1892.] 



connected with Cloudy Condensation. 



417 



the temperature of the mixed cold air and steam some alteration had 

 taken place in the surface films of the water drops. The jet looked as 

 if something came into action at that temperature which prevented 

 the drops coalescing when they came into collision, or, what would 

 amount to the same thing, that at high temperatures there was no 

 tendency for the drops to recoil after impact, and that when the 

 temperature fell this property made its appearance, and prevented 

 contact in the same way as we have supposed the electrification does. 



The simplest way of testing this explanation was to repeat Lord 

 Hayleigh's experiment with water jets, but in place of cold water 

 using hot. The result is, the experiment entirely confirms this ex- 

 planation. So long as the water in the jet is above a certain tem- 

 perature there is no scattering whatever, but perfect coalescence of 

 the drops on contact. As a consequence the jet is not influenced in 

 the slightest degree by the presence of an electrified body. It is only 

 after the temperature falls below a certain point that the scattering 

 commences, and electricity begins to have an influence. 



This experiment shows that it is only when the drops are below a 

 certain temperature that their surface films act in the way we are 

 accustomed to observe at ordinary temperatures, that is, repel each 

 other; and that when the temperature is high there is an entire 

 change, and the surface films no longer repel, but coalescence of the 

 drops takes place at each collision. It will be noticed that the point 

 here is, not the appearance of any new influence with the low tem- 

 perature, as the films are then in the condition with which we are 

 acquainted ; it is at the high temperature that the new condition 

 comes into action, and the films lose the resisting action with which 

 we are acquainted. 



Now it seems extremely probable that the change in the appearance 

 of the steam jet when the temperature of the air is lowered is due to 

 the temperature of the jet falling to the temperature at which this 

 repulsive action makes its appearance. 



There is, however, an experimental link wanting to bind these two 

 phenomena together, which I have desired to complete, but unfor- 

 tunately experimental difficulties stop the way. The link wanting is 

 some experimental proof that the jet gets dense at the same tem- 

 perature that the water jet begins to scatter. On attempting to take 

 the temperature of the jet difficulties presented themselves. If it is 

 to be taken with a thermometer, where is it to be placed ? A very 

 slight change in the position of the bulb of a thermometer placed 

 in the jet gives a different reading. It does not matter whether the 

 change be made nearer or further from the centre of the jet, or 

 nearer or further from the nozzle : in all cases a very slight change 

 gives a considerable difference of temperature. It may, however, be 

 stated that when the bulb was placed in the centre of the jet, and 



