enter into double decomposition with them, such reactions taking place in solutions in reality present very complex cases. In this sense the reaction between alcohols and acids is much more simple, and therefore its significance in confirmation of Berthollet's doctrine is of particular importance. The only cases which can be compared with these reactions for simplicity are those exchange decompositions investigated by G. G. Gustavson, which take place between CCl 4 and RBr n on the one hand, and CBr 4 and RCl n on the other. This case is convenient for investigation inasmuch as the RCl n and RBr n taken (such as BCl 3 , SiCl 4 , TiCl 4 , POCl 3 , and SnCl 4 ) belong to those substances which are decomposed by water, whilst CCl 4 and CBr 4 are not decomposed by water; and therefore, by heating, for instance, a mixture of CCl 4 + SiBr 4 it is possible to arrive at a conclusion as to the amount of interchange by treating the product with water, which decomposes the SiBr 4 left unchanged and the SiCl 4 formed by the exchange, and therefore by determining the composition of the product acted on by the water it is possible to form a conclusion as to the amount of decomposition. The mixture was always formed with equivalent quantities—for instance, 4BCl 3 + 3CBr 4 . It appeared that there was no exchange whatever on simple intermixture, but that it proceeded slowly, when the mixture was heated (for example, with the mixture above mentioned at 123° 4·86 per cent. of Cl was replaced by Br after 14 days' heating, and 6·83 per cent. after 28 days, and 10·12 per cent. when heated at 150° for 60 days). A limit was always reached which corresponded with that of the complemental system; in the given instance the system 4BBr 3 + 3CCl 4 . In this last 89·97 per cent. of bromine in the BBr 3 was replaced by chlorine; that is, there were obtained 89·97 molecules of BCl 3 and there remained 10·02 molecules of BBr 3 , and therefore the same state of equilibrium was reached as that given by the system 4BCl 3 + 3CBr 4 . Both systems gave one and the same state of equilibrium at the limit, which is in agreement with Berthollet's doctrine.
Thus we now find ample confirmation from various quarters for the following rules of Berthollet, applying them to double saline decompositions: 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 is determined both by the measure of the active affinities and by the relative masses of the substances as 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.34
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, an insoluble substance, while the other three substances remain in solution, 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