1890.] on Foam. 93 



again. The first result is not surprising, but why does the dusty 

 surface come back ? Such return is opposed to what we should 

 expect from any kind of viscosity, and proves that there must be some 

 force directly tending to produce that particular motion. It is the 

 superior tension of the clean surface. No oil has been added here, 

 but then no water surface is ever wholly free from contamination ; 

 there may be differences of degree, but contamination is always 

 present to some extent. I now make the surface more dirty and 

 greasy by contact of the finger, and the experiment no longer succeeds, 

 because the jet of wind is not powerful enough to cleanse the place 

 on which it impinges ; the dirty surface refuses to go away, or if it 

 goes in one direction it comes back in another. 



I want now to bring to your notice certain properties of soap 

 solutions, which, however, are not quite so novel as I thought when 

 I first came upon them in my own inquiries. * If we measure by 

 statical or slow methods the surface tension of soapy water, we 

 find that it is very much less than that of clean water. We can prove 

 this in a very direct manner by means of capillary tubes. Here, 

 shown upon the screen, are two tubes of the same diameter, in which, 

 therefore, if the liquids were the same, there would be the same eleva- 

 tion ; one tube dips into clean water, and the other into soapy water, 

 and the clean water rises much (nearly three times) higher than the 

 soapy water. 



Although the tension of soapy water is so much less than that of 

 pure water when measured in this way, I had some reason to suspect 

 that the case might be quite different if we measured the tensions 

 immediately after the formation of the surfaces. I was led to think 

 so by pondering on Marangoni's view that the behaviour of foaming 

 liquids w^as due to the formation of a pellicle upon their surfaces ; 

 for if the change of property is due to the formation of a pellicle, it 

 is reasonable to suppose that it will take time, so that if we can make 

 an observation before the surface is more than say y^ of a second 

 old, we may expect to get a different result. That may seem an 

 impossible feat, but there is really no diificulty about it ; all that is 

 necessary is to observe a jet of the substance in question issuing from 

 a fine orifice. If such a jet issues from a circular orifice it will be 

 cylindrical at first, and afterwards resolve itself into drops. If, how- 

 ever, the orifice is not circular, but elongated or elliptical, the jet 

 undergoes a remarkable transformation before losing its integrity. 

 As it issues from the elliptical orifice, it is in vibration, and trying to 

 recover the circular form ; it does so, but afterwards the inertia tends 

 to carry it over to the other side of equilibrium. The section oscil- 

 lates between the ellipse in one direction and the ellipse in the 

 perpendicular direction. The jet thus acquires a sort of chain-like 

 appearance, and the period of the movement represented by the 



* I here allude to the experiments of Dupre, and to the masterly theoretical 

 discussion of liquid films by Professor Willard Gibbs. 



