<848 On the Action between Metals and Acids. 



§ 16. Summary of Conclusions. — Pure mercury reacts with 

 acid solutions with displacement of hydrogen in the same 

 way as metals like zinc. 



The reaction stops before a perceptible quantity of hydrogen 

 is evolved because a very small quantity of mercury salt in 

 solution is sufficient to cause it to cease. 



The surface film of mercury which has been in contact 

 with the air probably contains more than enough oxygen to 

 oxidise to water all the hydrogen that would be displaced 

 before the direct action ceased. 



The amount of oxygen per sq. cm. of the surface film can 

 be reduced to a very small quantity by allowing the 

 mercury to escape from a containing tube in the form of a 

 narrow jet. 



But for certain counteracting influences due to the fact 

 that the substance displaced is a gas, this jet might be 

 used to obtain hydrogen from acid solutions of sufficient 

 strength. 



By the aid of the jet the direct action between mercury 

 and the acid, with displacement of hydrogen, can be made 

 continuous. The jet, when in direct communication with a 

 mercury surface at rest in the same solution, preA^ents the 

 concentration of the mercury salt, formed by the displace- 

 ment of hydrogen, remaining or becoming large enough at 

 the still surface to stop the evolution of gas. In consequence, 

 hydrogen escapes freely and can be collected and analysed. 



Owing to direct action at its own surface, the jet cannot 

 reduce the concentration of mercury salt round the still 

 surface below a certain limit. Further, the amount of 

 mercury salt which is sufficient to stop the direct action at 

 the still surface diminishes with the concentration of the 

 acid solution used. 



In consequence it will be impossible to decompose the acid 

 by means of the jet when the strength of the solution falls 

 below a certain limit. This limit might be expected to be 

 about the same for different acids. It w T as found to be about 

 the same for hydrochloric and sulphuric acids (roughly 

 6 gram equivalents per litre), although rather lower for the 

 former than for the latter. 



The various reactions which occur when metals are placed 

 in contact with concentrated sulphuric acid are elucidated 

 by the experiments described (§ 12). 



I am very much indebted to Mr. J. S. G. Thomas, B.Sc, 

 for frequent and valued help while performing the experi- 

 ments I have described. 



