418 Mr. J. Aitken. On some Phenomena [Apr. 28, 



near the nozzle, it showed a temperature of about 130°, but that 

 figure can only be looked upon as a very rough approximation to the 

 true temperature. 



One or two attempts'Vere, however, made to find the temperature 

 at which water films cease to have any repulsive action. This was 

 done by means of a small water jet ; and it was found that above 

 155° there was no scattering. It was not till the temperature fell 

 below that point that electrification had any effect. This was the 

 temperature of the drops themselves, not of the supply for the jet ; 

 and it may not be quite accurate, as the drops tend to cool very 

 quickly. Another method of finding this temperature was to observe 

 the highest temperature at which the mist drops floated on water, in 

 the experiment previously described. This method is not very satis- 

 factory, on account of the difficulty of seeing the drops when the 

 temperature is high, owing to the amount of condensed steam 

 hanging over the water. It is also difficult to keep the surface of 

 the water clean. The tests by this method gave a temperature con- 

 siderably higher than that given by the water jet. Neither of these 

 methods, however, promises to give satisfactory information on this 

 point ; but, if it were desired, the effect of temperature on the contact 

 of films could be studied in a more accurate way. 



It is difficult to imagine any sudden change in the action of the 

 films at or about the temperatures indicated. There is no corre- 

 sponding change, so far as I am aware, in the surface tension. We 

 might picture to ourselves the change to be brought about by the 

 alteration which takes place in the intervening gases. When the 

 drops are cold, the bounding surfaces are water and air with very 

 little vapour in it. And perhaps we may be permitted to assume 

 that the surface-film has a layer of air condensed on it, and it may 

 be this condensed layer of air which prevents contact when the drops 

 come into collision. But when the temperature is high, the condi- 

 tions are changed. The bounding surfaces are now water and air 

 with a large amount of vapour in it, and this vapour may play an 

 important part in bringing about the contact, by the violent inter- 

 change of water molecules taking place at the surfaces of the films, 

 and weakening the condensed films of air. If this explanation be 

 correct, then there is really no sudden change in the action of the 

 films, and the repulsion is a gradually increasing one with fall of 

 temperature. Though the somewhat sudden change in the appear- 

 ance of this jet might seem to indicate a sudden change in the action 

 of the films, yet the change may be really a slowly increasing one, and 

 the sudden change in the appearance of the jet may be due to the 

 repulsion rising to such an amount that the very small particles are 

 prevented from coalescing. If the relative temperatures given for 

 the coalescence of water drops and mist drops be correct ; then the 



