94 Mr. M. Carey Lea on the 



formation of acid sulphate was always endotherinic, was 

 accompanied by the absorption of 1'05 cal. in the case of 

 NaHS0 4 . He especially remarks that when a very small 

 proportion of acid is added, the tendency is to a complete 

 combination in the form of acid salt *. 



This last-mentioned result is contrary to that which I have 

 been able to observe, and I will therefore mention my re- 

 actions with some particularity. 



I invariably found that when a trace of sulphuric acid was 

 added to the solution of a neutral alkaline sulphate, it reacted 

 as free acid. The matter was examined especially in the 

 following manner. To 40 cub. centim. of distilled water 2 

 drops of sulphuric acid were added; the liquid was divided 

 into two equal parts, in one of which was dissolved a gramme 

 of neutral sodium sulphate. These two liquids were then 

 examined with the test solution, and the detection of the acid 

 was found to be fully as easy in the presence of the large 

 excess of sodium sulphate as in the case of the acid alone. 



It is true that this result is open to the following criticism. 

 Alcohol extracts sulphuric acid from solid acid sodium sul- 

 phate, showing thereby, as Mendeleef remarks, the extremely 

 weak affinity which unites it to the neutral salt. In the 

 present examination the test can only be used in alcoholic 

 solution. It may be said that the alcohol first precipitates 

 the solid acid sulphate, and then abstracts free acid from it. 

 There is force in this objection, but I think it may be 

 answered in the following manner. We first reduce sul- 

 phuric acid nearly to the limit of dilution and quantity at 

 which it can be detected. We then find that the addition of 

 a comparatively large quantity of neutral sulphate makes 

 absolutely no change in the reaction. It seems therefore 

 legitimate to conclude that no combination has taken place, 

 because otherwise it would be necessary to admit that alcohol 

 can abstract all the acid united with the solid neutral 

 sulphate, which is highly improbable. The weight of the 

 evidence seems therefore to favour complete decomposition 

 by solution into neutral salt and free acid. 



When herapathite is dehydrated either by long keeping 

 over oil of vitriol, or by exposure to a temperature of 100° C. 

 for several hours, its colour changes from green to dark 

 brown and its lustre is lost. In this condition it dissolves 

 sparingly in boiling absolute alcohol, and separates by cooling 

 in blackish particles which show little trace of crystallization. 



When the brown substance is placed in a vial with barium 



* Mech. Chim. ii. p. 323. 



