506 Sir William Thcmison [Jan. 29, 



down the side and give a fringe-like appearance to the space through 

 which the rising ring has passed. 



These phenomena may also be observed by using, instead of 

 alcohol, ether, which has a surface-tension equal to about three- 

 fourths of that of alcohol. In using ether, however, this very curious 

 effect may be seen.* I dip the brush into the ether, and hold it 

 near to but not touching the water-surface. Now I see a hollow 

 formed, which becomes more or less deep according as the brush is 

 nearer to or farther from the normal water surface, and it follows the 

 brush about as I move it so. 



Here is an experiment showing the effect of heat on surface- 

 tension. Over a portion of this tin plate there is a thin layer of 

 resin. I lay the tin plate on this hot copper cylinder, and we at once 

 see the fluid resin drawing back from the portion of the tin plate 

 directly over the end of the heated copper cylinder, and leaving a 

 circular space on the surface of the tin plate almost clear of resin, 

 showing how very much the surface-tension of hot resin is less than 

 that of cold resin. 



Note of January 30, 1886. — The equations (8) and (9) on p. 59 

 of Clerk-Maxwell's article on " Capillary Attraction " in the ninth 

 edition of the " Encyclopaedia Britannica " do not contain terms 

 depending on the mutual action between the two liquids, and the 

 concluding expression (10), and the last small print paragrajDh of the 

 page are wholly vitiated by this omission. The paragraph immedi- 

 ately following equation (10) is as follows : — 



" If this quantity is positive, the surface of contact will tend to 

 contract, and the liquids will remain distinct. If, however, it 

 were negative, the displacement of the liquids which tends to enlarge 

 the surface of contact would be aided by the molecular forces, so that 

 the liquids, if not kept separate by gravity, would become thoroughly 

 mixed. No instance, however, of a phenomenon of this kind has been 

 discovered, for those liquids which mix of themselves do so by 

 the process of diffusion, which is a molecular motion, and not by the 

 6j)ontaneous puckering and replication of the boundary surface 

 as would be the case if T were negative." 



It seems to me that this view is not correct ; but that on the 

 contrary there is this " puckering " as the very beginning of diffusion. 

 What I have given in the lecture as reported in the text above seems 

 to me the right view of the case as regards diffusion in relation 

 to interfacial tension. 



It may also be remarked that Clerk-Maxwell, in the large print 

 paragraph of p. 59, preceding equation (1), and in his application of 

 the term potential energy to E in the small print, designated by 

 energy what is in reality exhaustion of energy or negative energy ; 



* See Clerk-Maxwell's article (p. 65) on " Capillary Attraction " C Encyclo- 

 pscdia Britannica,' 9tli edition). 



