I NTRODUCTION 33 



in contact with hydrogen at the same temperature, is able to produce 

 iron and water. It is evident that if two substances, A and B, give 

 two others C and D, and the reaction be reversible, then C and D will 

 form A and B, and, consequently, by taking a definite mass of A, 

 and B, or a corresponding mass of C and D, we shall obtain, in each 

 case, all four substances that is to say, there will be a state of chemical 

 equilibrium between the reacting substances. By increasing the mass 

 of one of the substances we obtain a new condition of equilibrium, so 

 that reversible reactions present a means of studying the influence of 

 mass on the imnJnx operand* of chemical changes. Many of those 

 reactions which occur with very complicated compounds or mixtures 

 may serve as examples of non-reversible reactions. Thus many of the 

 compound substances of animal and vegetable organisms are broken 

 up by heat, but cannot be re-formed from their products of decomposi- 

 tion at any temperature. Gunpowder, as a mixture of sulphur, nitre, 

 and carbon, on burning, forms gases from which the original substances 

 cannot be re-formed at any temperature. In order to obtain them, re- 

 course must be had to an indirect method of combination at the moment 

 of separation. If A does not under any circumstances combine directly 

 with B, it does not imply that it cannot give a compound A B. For 

 A can often combine with C and B with D, and if C has a great 

 affinity for D, then the reaction of A C on B D produces not only C D, 

 but also A B. As on the formation of C D, the substances A and B 

 (previously in A C and B D) are left in a peculiar state of separation, 

 it is supposed that their mutual combination occurs because they meet 

 together in this nascent state at the moment of separation (in statu 

 nascendi). Thus chlorine does not directly combine with charcoal, 

 graphite, or the diamond, nevertheless there are compounds of chlorine 

 with carbon and many of them are distinguished by their stability. 

 They are obtained during the action of chlorine on hydrocarbons, as 

 the separation products from the direct action of chlorine on hydrogen. 

 Chlorine takes up the hydrogen, and the freed carbon at the moment 

 of its separation enters into combination with another portion of the 

 chlorine, so that in the end the chlorine is combined with both the 

 hydrogen and the carbon. 35 



""' Itis possible to imagine that the cause of a great many of such reactions is, that sub- 

 stances taken in a separate state, for instance, charcoal, present a complex molecule 

 composed of separate atoms of carbon which are fastened together (united, as is usually 

 said) by a considerably affinity ; for atoms of the same kind, just like atoms of different 

 kinds, possess a mutual affinity. The affinity of chlorine for carbon, although unable 

 to break this bond asunder, may be sufficient to form a stable compound with already 

 separate atoms of carbon. Such a view of the subject presents a hypothesis which, 

 although dominant at present, is without sufficiently firm foundation. Were the matter 



VOL. I. D 



