so PI I'M CHLORIDE BERTHOLLET'S LAWS 437 



Thus we now find ample confirmation from various quarters for the 

 following rules of Berthollet, on referring them to double saline decom- 

 positions : 1. From two salts MX and NY containing different haloids 

 and metals there result from their reaction two others, MY and NX, 

 but such a substitution will not proceed to the end unless one product 

 passes from the sphere of action. 2. This reaction is limited by the 

 existence of an equilibrium between MX, NY, MY, and NX, because a 

 reverse reaction is quite as possible as the direct reaction. 3. This limit 

 i* letermined by both the measure of the active affinities and by the rela- 

 tive masses of the substances measured by the number of the reacting 

 molecules. 4. Other conditions being constant, the chemical action is 

 proportional to the product of the chemical masses in action. 2!) 



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 a precipitate, as 

 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 



carbon, and there is an exchange of atoms between different molecules in a homogeneous 

 medium ditto. This hypothesis may, in my opinion, explain certain phenomenaof disso- 

 ciation, but in mentioning it I do not consider it possible to linger on it. I will only 

 observe that a similar hypothesis suggested itself to me in my researches on solutions, 

 and that Pfaundler essentially enunciated a similar hypothesis, and in recent times a 

 like view is beginning to diffuse itself with respect to the electrolysis of saline solutions. 

 29 Berthollet's doctrine can hardly be in any way undermined when it is shown that 

 there are cases in which there is no decomposition between salts, because in principle 

 the affinity may be so small that a large mass would still give no observable displacements. 

 The fundamental condition of the adaptability of Berthollet's doctrine, as well as 

 Deville's doctrine, of dissociation lies in the reversibility of reactions. As there are prac- 

 tically unreversible reactions (for instance, CC1 4 + 2H.,O = CO., + 4HC1), and as non- volatile 

 substances do exist, so whilst accepting the doctrine of reversible reactions and retaining 

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

 volatile substances, and in just the same way of reaction without any visible conformity 

 to Berthollet's doctrine. This doctrine evidently approaches nearer than the opposite 

 doctrine of Bergman to the solution of the complex problems of chemical mechanics, 

 for the successful resolution of which at the present time the richest fruits are to be 

 expected from the working out of data concerning dissociation, the influence of mass, 

 and the equilibrium and velocity of reactions. 



