86 Lord Bayleigh [Marcli 28, 



find a result very different. I have here some water containing a 

 very small fraction of ether, about l-240th part. If I shake it up, 

 it scarcely foams at all ; but another mixture made in the same 

 proportion from another sample shows more tendency to foam. This 

 is rather curious, because both ethers were supposed to be of 

 the same quality ; but one had been in the laboratory longer than 

 the other, and perhaps contained more greasy matter in solution. 



Another liquid which foams freely is water impregnated with 

 camphor. Camphor dissolves sparingly ; but a minute quantity of it 

 quite alters the characteristics of water in this respect. Another 

 substance, very minute quantities of which communicate the foaming 

 property to water, is glue or gelatine. This liquid contains only 

 3 parts in 100,000 of gelatine, but it gives a froth entirely different 

 from that of pure water. Not only are there more bubbles, but the 

 duration of the larger bubbles is quite out of proportion to that of 

 water-bubbles. This sample contains 5 parts in 100,000, nearly 

 double as much ; but even with but 1 part in 100,000, the foaming 

 property is so evident as to suggest that it might in certain cases 

 prove valuable for indicating the presence of minute quantities of 

 impurities. I have been speaking hitherto of those things which 

 foam slightly. They are not to be compared with, say, a solution of 

 soap in w^ater, which, as is well known to everybody, froths very 

 vigorously. Another thing comparable to soap, but not so well 

 known, is saponine. It may be prepared from horse chestnuts 

 by simply cutting them in small slices and making an infusion with 

 water. A small quantity of this infusion added to water makes 

 it foam strongly. The quantity required to do this is even less than 

 in the case of soap ; so the test is more delicate. It is well known 

 that rivers often foam freely. That is no doubt due to the effect of 

 saponine or some analogous substance. Sea-water foams, but not, 

 I believe, on account of the saline matter it contains ; for I have 

 found that even a strong solution of pure salt does not foam much. 

 I believe it has been shown that the foaming of sea-water, often so 

 conspicuous, is due to something extracted from seaweeds during the 

 concussion which takes place under the action of breakers. 



Now let us consider for a moment what is the meaning of foaming. 

 A liquid foams when its films have a certain durability. Even in 

 the case of pure water, alcohol, and ether, these films exist. If a 

 bubble rises, it is covered for a moment by a thin film of the liquid. 

 This leads us to consider the properties of liquid films in general. 

 One of their most important and striking properties is their tendency 

 to contract. Such surfaces may be regarded as being in the condition 

 of a stretched membrane, as of india-rubber, only with this difference, 

 that the tendency to contract never ceases. We may show that by 

 blowing a small soap bubble, and then removing the mouth. The 

 air is forced back again by the pressure exerted on the bubble by the 

 tension of the liquid. This ancient experiment suffices to prove con- 

 clusively that liquid films exercise tension. 



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