Nature of certain Solutions. 91 



1. Sulphates of the Type W 2 , S0 4 , or B /; S0 4 



I find that these salts dissolve in water and exist in solution 

 as such and absolutely without separation of sulphuric acid 

 even in those cases in which the solution is acid to litmus. 

 The substances examined were the alkaline sulphates and 

 the sulphates of Mg, Zn, Od, Cu, Ni, Co, Mn, Tl; also the 

 sulphates of morphia, quinia, strychnia, and brucia. To this 

 series of reactions there is a single exception of an interesting 

 character. 



A solution of ferrous sulphate invariably contains free 

 acid, no matter how often recrystallized or under what cir- 

 cumstances prepared. A small flask was filled with freshly 

 distilled water and was well boiled, corked, and set aside to 

 cool. In this ferrous sulphate was dissolved and potash was 

 added in quantity sufficient to precipitate a considerable pro- 

 portion of oxide. Even this solution gave the reaction in- 

 dicating the presence of free acid. So, too, the double salts 

 of ferrous oxide with ammonia and with magnesia, their 

 solutions always contain free acid, no matter how often they 

 may be recrystallized or purified by precipitation of their 

 solutions in water by alcohol. This exceptional behaviour 

 probably results from the great tendency of ferrous solutions 

 to rapid absorption of oxygen from the air, and, as will pre- 

 sently appear, sesquisulphates are dissociated in solution. 



With this one exception, the heavy metallic sulphates above 

 mentioned dissolve in water without decomposition, although 

 their solutions redden litmus. The alkaloids mentioned yield 

 sulphates, which, after suitable purification, are perfectly 

 neutral to litmus. 



To explain this contrast it is necessary to remember that a 

 salt reddens litmus whenever the affinity of its acid for the 

 potash in litmus is not held in check by two affinities which 

 oppose it, that of the base for its acid, and that of the litmus 

 dye for its potash. If the base is sufficiently weak, the affinity 

 of its acid for potash preponderates*. The whole question 

 therefore reduces itself to that of the strength of the base 

 with which the acid is united ; and it consequently follows 

 irresistibly that even very weak alkaloids are stronger bases 

 than such metallic oxides as those of zinc, copper, cobalt, &c. 



Although this deduction seems clear, it may be supported 



* It is of interest to observe that the reaction may be changed by the 

 presence of an additional substance, although the latter may be quite 

 neutral. Thus mercuric chloride is faintly acid to litmus, but not after 

 the addition of potassium chloride. The tendency to form a double salt 

 changes the balance of affinities. 



