30 PRINCIPLES OF CHEMISTUY 



As the chemical changes to which substances are liable proceed 

 from internal forces proper to these substances, as chemical phenomena 

 certainly consist of movements of material parts (from the laws of the 

 indestructibility of matter and of elements), and as the investigation 

 of mechanical and physical phenomena proves the law of the indestruc- 

 tibility of forces, or the conservation of energy that is, the possibility 

 of the transformation of one kind of movement into another (of visible 

 or mechanical into invisible or physical) we are inevitably obliged to 

 acknowledge the presence in substances (and especially in. the elements 

 of which all others are composed) of a store of chemical energy or in- 

 visible movement inducing them to enter into combinations. If heat be 

 evolved in a reaction, it means that a portion of chemical energy is 

 transformed into heat ; 32 if heat be absorbed in a reaction, 33 that it is 



32 The theory of heat gave the idea of a store of internal movement or energy, and 

 therefore with it, it became necessary to acknowledge chemical energy, but there is no 

 foundation whatever for identifying heat energy with chemical energy. It may be sup- 

 posed, but not positively affirmed, that heat movement is proper to molecules and 

 chemical movements to atoms, but that as molecules are made up of atoms, the movement 

 of the one passes to the other, and that for this reason heat strongly influences reaction 

 and appears or disappears (is absorbed) in reactions. These relations, which are, 

 apparent and hardly subject to doubt on general lines, still present much that is doubtful 

 in detail, because all forms of molecular and atomic movement are able to pass into 

 each other. On broad general lines it must be acknowledged that as mechanical energy 

 can entirely pass into heat energy (but the reverse transition is accomplished only 

 partially, according to the second law of heat), so also heat energy may pass into 

 chemical energy, but it is doubtful, and even unlikely, that chemical energy passes 

 altogether into heat energy. Therefore, the heat evolved in chemical reactions cannot 

 serve as the total measure of chemical energy, more especially as there are a number of 

 reactions of combination in which heat is absorbed ; for instance, the combination of 

 charcoal with sulphur is accompanied by an absorption of heat probably because the 

 molecules of charcoal are complex, and those of carbon bisulphide less so, and the break- 

 ing up of the complex molecules of charcoal requires a large absorption of heat (whose 

 measure we do not know) and whilst the combination of charcoal with sulphur is accom- 

 panied by an evolution of heat, yet we only observe the difference of these two heat 

 effects. 



33 The reactions which take place (at the ordinary or at a high temperature) directly 

 between substances may be clearly divided into exothermal, which are accompanied by 

 an evolution of heat, and endothermal, which are accompanied by an absorption of heat. 

 It is evident that the latter require a source of heat. They are determined either by the 

 directly surrounding medium (as in the formation of carbon bisulphide from charcoal and 

 sulphur, or in decompositions which take place at high temperatures), or else by a 

 simultaneously proceeding secondary reaction. So, for instance, hydrogen sulphide is 

 decomposed by iodine in the presence of water at the expense of the heat which is 

 evolved by the solution in water of the hydrogen iodide produced. This is the reason why 

 this reaction, as exothermal, only takes place in the presence of water ; otherwise it would 

 be accompanied by a cooling effect. As in the combination of dissimilar substances, the 

 bonds existing between the molecules and atoms of the homogeneous substances have to 

 be broken asunder, whilst in reactions of rearrangement the formation of any one sub- 

 stance proceeds parallel with the formation of another, and, as in reactions, a series of 

 physical and mechanical changes take place, it is impossible to separate the heat directly 

 depending on a given reaction from the total sum of the observed heat effect. For this 



