424 PRINCIPLES OF CHEMISTRY 



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 

 admitted that neither MN nor XY, nor any other substances, are pro- 

 duced, which may for the present 24 be admitted in the case of the 

 double decomposition of salts, for which M and N are metals and X 

 and Y haloids. As the ordinary double decomposition consists exclu- 

 sively in the exchange of metals, the above simplification is applicable 

 in this case. The sum total of the existing data concerning the double 

 decomposition of salts leads to the conclusion that from salts MX + NY 

 there always proceeds a certain quantity of NX and MY, as it should 

 be according to Berthollet's doctrine. A portion of the historical data 

 concerning this subject will be afterwards mentioned, and we will now 

 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 under a condition of sufficiently close contact (which requires 

 time, a finely-divided state, and intimate mixture). Spring took two 

 non-hygroscopic salts, potassium nitrate, KNO 3 , and well-dried sodium 

 acetate, C 2 H 3 NaO 2 , 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 vigorously attracted the moisture 

 of the air owing to the formation of sodium nitrate, NaN0 3 , and 

 potassium acetate, C 2 H 3 K0 2 , both of which are highly hygroscopic. 24& 



When Berthollet enunciated his doctrine the present views of atoms 

 and molecules had yet to be developed, but it is now necessary to sub- 

 mit the examination of the matter to these conceptions, and we will 

 therefore consider the reaction of salts, taking M and N, X and Y as 



24 If M X and N Y represent the molecules of two salts, and if there be no third 

 substance present (such as water in a solution), the formation of X Y would also be pos- 

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

 and with the complex groups which play the part of haloids in salts. 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 contained be different, as in NaCl + H 2 SO 4 , 

 where Cl is a univalent haloid and SO 2 is bivalent, then the matter may be complicated 

 by the formation of other compounds besides MY and NX, and when a solvent partici- 

 pates in the action, and especially if in a large proportion, then the phenomena must 

 evidently become still more complex ; and this is actually the case in reality. Therefore 

 in placing before the reader a certain portion of the existing store of matter concerning 

 the phenomena of double saline decompositions I cannot consider the theory of the sub- 

 ject as complete, and have therefore limited myself to a few data, the completion of which 

 must be looked for, without losing sight of what has been said above, in more detailed 

 works on the subject of theoretical chemistry. 



246 "WTien the mixture of potassium nitrate and sodium acetate was heated by Spring 

 to 100 it was completely fused into one mass, although potassium nitrate fuses at about 

 840, and sodium nitrate at about 320. 



