772 THE POPULAR SCIENCE MONTHLY. 



upon each other, then the chemical action is accompanied by an evolu- 

 tion of heat ; in the opposite case, an absorption of heat takes place. 

 The greater the difference, the greater also will be the change of 

 temperature accompanying the process, so that, if the action takes 

 place rapidly, the temperature may reach that of a red heat, as a result 

 of which the chemical action assumes the character of combustion in 

 the more limited sense of the word. 



The aim of quantitative ther mo -chemical investigation is now, in 

 the first place, to measure those quantities of heat which are evolved or 

 absorbed in chemical actions. It is true that these values furnish no 

 direct information as to the magnitude of the forces which are con- 

 cerned in the chemical action, partly because they are merely an ex- 

 pression for the difference between the energy of the molecules decom- 

 posed and that of the molecules formed, partly because they are often 

 affected by other actions which accompany the chemical process ; they 

 furnish, however, the material for theoretical investigations for the 

 higher aim of thermo-chemistry is to establish the dynamical laws of 

 chemical action and to afford an insight into the mysterious region 

 of the constitution of chemical compounds that is, of the molecules. 



Up to the present time, an almost impenetrable veil has enveloped 

 the internal structure of the molecules and the true nature of the atoms : 

 we know, at the most, the relative number of the different atoms in 

 the molecule, the relative mass of the molecules, and of the individual 

 atoms, and the presence of certain groups of atoms (radicals) in the 

 molecules ; but we know almost nothing about the nature of the forces 

 which dominate in the molecule, and which cause the formation and 

 decomposition of compounds. 



Experience teaches that the different atoms exert an influence upon 

 each other, which seems to be independent of their mass and which 

 appears now as attraction, now as repulsion, and that the combining 

 capacity of the atoms does not extend beyond a certain limit ; still, up 

 to the present time, no satisfactory explanation has been offered for 

 these, the chief phenomena of chemistry. Chemical processes conse- 

 quently do not as yet admit of a mathematical discussion in their 

 entire extent, as is the case, for example, with the phenomena of physics 

 and astronomy ; for the general mathematical discussion of chemieal 

 phenomena we lack that which is most important as a basis, namely, a 

 knowledge of the fundamental laws which govern the actions of the 

 atoms. With each decade, however, chemistry approaches nearer and 

 nearer the exact sciences, and already many laws of wider or narrower 

 application are being established on the basis of experiment. The ex- 

 tremely rich and varied material of chemistry now arranges itself in 

 large groups, the members of which follow certain common rules or 

 laws, with reference to their formation and decomposition, and whose 

 properties may be, to a certain extent, deduced from the composition 

 of their molecules. 



