October 15, 1915] 



SCIENCE 



511 



ture coefiSeient. This is unusual ; but, if we 

 remember that the interface between a 

 liquid and its vapor disappears when the 

 temperature rises to the critical point, and 

 with it, of course, all phenomena at the 

 boundary surface, the fact is not surprising 

 that there is a diminution of these phenom- 

 ena as the critical temperature is ap- 

 proached. 



Perhaps that result of surface energy 

 known as "adsorption" is the one in which 

 the conditions present at phase boundaries 

 make themselves most frequently obvious. 

 Since the name has been used somewhat 

 loosely, it is a matter of some consequence 

 to have clear ideas of what is meant when 

 it is made use of. Unless it is used to de- 

 scribe a definite fact, it can only be mis- 

 chievous to the progress of science. 



Permit me, then, first to remind you of 

 that fact of universal experience, known as 

 the "dissipation of energy," which is in- 

 volved in the second law of energetics. 

 Free energy — that is, energy which can be 

 used for the performance of useful work — 

 is invariably found to diminish, if the con- 

 ditions are such that this is possible. If 

 we have, therefore, a system in which, by 

 any change of distribution of the constitu- 

 ents, free energy can be decreased, such a 

 change of distribution wiU take place. This 

 is one form of the well-known "Principle 

 of Carnot and Clausius." 



Now, practically any substance dissolved 

 in water lowers the surface tension present 

 at the interface between the liquid and 

 another solid or liquid phase with which 

 it is in contact. Moreover, up to a certain 

 limit, the magnitude of this effect is in pro- 

 portion to the concentration of the solute. 

 Therefore, as was first pointed out by Wil- 

 lard Gibbs, concentration of a solute at an 

 interface has the effect of reducing free 

 energy and will therefore occur. This is 

 adsorption. As an example, we may take 



the deposition of a dyestuff on the surface 

 of charcoal, from which it can be removed 

 again, unaltered, by appropriate means, 

 such as extraction with alcohol. Charcoal 

 plus dye may, if any satisfaction is derived 

 from the statement, be caUed a compound. 

 But, since its chemical composition depends 

 on the concentration of the solution in 

 which it was formed, it is much more accu- 

 rate to qualify the statement by calling it 

 an ' ' adsorption-compound. ' ' Moreover, the 

 suggestion that the union is a chemical 

 one tends to deprive the conception of chem- 

 ical combination of its characteristic qual- 

 ity, namely, the change of properties. 

 Dye-stuff and charcoal are chemically un- 

 changed by adsorption. 



The origin of adsorption from surface 

 tension is easily able to explain why it is 

 less as the temperature rises, as we find 

 experimentally. As we have just seen, sur- 

 face tension diminishes with increase of 

 temperature. 



Let us next consider what wiU happen if 

 the liquid phase contains in solution a 

 substance which lowers surface tension and 

 is also capable of entering into chemical re- 

 action with the material of which the other, 

 solid, phase consists. For example, a solu- 

 tion of caproic acid in contact with par- 

 ticles of aluminium hydroxide. On the 

 surface of the solid, the concentration of 

 the acid will be increased by adsorption, 

 and, in consequence, the rate of the reac- 

 tion with it will be raised, according to the 

 law of mass action. Further, suppose that 

 the liquid phase contains two substances 

 which react slowly with each other, but not 

 with the solid phase. They will be brought 

 into intimate contact with each other on 

 the surface of the solid phase, their con- 

 centration raised and the rate of their in- 

 teraction increased. One of the reagents 

 may clearly be the solvent itself. But in aU 

 these eases the rate of the reaction can not 



