SECTION PHYSICS (including Astronomy). 



May 24/7*, 1876. 



The PRESIDENT : The first paper on our list to-day is that by Pro- 

 fessor Clerk-Maxwell, on the Equilibrium of Heterogeneous Bodies. 

 He is here now to speak for himself, and I beg to call upon him to 

 give us his communication. 



ON THE EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



Professor J. CLERK-MAXWELL, M.A., F.R.S. : The warning which 

 Cointe addressed to his disciples, not to apply dynamical or physical 

 ideas to chemical phenomena, may be taken, like several other warn- 

 ings of his, as an indication of the direction in which science was 

 threatening to advance. 



We can already distinguish two lines along which dynamical science 

 is working its way to undermine at least the outworks of Chemistry, 

 and the chemists of the present day, instead of upholding the mystery 

 of their craft, are doing all they can to open their gates to the enemy. 



Of these two lines of advance one is conducted by the help of the 

 hypothesis that bodies consist of molecules in motion, and it seeks to 

 determine the structure of the molecules and the nature of their motion 

 from the phenomena of portions of matter of sensible size. 



The other line of advance, that of Thermodynamics, makes no 

 hypothesis about the ultimate structure of bodies, but deduces relations 

 among observed phenomena by means of two general principles the 

 conservation of energy and its tendency towards diffusion. The ther- 

 rnodynamical problem of the equilibrium of heterogeneous substances 

 was attacked by Kirchhoff in 1855, when the science was yet in its in- 



