46 THE POPULAR SCIENCE MONTHLY 



Dalton's law as implying that " every gas is as a vacuum to every 

 other gas/' 110 his anticipation of van't Hoff s equation in the form of 

 Henry's law for dilute solutions of gases in liquids 111 and his genial 

 discussion of gas-mixtures, known in Germany as, 



The Paradox of Gioos. 112 — If two different gases which can be 

 separated reversibly by quicklime or other process are allowed to mix, 

 a certain definite amount of work or available energy 'will be gained ; 

 but if two gases, which are in every respect identical, are allowed to 

 mix, they could not be separated by any reversible process and there 

 would consequently be no gain of available energy in their mixing nor 

 any dissipation of energy (increase of entropy). But if we suppose two 

 gases which differ only infinitesimally to mix, the first condition would 

 still obtain and there would still be a certain gain of available energy. 

 The question arises, what will happen if we proceed to the limit ? Max- 

 well explained this paradox by saying that our ideas of dissipation of 

 energy depend upon the extent of our knowledge of the subject. Could 

 we invoke Maxwell's demon and borrow his gift of molecular vision, 

 we should perceive that when two identical gases mix there is in reality 

 a complete dissipation of energy, which the demon's intelligence might 

 turn into available energy if he liked ; for " it is only to a being in the 

 intermediate stage who can lay hold of some forms of energy, while 

 others elude his grasp, that energy appears to be passing inevitably 

 from the available to the dissipated state." 113 In the reasoning of 

 energetics, the paradox is explained by saying 11 * that the more nearly 

 alike the gases are, the slower will be the process of diffusion, so that 

 work or available energy might indeed be gained, but only after the 

 lapse of indefinite or infinite time, if we have such time at our disposal. 



Theory of Capillarity, Liquid Films and Interfacial Phenomena. — 

 There are two important theories of capillary action, that of Laplace, 

 based upon the assumption that the play of molecular forces in a liquid 

 is only possible at insensible or ultra-micrometric distances, and that 

 of Gauss, based upon the doctrine of energy. Gibbs's exhaustive dis- 

 cussion of capillarity, which takes up at least one third of his memoir, 

 is the thermodynamic or chemical completion of the purely dynamic 

 theory of Gauss. A capillary film or interfacial layer forms a new 

 " phase " between the two substances on either side of it, and the 

 mathematical condition for the formation of a new chemical substance 

 at such an interface or " surface of discontinuity " is expressible as an 

 algebraic relation between the surface tensions of the three layers of 

 substance and the pressure of the three phases, 115 the surface tensions 



uo Ibid., 218. 



111 Ibid., 194-7, 225-7. 



^ Ibid., 227-9. 



113 Maxwell, " Encycl. Britan.," 9th ed., VII., 220, sub voce " Diffusion." 



m Larmor, "Encycl. Britan.'," 10th ed., XXVIII., 171. 



115 Tr. Connect. Acad., III., 391-416. 



