nN 31 



partly transformed (rendered latent) into chemical energy. The store 

 of force or energy going to the formation of new compounds may, after 

 several combinations, accomplished with an absorption of heat, at last 

 diminish to such a degree that indifferent compounds will be obtained, 

 although these sometimes, by combining with energetic elements or 

 compounds, give more complex compounds, which may be capable of 

 entering into chemical combination. Among elements gold, platinum, 

 and nitrogen have but little energy, whilst potassium, oxygen, and 

 chlorine have a very marked degree of energy. When dissimilar sub- 

 stances enter into combination they often form substances of diminished 

 energy. Thus sulphur and potassium when heated easily burn in air, 

 but when combined together their compound is neither inflammable nor 

 burns in air like its component parts. Part of the energy of the 

 potassium and of the sulphur was evolved in their combination in the 

 form of heat. Just as in the passage of substances from one physical 

 state into another a portion of their store of heat is absorbed or 

 evolved, so in combinations or decompositions and in every chemical 

 process, there occurs a change in the store of chemical energy, and at 

 the same time an evolution or absorption of heat. 34 



For the comprehension of chemical phenomena in a mechanical 

 sense i.e., in the study of the modus operandi of chemical phenomena- 

 it is at the present time most important to consider : (1) the facts 

 gathered from stoichiometry, or that part of chemistry which treats of 

 the quantitative relation, by weight or volume, of the. reacting sub- 

 stances ; (2) the distinction between the different forms and classes of 

 chemical reactions ; (3) the study of the changes in properties produced 

 by alteration in composition ; (4) the study of the phenomena which 

 accompany chemical transformation ; (5) a generalisation of the con- 

 ditions under which reactions occur. As regards stoichiometry, this 

 branch of chemistry has been worked out most thoroughly, and embraces 

 laws (of Dalton, A vogadro- Gerhard t, and others) which bear so deeply 

 on all parts of chemistry that its entire contemporary standing may be 



reason, thermo-chemical data are very complex, and cannot by themselves give the key 

 to many chemical problems, as it was at first supposed they might. They ought to form 

 a part of chemical mechanics, but alone they do not constitute it. 



3 * As chemical reactions are effected by heating, so the heat absorbed by substances 

 before decomposition or change of state, and called ' specific heat,' goes in many cases to the 

 preparation, if it may be so expressed, of reaction, even when the limit of the temperature 

 of reaction is not attained. The molecules of a substance A, which is able to react on a 

 substance B below a temperature t by being heated from a somewhat lower temperature to 

 /, undergoes that change which had to be arrived at for the formation of A B. This 

 idea is often extended ; for instance, it is supposed that a given sul>-tance in its passage 

 from a liquid to a gaseous state gives chemically or materially new, lighter, and simpler 

 molecules (is depolymerised, according to De Haen). 



