846 Dr. S. TV. J. Smith on the 



within the acid like that of: the air buhbles before. A con- 

 siderable quantity of gas accumulated at the top of the 

 apparatus ; but it may have resulted from some chemical 

 reaction in which the benzene took pari. The evolution was 

 even more noticeable when the benzene was replaced by 

 pentane. 



It has already been pointed out that the escape o£ hydrogen 

 at the jet may be impossible although it takes place at S (§ 9). 

 At the latter the effect is cumulative and all the hydrogen 

 is evolved at the same surface. Here minute bubbles can 

 coalesce into larger ones in which the pressure is not greatly 

 above that of the air. At the jet, however, the hydrogen 

 which mercury replaces must first reach a certain concen- 

 tration in every fresh element of the jet and of the solution 

 surrounding it. Then it must overcome the resistance to the 

 formation of a minute bubble before it can escape as gas. 



That direct action between the jet and the acid might 

 result in visible production o£ hydrogen, but for the counter- 

 actions just described, can be seen by allowing the jet to 

 break in the surface of a concentrated solution of sulphate of 

 copper. The surface of the mercury which collects on the 

 bottom of the vessel containing the sulphate presents a 

 tarnished appearance like that produced by the addition of 

 copper. This result, it will be seen, is in close accord with 

 the present point of view and suggests the need for qualifi- 

 cation of the familiar statement : " The more electropositive 



metals, Gu , precipitate the less electropositive metal 



Hg." It is probable that other metals, such as Pb and even 

 Cd, could be precipitated by mercury, from solutions of their 

 salts, in a similar way. 



§ 15. .A kinetic representation of § 3. — The action between 

 an acid and a metal is formulated in § 3 in a way which 

 avoids the necessity of dealing with the kinetics of the 

 process by which equilibrium is attained. The following is 

 perhaps the simplest picture of what actually happens. The 

 metal is assumed to be monovalent, but it is easy to see what 

 change must be made when the valency is n. 



Into the space (the " border layer ") which separates acid 

 and metal, Faraday tubes can stretch from the acid and from 

 the metal respectively. Of these, a possible pair forming 

 side by side will be (1) a tube stretching from the acid 

 nearly across the border layer, with its negative end (an 

 anion of the acid) in the solution and its positive end (an ion 

 of hydrogen) near the metal, (2) a tube stretching from the 

 metal, with its negative end on the metal and its positive 

 end (a metal ion) almost reaching the solution. 



