574 SCIENCE PROGRESS. 



heats of the Hquid substances or their compounds under 

 comparable or equiv^alent conditions apparently will it be 

 possible to re-cast the original law into a form in which it 

 will hold with accuracy. Naturally, the same kind of 

 criticism is applicable to many other properties, such, for 

 example, as the electrical conductivity ; since all the solid 

 metals and most other substances we have to deal with are 

 crystalline, composed of minute crystals of which the con- 

 ductivity varies with the direction and of which the average 

 conductivity is presumably dependent on the crystalline 

 form, no general quantitative law connecting the electrical 

 conductivity and the chemical nature of the material can be 

 formulated until the influence of the crystalline form of the 

 tiny individual crystals can be expressed in figures and 

 eliminated. 



Many other instances might be quoted in which crystallo- 

 graphic causes operate to render obscure the laws governing 

 various classes of chemical phenomena ; an instructive case 

 of the kind arises in connection with the heat of formation. 

 During the formation of a chemical compound by the union 

 of certain materials a perfectly definite quantity of heat — 

 the so-called heat of formation — is absorbed or liberated ; 

 this heat of formation is of course a measure of the energy- 

 content of the compound. The heats of formation of sub- 

 stances have long been regarded as a highly constitutive 

 property, the continued study of which cannot fail to yield 

 important information respecting the manner in which the 

 atoms constituting a chemical molecule are linkeel together ; 

 large numbers of organic compounds, — gaseous, liquid and 

 solid— have therefore been examined in order to ascertain 

 the exact nature of the relation holding between chemical 

 constitution and heat of formation. The results up to the 

 present obtained have hardly been commensurate with the 

 work done on the subject owing to the many inherent 

 difficulties which have been encountered. It is, however, 

 somewhat remarkable that the influence which the crystalline 

 form of a solid exerts on its heat of formation has to a large 

 extent escaped consideration so that we find, for instance, the 

 heat of formation of solid substances compared with those of 



