330 Lord Rayleigh : Investigations in Capillarity. 



It would seem that the electrical forces a^t with peculiar 

 advantage. If we suppose that upon the whole the air 

 cannot be removed, so that the mean distance between the 

 opposed surfaces remains constant, the electric attractions 

 tend to produce an instability whereby the smaller intervals 

 are diminished while the larger are increased. Extremely 

 local contacts of the liquids, while opposed by capillary 

 tension which tends to keep the surfaces flat, are thus 

 favoured by the electrical forces, which moreover at the 

 small distances in question act with exaggerated power. 



It is probably by promoting local approximations in oppo- 

 sition to capillary forces that dust, finding its way to the 

 surfaces, brings about union. 



A question remains as to the mode of action of milk or 

 soapy turbidity. The observation, formerly recorded, that it 

 is possible for soap to be in excess may here have significance. 

 It would seem that the surfaces, coming into collision within 

 a fraction of a second of their birth, would still be subject to 

 further contamination from the interior. A particle of soap 

 rising accidentally to the surface would spread itself with 

 rapidity. Now such an outward movement of the liquid is 

 just what is required to hasten the removal of the intervening- 

 air. It is obvious that this effect would fail if the con- 

 tamination of the surface had proceeded too far previously to 

 the collision. 



In order to illustrate the importance of the part played 

 by the intervening gas, I thought that it would be interesting 

 to compare the behaviour of the jets when situated in atmo- 

 spheres of different gases. It seemed that gases more freely 

 soluble in water than the atmospheric gases would be more 

 easily got rid of in the later stages of the collision, and that 

 thus union might more readily be brought about. This 

 expectation has been confirmed in trials made on several 

 different occasions. It was found sufficient to allow a pretty 

 strong stream of the gas under examination to play upon 

 the jets at and above the place of collision. Jets of air, of 

 oxygen, and of coal-gas were found to be without effect. 

 On the other hand, carbonic acid, nitrous oxide, sulphurous 

 anhydride, and steam at once caused union. Only in the 

 case of hydrogen was there an ambiguity. On some 

 occasions the hydrogen appeared to be without effect, but on 

 others (when perhaps the pressure of collision was higher) 

 union uniformly followed. Care was taken to verify that air 

 blown through the same tube as had supplied the hydrogen 

 was inactive, so that the effect of the hydrogen could not be 

 attributed to dust. The action of hvdro&en cannot be 



