210 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



The siuiplest way of testing this exphiiiatioii was to repeat Lord 

 Rayleigh's experiment with water Jets, but in pUice of cold water using 

 hot. The result is, the experiment entirely confirms this explanation. 

 So long as the water in the jet is above a certain temperature there is 

 no scattering whatever, but perfect coalescence of the drops on con- 

 tact. As a consequence the jet is not intiueuced in the slightest degree 

 by the presence of an electrified body. It is only after the tempera- 

 ture falls below a certain point tliat the scattering commences and 

 electricity begins to have an mfiuence. 



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

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

 accustomed to observe at ordinary temperatures — that is, rei)el each 

 other; and that when tlie temperature is high there is an entire change, 

 and the surface films no hunger repel, but coalescence of the drops takes 

 place at each collision. It aviII be noticed that the point here is, not 

 the appearance of any new infiuence with the low temperature, 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 temi)erature of the air is lowered is due to 

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

 sive action makes its ap[)earance. 



There is however an experimental link wanting to bind these two 

 Ijhenomena together, which I have desired to complete, but unfortu- 

 nately experimental difficulties stopped the way. The link wanting is 

 some exiteriinental jjioof that the Jet gets dense at the same temper 

 ature that the water Jet begins to scatter. On attempting to take the 

 tenipciature of the jet difticiilties presented themselves. If it is to be 

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

 change in the position of the bulb of a thermometer ])laced in the Jet 

 gives a different reading. It does not matter whether the change be 

 made nearer or further from the center of the jet or nearer or further 

 from the no/zle. In all cases a very slight change gives a considerable 

 dift'erence of temperature. It may however be stated that when the 

 bulb was placed in the center of the Jet and 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 atteiui)ts were however made to find the temperature at 

 which water films ceased 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 temi)erature fell below that 

 point that electrification had any eft'ect. This was the temperature of 

 the drops themselves, not of the supply for the Jet, and it may not be 

 quite ac<'urate, as the drops tend to cool very quickly. Another method 

 of finding this temperature Mas to observe the highest temperature at 



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