484 PRINCIPLES OF CHEMISTRY 



the double decomposition of salts in which M and N 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 4- 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 Mata. 

 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, KNO 3 , and well-dried sodium 

 acetate, C 2 H 3 Na0 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 rapidly attracted the 

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

 and potassium acetate, C 2 H 3 KO2, both of which are highly hygro- 

 scopic. 24 Ws 



When Berthollet enunciated his doctrine the present views of atoms 

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

 mit 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, JCa or |Mg (bivalent elements) replace hydrogen. 



And since, according to Berthollet's doctrine, when mMX of one 

 salt comes into contact with nNY of another salt, a certain quantity 

 rcMY and ceNX 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 rcMY + nNX + (m w)MX that is, a portion of one 

 only of the salts taken would remain unchanged because the reaction 

 could only proceed between rcMX and wNY If x were actually equal 



when a solvent participates in the action, and especially if present in large proportion, the 

 phenomena must evidently become still more complex ; and this is actually the case in 

 nature. Hence while placing before -the reader a certain portion of the existing store 

 of knowledge concerning the phenomena of double saline decompositions, I cannot con- 

 sider the theory of the subject as complete, and have therefore limited myself to a few 

 data, the completion of which must be sought in more detailed works on the subject of 

 theoretical chemistry, without losing sight of what has been said above. 



ai bis When 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 

 340 and sodium nitrate at about 820 



