328 Lord Rayleigh : Investigations in Capillarity. 



attributing the activity of a non-porous surface to imperfect 

 adhesion. We have to consider in detail the course of events 

 when a surface, e. g. of glass, is introduced into the liquid.' If 

 the surface be clean, it is wetted by the water advancing over 

 it. whether there be a film of air condensed upon it or not, 

 and no gas is liberated from the liquid. But if the surface 

 be greasy, even in a very slight degree, the behaviour is 

 different. We know that a drop of water is reluctant to 

 spread over a glass that is not scrupulously clean. If a large 

 quantity of water be employed, some sort of spreading follows 

 under the influence of gravity, but there is no propsr adhesion, 

 at least for a time, as appears at once on pouring the water 

 off again. The precise character of the transition from glass 

 to water when there is grease between is not well understood. 

 It may be that there is something which can fairly be called 

 a film of air. If so, its existence is a consequence of the pre- 

 sence of. the grease. On the other hand, it appears at least 

 equally probable that air is not concerned, and that the activity 

 of the surface is directly due to the thin film of grease, whose 

 properties, as in the case of greased water surfaces, are mate- 

 rially different from those of a thick layer. 



On this principle, too, it is easier to understand the retention 

 of a visible bubble when formed— a retention which often lasts 

 for a long time. So soon as the gas is entirely surrounded 

 by liquid of thickness exceeding the capillary limit, the bubble 

 is bound to rise. It is difficult to see how the hypothetical 

 film of air explains the failure of the liquid to penetrate 

 between the bubble and the solid. 



Colliding Jets. 



In various papers (Proc. Roy. Soc. Feb. 1879, May 1879, 

 June 1882) I have examined the behaviour of colliding drops 

 and jets. Experiments with drops are very simply carried 

 out by the observation of nearly vertical fountains, rising say 

 to two feet from nozzles ^ inch in diameter. The scattering 

 of the drops, when the water is clean and not acted upon by 

 electricity, shows that collision is followed by rebound. If 

 the water is milky, or soapy with unclarified soap, or if the 

 jet, though clean, is under the influence of feeble electricity, 

 the apparent coherence and the heaviness of the patter made 

 by the falling water are evidence that rebound no longer 

 ensues, but that collision results in amalgamation. Eye 

 observation, or photography, with the instantaneous illumi- 

 nation of electric sparks renders the course of events perfectly 

 clear. 



The form of the experiment in winch are employed jets, 



