Action between Metals and Acids. 845 



experiments of § 10 show that this limit is reached at a con- 

 centration of about 6 equivalents per litre in the case of 

 each acid. 



Further, assuming that equally concentrated solutions of 

 the acids and of their corresponding mercury salts are 

 approximately equally dissociated, one would expect in virtue 

 of equation (i.) that the limiting concentrations of the two 

 acids would be approximately equal. This also agrees with 

 experiment. 



It happens, however, that the quantity y in (i. a) is greater 

 for HC1 than for H 2 S0 4 . It might therefore be anticipated 

 that if the corresponding dissociations were exactly equal, 

 the limiting concentration for HC1 would be rather less than 

 for H 2 S0 4 ; but although the results suggest the fulfilment 

 of this anticipation, the data are not sufficiently accurate to 

 make it worth while to attempt a quantitative proof — 

 especially as there are other possible explanations. 



§ 14. Possible effects at the Jet. — The explanation which 

 has been given of the behaviour of the still mercury surface 

 S supposes that the concentration of mercury salt round the 

 jet J is small. In fact the behaviour of S is (by hypothesis) 

 controlled by the rate of independent formation of mercury 

 salt at J. Consequently when hydrogen is evolved at S the 

 amount of mercury in solution round J should be insufficient 

 to prevent the evolution of hydrogen at J. 



If any such evolution takes place it is very difficult to 

 detect. Mercury was allowed to run for a long time from a 

 capillary tube fused into the top of a glass bulb which was 

 filled with concentrated sulphuric acid and terminated below 

 in a tube dipping into mercury. No trace of gas could be 

 seen. The level of the acid was lowered until the jet broke 

 in the surface, the space above being filled with air. In this 

 case, the first impression is that there is a copious evolution 

 of gas (cf. Paschen, Wied. Ann. vol. xli. p. 50, 1890). But 

 this is an illusion. The greater part, if not all, of the 

 gas which appears to form at the surface of the mercury 

 drops is air dragged in from above. This effect is very pro- 

 nounced in liquids of great viscosity. It is less conspicuous 

 in sulphuric acid than it is in glycerine. 



In consequence of this phenomenon it is impossible to tell 

 by inspection whether any gas is evolved as the result of 

 chemical action between the acid and the mercury. In a 

 further experiment benzene (which had previously been 

 shaken up with another sample of concentrated H 2 S0 4 and 

 then decanted) was substituted for the air above the acid. 

 There was now a continuous circulation of drops of benzene 



