HEAT. 403 



which represents graphically the results recorded in the table. The ele- 

 ments are arranged in the order of their atomic weights, and both sets of 

 data are drawn as abscissae, because this method of treatment will facili- 

 tate later comparison, and because it obviates certain irregularities due to 

 periodicity. Each division stands for ten cubic centimeters of contraction 

 on the left hand curve, and a hundred kilo joules of heat-energy on the 

 right hand curve. 



The correspondence is obviously too close to be the result of chance. 

 One is forced to believe that a fundamental relationship exists between 

 the two phenomena. 



In these curves the compressibility is ascribed wholly to chlorine, and 

 that of the other substance is neglected ; but when the latter is large, it 

 also must enter into the problem. Unfortunately our data concerning 

 compressibility are unusually limited ; but approximate calculations, 

 based upon such as are known or may be guessed, show that at least 

 some of the irregularities in the parallelism may be ascribed to this 

 source. 



We may thus formulate the following law : The work needed for the 

 compression involved in the formation, of one solid or liquid by the combi- 

 nation of two others is approximately proportional to the heat evolved. 



While the general tendency of the law is manifest, and a correction 

 for individual compressibilities would undoubtedly make it more so, there 

 are nevertheless several exceptions to be explained. These may arise 

 from several causes ; in the first place, many specific gravities of solids 

 are known only approximately ; * in the next place, it is important that 

 the same modifications of each substance should enter into each calcu- 

 lation. A plausible explanation has been found even for the excep- 

 tionally wide deviation exhibited by argentic chloride ; but this point 

 will not be dwelt upon now, since it is being submitted to the test of 

 experiment. 



The relation may be further illustrated by a table giving the data for 

 a few bromides, and of course many other data might also be given. In 

 order to eliminate as much as possible the contraction of the metal, it is 

 well to choose for comparison a common non-metal possessing a compara- 

 tively large coefficient of compressibility, hence both chlorine and 

 bromine serve well. 



As a final example, the case of a single metal combining with several 



* See Richards, These Proceedings, 31, 163 (1895); also Ostvvald, Zeitschr. 

 phys. Chem. 3, 143 (1889). 



