Action between Metals and Acids. 835 



rest of the system. The familiar parallel is the fact that a 

 gas will always expand spontaneously from higher pressure 

 to lower if the external constraints permit — the work which 

 it does being performed at the expense, if necessary, of its 

 own internal energy. 



It would appear, therefore, that every metal must be able 

 to displace to some extent the hydrogen of an acid solution. 

 If in any case the action is imperceptible, it must be because 

 the tendency of the metal to enter solution is very small. 

 The entry of an insignificant amount is then sufficient to 

 balance the tendency. After this, further solution of the 

 metal with escape of hydrogen will be a process like the 

 compressing of one quantity of gas by the expansion of 

 another — the constraints of the two quantities of gas being- 

 such that mutual expansion and contraction is the only 

 change possible. 



In such a case equilibrium is reached when the work which 

 could be done by any further expansion of the second gas 

 would be less than the work required to increase the com- 

 pression of the first. Similarly, the replacement of hydrogen 

 by a metal will cease at a point defined by the condition 

 that further escape of hydrogen would produce less available 

 work than would be required to cause the equivalent quantity 

 of metal to enter solution. 



§ 4. Symbolic expression of the argument of § 3. — 

 Assuming as a rough approximation that the process is 

 reversible and takes place isothermally, the conception may 

 be expressed symbolically as follows. 



The work done (diminution of available energy) when one 

 equivalent weight of hydrogen escapes from solution may be 



written in the form /x* — /i , in which fi h is a function of 

 the strength of the solution (increasing with the concentra- 

 tion), and \i h depends upon the pressure at which the 



hydrogen escapes (increasing with the pressure), 



Similarly the work done (gain of available energy) when 

 an equivalent of the metal enters solution may be expressed 



as a — a , in which ll is a function of the amount of 



metal already dissolved per ccm. near the interface, and 



fi n is a physical constant of M (at the temperature of the 



interaction and for a given curvature of surface if the metal 

 is fluid), which may be large or small according to the nature 

 of the metal. 



