204 SCIENCE PROGRESS 



as quite definitely settled, until, after the lapse of a number 

 of years, it is once more reopened in a new and acute 

 form. 



Twenty years ago P. Duhem ( i ), in his now classic re- 

 searches on the thermodynamic treatment of chemical changes, 

 had come to the conclusion that a false equilibrium could be 

 realised in place of a true one, owing to the existence of 

 viscous forces, which, by delaying the process, gave to the 

 meta-stable state a degree of permanence which had much 

 in common with the true equilibrium state. The distinction 

 between this false equilibrium and the true lay in the fact 

 that, on allowing the reverse process to occur, the position 

 finally reached did not coincide with that attained in the 

 direct process. The process was, in fact, thermodynamically 

 irreversible. 



This theoretical conclusion was apparently justified by a 

 number of experimental results obtained by Pelabon (2), who 

 investigated the reaction between hydrogen and selenium, and 

 that between hydrogen and sulphur. It is essential to observe 

 that in these reactions we are dealing with a solid or fused 

 substance which interacts with a gas, i.e. the reaction is 

 heterogeneous. Pelabon found that above 320 the true 

 equilibrium was attained, but that at lower temperatures 

 different end-points were reached according as to whether 

 he started with the system hydrogen + selenium or with the 

 system hydrogen selenide. Pelabon 's results were adversely 

 criticised, however, by Bodenstein (3), who reinvestigated the 

 same reactions and came to the conclusion that in all cases a 

 true equilibrium point was reached. Bodenstein regarded 

 the limits of formation of hydrogen selenide or hydrogen 

 sulphide found by Pelabon as simply arbitrarily selected 

 points from a reaction which was still going on. Duhem (4), 

 in reply to Bodenstein, pointed out that the actual limit of the 

 reaction depended upon the mass of selenium or sulphur 

 employed, the reaction being practically complete if sufficient 

 fused or solid material were present, and apparently in Boden- 

 stein 's experiments much more sulphur was used than in 

 those of Pelabon. Bodenstein (5), as a matter of fact, found 

 that the presence of excess of selenium accelerated the re- 

 action and permitted of a true equilibrium point being reached. 

 The whole controversy has been very generally regarded as 



