Action between Metals and Acids. 841 



hydrogen at S is exactly equivalent to the rate o£ removal of 

 mercury from solution at S by J. For every equivalent of 

 acid that disappears at S 5 an equivalent of the mercury salt 

 will appear in solution at J. 



§ 10. Results, — In the above way it is possible to conceive 

 that mercury will decompose sulphuric or hydrochloric acid 

 with evolution of hydrogen. Experiment justifies the con- 

 ception, for, if either acid (of sufficient strength) is used in 

 the vessel of fig. 1, a stream of hydrogen is evolved at S. 



If the point of the capillary (at J) is below the surface of 

 the acid the effect occurs most rapidly when the head of 

 mercury just suffices to produce a short, approximately 

 cylindrical, jet at J. (The ratio l/v for the jet has then a 

 minimum value because, with increase of head, the jet length 

 increases more rapidly than the velocity.) 



The effect occurred with the strongest sulphuric acid avail- 

 able ('pure redistilled'' s. G. about 1*84). If the acid is 

 diluted the same evolution of gas can be obtained, but, when 

 the density of the acid falls below about 1*25 the rate of 

 evolution is very slow and occurs only when the jet breaks in 

 the surface of the solution. Finally, when the density of 

 the acid is below about 1*19, there is no perceptible evolution 

 of gas in any position of the jet*. 



The experience with hydrochloric acid is similar. With 

 the most concentrated acid used (s. G. about 1'16) there was 

 a fairly rapid evolution of gas when the jet broke in the 

 surface. Gas ceased to come off when the density of the acid 

 fell below about 1*09. The molecular concentrations of these 

 limiting solutions are about the same — roughly 6 equivalent 

 gram mols. per litre — although that of the HC1 solution is 

 slightly the smaller f. (Cf. § 13 below.) 



§ 11. Proof of Evolution of Hydrogen. — The gas was not 

 proved to be hydrogen by a direct test in every case ; but 

 only in the case most liable to suspicion, viz. when concen- 

 trated sulphuric acid was employed ; and in one other, viz. 

 when equal volumes of this acid and water were mixed. It 

 was proved to be hydrogen in the following way : — Collected 

 in a small tube over the concentrated acid, it did not dissolve 

 appreciably in the latter, nor, subsequently, in recently boiled 

 distilled water by which the acid was displaced. It was 

 therefore neither H 2 S nor S0 2 . 



It was difficult to test the gas positively since with the 



* In practice it is easier to adjust J and S if separate columns of 

 mercury connected by a wire are used instead of the apparatus of fig. 1. 



t Mr. J. S. G. Thomas has since made a more exact determination 

 of the limiting- concentrations. He finds them to be 6"25 gram equi- 

 valents per litre for H 2 SOj and 5-75 gram equivalents per litre for HC1. 



