degrees of attraction existing between the elements M, N, X, and Y; but however great their mass may be, and however considerable 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 present27 be admitted in the case of the double decomposition of salts in which M and X are metals and X and Y haloids. As the ordinary double decomposition here consists merely in the exchange of metals, the above simplification is applicable. The sum total of existing data concerning the double decomposition of salts leads to the conclusion that from salts MX + NY there always arises a certain quantity of NX and MY, as should be the case according to Berthollet's doctrine. A portion of the historical data concerning this subject will be afterwards mentioned, but we will at once proceed to point out the observations made by Spring (1888) which show that even in a solid state salts are subject to a similar interchange of metals if in a condition of sufficiently close contact (it requires time, a finely divided state, and intimate mixture). Spring took two non-hygroscopic salts, potassium nitrate, KNO3, and well-dried sodium acetate, C2H3NaO2, and left a mixture of their powders for several months in a desiccator. An interchange of metals took place, as was seen from the fact that the resultant mass rapidly attracted the moisture of the air, owing to the formation of sodium nitrate, NaNO3, and potassium acetate, C2H3KO2, both of which are highly hygroscopic.28
When Berthollet enunciated his doctrine the present views of atoms and molecules had yet to be developed, and it is now necessary to submit the matter to examination in the light of these conceptions; we will therefore consider the reaction of salts, taking M and N, X and Y as equivalent to each other—that is, as capable of replacing each other ‘in toto,’ as Na or K,, ½Ca or ½Mg (bivalent elements) replace hydrogen.
And since, according to Berthollet's doctrine, when m MX of one salt comes into contact with n NY of another salt, a certain quantity x MY and x NX is formed, there remains m - x of the salt MX, and n - x of the salt NY. If m be greater than n , then the maximum interchange could lead to x = n , whilst from the salts taken there would be formed n MY + n NX + ( m - n )MX—that is, a portion of one only of the salts taken would remain unchanged because the reaction could only proceed between n MX and n NY. If x were actually equal to n , the mass of the salt MX would not have any influence on the modus operandi of the reaction, which is equally in accordance with the teaching of Bergmann, who supposed double reactions to be independent of the mass and determined by affinity only. If M had more affinity for X than for Y, and N more affinity for Y than for X, then according to Bergmann there would be no decomposition whatever, and x would equal 0. If the affinity of M for Y and of N for X were greater than those in the original grouping, then the affinity of M for X and of N for Y would be overcome, and, according to Bergmann's doctrine, complete interchange would take place— i.e. x would equal n . According to Berthollet's teaching, a distribution of M and N between X and Y will take place in every case, not only in proportion to the degrees of affinity, but also in proportion to the masses, so that with a small affinity and a large mass the same action can be produced as with a large affinity and a small mass. Therefore, (1) x will always be less than n and their ratio x / n less than unity—that is, the decomposition will be expressed by the equation, m MX + n NY = ( m - x )MX + ( n - x )NY + x MY + x NX; (2) by increasing the mass m we increase the decomposition—that is, we increase x and the ratio