INTRODUCTION 35 



by reason of the difference of masses, the composition of the sub- 

 stances concerned, the distribution of their parts, and their affinity or 

 chemical energy, but also by reason of the conditions under which the 

 substances occur, and these conditions differ for every particular reac- 

 tion. In order that a certain chemical reaction may take place between 

 substances which are capable of reacting on each other, it is often 

 necessary to have recourse to conditions which are sometimes very 

 different from those in which the substances usually occur in nature. 

 For example, not only is the presence of air (oxygen) necessary for the 

 combustion of charcoal, but the latter must also be heated to redness. 

 The red-hot portion of the charcoal burns i.e., combines with the 

 oxygen of the atmosphere and in doing so evolves heat, which heats 

 the adjacent parts of charcoal, which are thus able to burn. Just as 

 the combustion of charcoal is dependent on its being heated to red- 

 ness, so also every chemical reaction only takes place under certain 

 physical, mechanical, or other conditions. The following are the 

 chief conditions which exert an influence on the progress of chemical 

 reactions. 



(a) Temperature. Chemical reactions of combination only take 

 place within certain definite limits of temperature, and cannot be 

 accomplished outside these limits. As examples we may cite, not only 

 that the combustion of charcoal begins at a red heat, but also that 

 chlorine and salt only combine with water at a temperature below 0. 

 These compounds cannot be formed at a higher temperature, for they 

 are then wholly or partially broken up into their component parts. 

 A certain rise in temperature is necessary to start, combustion. In 

 certain cases the effect of this rise may be explained as causing one 

 of the reacting bodies to change from a solid into a liquid or gaseous 

 form. The transference into a fluid form facilitates the progress of 

 the reaction, because it aids the intimate contact of the particles acting 

 on each other. Another reason, to which must be ascribed the chief 

 influence of heat in exciting chemical action, is that the physical cohe- 

 sion, or the internal chemical union, of homogeneous particles is thereby 

 weakened, and therefore the separation of the particles of the sub- 

 stances taken, and their transference into new compounds, is rendered 

 easier. When a reaction absorbs heat as in decomposition, where the 

 heat is transformed into latent chemical energy the reason why heat 

 is necessary is self-evident. 



It is most important to observe the effect of an elevation of tem- 

 perature on all compounds, as there is reason to believe that they are 

 all decomposed at a more or less high temperature. We have already 

 seen examples of this in describing the decomposition of mercury oxide 



D 2 



