1890.] on Foam. 91 



contrary, because we find that soap can do so mucli more than oil. 

 There is indeed, something more or less corresponding to the decom- 

 position of the soap and the formation of a superficial layer of oil. 

 But the decomposition takes place in a very peculiar manner, and 

 under such conditions that there is a gradual transition from the soapy 

 liquid in the interior to the oily layer at the top, and not, as when we 

 float a layer of oil on water, two sudden transitions, first from water to 

 oil, and secondly from oil to air. The difference is important, because, 

 as I showed some years ago, capillary tension depends on the sudden- 

 ness of change. If we suppose that the change from one liquid to 

 another takes place by slow stages, though the final change may be as 

 before, the capillary tension would absolutely disappear. 



There is another very interesting class of phenomena due to oil 

 films, which I hope to illustrate, though I am conscious of the diffi- 

 culty of the task, — namely, the action of oil in preventing the forma- 

 tion of waves. From the earliest times we have records of the effect 

 of oil in stilling waves, and all through the Middle Ages the effect 

 was recognised, though connected with magic and fanciful explanations. 

 Franklin, than whom, I suppose, no soberer inquirer ever existed, 

 made the thing almost a hobby. His attention was called to it 

 accidentally on board ship from noticing the effect on the waves 

 caused by the greasy debris of a dinner. The captain assured him 

 that it was due to the oil spread on the water, and for some time after- 

 wards, Franklin used to carry oil about with him, so as never to miss 

 a chance of trying an experiment. A pond is necessary to illustrate 

 the phenomena properly, but we shall get an idea of it by means of 

 this trough six feet long, containing water.* Along the surface 

 of the water we shall make an artificial wind by means of a fan,f 

 driven by an electro motor. In my first experiments I used wind 

 from an organ bellows, which is not here available. Presently we 

 shall get up a ripple, and then we will try the effect of a drop of oil 

 put in to windward. I have now put on the drop, and you see a 

 smooth place advancing along. As soon as the waves come up again, 

 I will repeat the experiment. While the wind is driving the oil away, 

 I may mention that this matter has been tested at Peterhead. Experi- 

 ments were there made on a large scale to show the effect of oil in 

 facilitating the entrance of ships into harbour in rough weather. 

 Much advantage was gained. But here a distinction must be observed. 

 It is not that the large swell of the ocean is damped down. That 

 would be impossible. The action in the first instance is upon the 

 comparatively small ripples. The large waves are not directly affect- 

 in by the oil ; but it seems as if the power of the wind to excite and 

 maintain them is due to the small ripples which form on their backs, 



* The width is 8 inches, and the depth 4 inches. The sides are of glass ; the 

 bottom and ends of wood, painted white. 



t For tliis fan and its fittinga the Institution is indebted to the liberality of 

 the Blackmau Ventilating Company. 



