Lord Rayleigh : Investigations in Capillarity. 335 



72 per cent, would become 78 per cent., and the difference 

 seems larger than can well be explained as an alteration of 

 standard in judging when the iragments are nearly dead. 



One of the most striking conclusions to be drawn from an 

 inspection of the curves is the slowness of the fall of tension 

 which sets in soon after passing the camphor-point. On a 

 rough view it would seem as if a second limit were being 

 approached. But this idea is scarcely confirmed by actual 

 further additions of oil, for the tension continues to fall 

 slightly after each addition, even when large quantities are 

 already present. But there is one peculiarity in the behaviour 

 of the oil which suggests that the failure to reach a limit may 

 be due to want of homogeneity. As is well known, the disk 

 into which a drop deposited upon an already oiled surface at 

 first spreads, soon breaks up, and the superfluous oil collects 

 itself into little lenses. After this stage is reached it would 

 be natural to suppose that the affinity of the surface for oil 

 was fully satisfied, and that no further alteration in tension 

 could occur. And in fact the balance usually indicated the 

 absence of immediate effect. But if the surface were ex- 

 panded so as to spread the added oil more effectively and 

 then contracted again, a fall in tension was almost always 

 observed. It would seem as if the surface still retained an 

 affinity for some minor ingredient capable of being extracted, 

 though satiated as regards the principal ingredient. 



The comparison of the present with former results throws 

 an interesting light upon molecular magnitudes. It has been 

 shown (Proc. Roy. Soc. March 1890) that the thickness of 

 the film of olive-oil, calculated as if continuous, which 

 corresponds to the camphor-point, is about 2*0 ftp*; while 

 from the present curves it follows that the point at which 

 the tension begins to fall is about half as much, or l'O /a//.. 

 Now this is only a moderate multiple of the supposed diameter 

 of a gaseous molecule, and perhaps scarcely exceeds at all the 

 diameter to be attributed to a molecule of oil. It is obvious 

 therefore that the present phenomena lie entirely outside the 

 scope of a theory such as Laplace's, in which matter is 

 regarded as continuous, and that an explanation requires a 

 direct considerntion of molecules. 



If we begin by supposing the number of molecules of oil 

 upon a water surface to be small enough, not only will every 

 molecule be able to approach the water as closely as it desires, 

 but any repulsion between molecules will have exhausted 

 itself. Under these conditions there is nothing to oppose the 



HH = micronrillimefre = 10 — b mm. 



