1890.] Lord Bayleigh on Foam. 85 



WEEKLY EVENING MEETING, 



Friday, March 28, 1890. 



Sir Frederick Bramwell, Bart. D.C.L. F.E.S. Honorary Secretary 

 and Vice-President, in the Chair. 



The Right Hon. Lord Eayleigh, M.A. D.C.L. LL.D. F.E.S. M.B.L 

 Professor of Natural Philosophy, E.I. 



Foam. 



When I was turning over in my mind the subject for this evening, it 

 occurred to me to take as the title of the lecture, " Froth." But I 

 was told that a much more poetical title would be " Foam," as 

 it would so easily lend itself to appropriate quotations. I am afraid, 

 however, that I shall not be able to keep up the poetical aspect of 

 the subject very long ; for one of the things that I shall have most 

 to insist upon is that foaming liquids are essentially impure, 

 contaminated — in fact, dirty. Pure liquids will not foam. If I take 

 a bottle of water and shake it up, I shall get no appreciable foam. 

 If, again, I take pure alcohol, I get no foam. But if I take a mixture 

 of water with 5 per cent, of alcohol there is a much greater tendency. 

 Some of the liquids we are most familiar with as foaming, 

 such as beer or ginger-beer, owe the conspicuousness of the 

 property to the development of gas in the interior, enabling the 

 foaming property to manifest itself ; but of course the two things are 

 quite distinct. Dr. Gladstone proved this many years ago by showing 

 that beer from which all the carbonic acid had been extracted in 

 vacuo still foamed on shaking up. I now take another not quite 

 pure but strong liquid, acetic acid, and from it we shall get no more 

 foam than we did from the alcohol or the water. The bubbles, as 

 you see, break up instantaneously. But if I take a weaker acid, the 

 ordinary acid of commerce, there is more, though still not much, 

 tendency to foam. But with a liquid which for many purposes may 

 be said to contain practically no acetic acid at all, seeing that it 

 consists of water with but 1-lOOOth part of acid, the tendency 

 is far stronger ; and we get a very perceptible amount of foam. 

 These tests with the alcohol and acetic acid are sufficient to illustrate 

 the principle that the property of foaming depends on contamination. 

 In pure ether we have a liquid from which the bubbles break even 

 more quickly than from alcohol or water. They are gone in a 

 moment. In some experiments I made at home I found that water 

 containing a small proportion of ether foamed freely ; but on attempting 

 two or three days ago to repeat the experiment, I was surprised to 



