446 PRINCIPLES OF CHEMISTRY 



Thus if the salts MX and NY after reaction partly formed salts MY 

 and NX, then a state of equilibrium is reached and the reaction ceases ; 

 but if one of the resultant compounds, in virtue of its physical properties, 

 passes from the sphere of action of the remaining substances, then the 

 reaction will continue. This exit from the sphere of action depends on 

 the physical properties of the substance and on the conditions under 

 which the reaction takes place. Thus, for instance, the salt NX may, 

 in the case of reaction between solutions, separate as & precipitate, 

 an insoluble substance, while the other three substances remain in solu- 

 tion, or it may pass into vapour, and in this manner also pass away 

 from the sphere of action of the remaining substances. Let us now 

 suppose that it passes away in some form or other from the sphere of 

 action of the remaining substances for instance, that it is transformed 

 into a precipitate or vapour then a fresh reaction will set in and a 

 re-formation of the salt NX. If this be removed, then, although the 

 quantity of the elements N and X in the mass will be diminished, still, 

 according to Berthollet's law, a certain amount of NX should be again 

 formed. When this substance is again formed, then, owing to its 

 physical properties, it will again pass away ; hence the reaction, 

 in consequence of the physical properties of the resultant substances, is 

 able to proceed to completion notwithstanding the possible weakness of 

 the attraction existing between the elements entering into the com- 

 position of the resultant substance NX. Naturally, if the resultant 

 substance is formed of elements having a considerable degree of 

 affinity, then the complete decomposition is considerably facilitated. 



Such a representation of the modus operandi of chemical trans- 

 formations is applicable with great clearness to a number of re- 

 actions studied in chemistry, and, what is especially important, the 

 application of this aspect of Berthollet's teaching does not in any way 

 require the determination of the measure of affinity acting between the 

 substances present. For instance, the action of ammonia on solutions 



small that eveu a large mass would still give no observable displacements. The funda- 

 mental condition for the application of Berthollet's doctrine, as well as Deville's doctrine 

 of dissociation, lies in the reversibility of reactions. There are practically irreversible 

 reactions (for instance, CCl4 + 2H 2 O = CO 2 H-4HCl), just as there are non- volatile sub- 

 stances. But while accepting the doctrine of reversible reactions and retaining the 

 theory of the evaporation of liquids, it is possible to admit the existence of non- volatile 

 substances, and in just the same way of reactions, without any visible conformity to 

 Berthollet's doctrine. This doctrine evidently comes nearer' than the opposite doctrine 

 of Berginann to solving the complex problems of chemical mechanics for the successful 

 solution of which at the present time the most valuable help is to be expected from the 

 working out of data concerning dissociation, the influence of mass, and the equilibrium 

 and velocity of reactions. But it is evident that from this point of view we must not 

 regard a solvent as a non-participant space, but must take into consideration the 

 chemical reactions accompanying solution, or else bring about reactions without solutior 



