SODIUM CHLORIDE BERTHOLLKT s LAWS 423 



hydrochloric acid, and the residue will be entirely free from them. Hence 

 in the above reaction, non- volatile or sparingly- volatile table salt and 

 sparingly-volatile sulphuric acid are taken, and as the result of their re- 

 action, after the hydrogen and sodium have exchanged places, there is 

 obtained non-volatile sodium sulphate and gaseous hydrochloric acid. 

 The fact of the latter being a gaseous substance forms the main reason 

 for the reaction proceeding to the very end. The mechanism of this 

 kind of double decomposition, and the cause of the course of the reac- 

 tion, are exactly the same as those we saw in the decomposition of 

 nitre (Chapter VI.) by the action of sulphuric acid. The sulphuric acid 

 in each displaces the other, volatile, acid. 



Not only in these two, but in every instance, if a volatile acid can 

 be formed through the substitution for a metal of the hydrogen of sul- 

 phuric acid, then this volatile acid will be formed. From this it may be 

 concluded that the volatility of the acid should be considered as the 

 cause of the progress of the reaction ; and, indeed, if the acid be soluble 

 but not volatile, or if the reaction takes place in an enclosed space 

 where the resulting acid cannot volatilise, or at the ordinary tempera- 

 ture when it does not pass into an elastic state of vapour then the 

 decomposition does not proceed to the end, but only up to a certain 

 limit. In this respect the explanations given at the beginning of this 

 century by the French chemist Berthollet in his work ' Essai de Statique 

 Chimique,' are very important. The doctrine of Berthollet starts from 

 the supposition that the chemical reaction of substances is accomplished 

 in consequence not only of the measure of affinity between the different 

 parts, but also under the influence of the relative masses of the reacting 

 substances and of those physical conditions under which the reaction 

 takes place. Two substances containing the elements MX and NY, 

 being brought into mutual contact, form by double decomposition the 

 compounds MY and NX, but the formation of these two new compounds 

 will not proceed to the end unless one of the resulting substances is 

 removed from the sphere of action. But it can only be removed if it 

 possesses different physical properties from those of the other substances 

 which are present together with it. Either it will be a gas while the 

 others are liquid or solid, or it will be an insoluble solid while the others 

 are liquid or soluble. The relative amounts of the resultant substance 

 depend, if nothing be separated out from their mutual contact, only on 

 the relative quantities of the substances MX and NY, and upon the 

 measure of attraction existing between the elements M, N, X, and Y ; 

 but however great their mass may be, and however considerable the 

 attraction, still in any case, if nothing be separated out from the sphere 

 of action, the decomposition will cease, a state of equilibrium will be 



