SODIUM CHLORIDE-^BERTHQLLET'S LAWS 433 



Not only in these two instances, but in every instance, if a volatHe acid 

 can he formed by the substitution of the hydrogen of sulphuric acid for 

 a metal, 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 take place in an enclosed space 

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

 ture when it does not pass into the state of elastic 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 determined 

 not only by the degrees of affinity between the different parts, but also 

 by the relative masses of the reacting substances and by those physical 

 conditions under which the reaction takes place. Two substances 

 containing the elements MX and NY, being brought into contact 

 with each other, 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 them 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 with it. Either it 

 must be a gas while the others are liquid or solid, or an insoluble solid 

 while the others are liquid or soluble. The relative amounts of the 

 resultant substances, if nothing separates out from their intermixture, 

 depend only on the relative quantities of the substances MX and NY, 

 and upon the degrees of attraction existing between the elements M, 

 N, X, and Y ; but however great their mass may be, and however con- 

 siderable the attractions, still in any case if nothing separates out from 

 the sphere of action the decomposition will presently cease, a state of. 

 equilibrium will be established, and instead of two there will remain four 

 substances in the mass : namely, a portion of the original bodies MX and 

 NY, and a certain quantity of the newly formed substances MY and NX, 

 if it be assumed that neither MN or XY nor any other substances 

 are produced, and this may for the present 24 be admitted in the case of 



* 



24 If MX and NY represent the molecules of two salts, and if there be ;K> third 

 substance present (such as water in a solution), the formation of XY would also be 

 possible ; for instance, cyanogen, iodine, &c. are capable of combining with simple haloids, 

 as well as with the complex groups which in certain salts play the part of haloids. Besides 

 which the salts MX and NY or MY with NX may form double salts. If the number of 

 molecules be unequal, or if the valency of the elements or groups contained in them be 

 different, as in NaCl + H 2 S0 4 , where Cl is a univalent haloid and SO 4 is bivalent, then the 

 matter may be complicated by the formation of other compounds besides MY and NX, and 



