M. C. Lea — Relative Affinities of certain Acids. 449 



In the second column these numbers are modified in such 

 manner as to cause them to justly represent the comparative 

 affinity of the acid. With monobasic acids the number of 

 molecules is divided by 2, with quadrobasic acids it is multi- 

 plied by 2. For a tribasic acid it is multiplied by §. Bibasic 

 acids only, remain unchanged. Next, unity is subtracted be- 

 cause it is always the excess of the salt which must be present 

 in order to keep the sulphuric acid saturated with base that 

 gives the measure of the affinity of the acid. Were this cor- 

 rection not applied the entire result would be vitiated. 



The third column gives the numbers as they appear when 

 hydrochloric acid is taken as 100. 



Instead of adding salts of different acids to sulphuric acid, 

 we may add various acids to a salt formed by the union of 

 sulphuric acid to a strong base, for example, to sodium sul- 

 phate. 



Sulphuric acid is now recognized as being a weaker acid 

 than hydrochloric, and yet we have seen that it is able to de- 

 tach a certain quantity of base from a chloride. Further, that 

 if the chloride is present in sufficient excess, the sulphuric 

 acid may take up enough base to completely saturate itself. 

 The general fact that a certain quantity of an acid may be 

 expelled from a salt by another acid, even much weaker than 

 the first, has been shown by the researches of Thomsen and 

 of Ostwald. So that if for example we add acetic acid to a 

 solution of sodium sulphate a distinctly recognizable quantity 

 of sulphate is decomposed and converted into acetate. A con- 

 dition of equilibrium is produced in which the liquid contains 

 both acids in a free state and both salts. In some way that we 

 do not yet understand the presence of the free acid maintains 

 the combined acid in its combination. The sodium acetate 

 exists only by virtue of the free acetic acid present. 



The existence of this state of equilibrium was first proved 

 by Thomsen who deduced it from the thermochemical changes 

 which took place on mixing the solutions. Ostwald reached 

 similar conclusions by making accurate determinations of the 

 changes of volume and consequently of specific gravity which 

 resulted from the mixing of the solutions, and in other ways. 



In both these cases the conclusions are reached by logical 

 deductions from the phenomena observed. But with the aid 

 of the herapathite test the expulsion of sulphuric acid by a 

 very much weaker acid can be rendered immediately evident 

 to the eye. Thus if to the solution of sodium sulphate we 

 add acetic acid and place two or three drops of the mixture in 

 a warm porcelain basin and add some of the test liquid to it, 

 in a few minutes we have great number of small black rosettes 



