562 Mr. Carey Lea on a New Method of Determining 



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 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 condition 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 a great number of small black rosettes 

 of herapathite which crystallize out. Solution of sodium 

 sulphate not containing acetic acid gives no such reaction 

 with the herapathite test. It dries up to a pale yellow residue. 



Acids vary very much in their ability to detach sulphuric 

 acid from soda. The following acids, when added to sodium 

 sulphate and tested by the herapathite test, give the results 

 here noted. 



Malic acid, gives an abundant crystallization: 



Succinic acid, acts similarly. 



Lactic acid, a moderate reaction. 



Mucic acid, about the same as lactic. 



Vanadic acid, traces. 



Arsenic acid, abundant crystallization. 



Hippuric acid, distinct traces. 



Salicylic acid, distinct crystallization. 



Of course the stronger organic acids, tartaric, oxalic, and 

 citric, separate sulphuric acid with abundant crystallizations 

 of herapathite when they are made to act on sodium sulphate 



