144 



On Liquid Jets. 



[June 15, 



but apparently only because, owing to its lightness, it was difficult to 

 ensure its falling upon the jets. Whenever we were sure it did so, 

 union followed." 



When mixed with the water, powders acted differently. Emery and 

 putty powder were not effective, but sulphur caused immediate union. 

 Much probably depends upon the extent to which the extraneous 

 matter is wetted. A precipitate of chloride of silver, formed in the 

 liquid itself, seemed to be without influence. 



Acting upon this hint, Mrs. Sidgwick made an extended series of 

 observations upon the behaviour of jets composed of water which had 

 been allowed to settle thoroughly, and which were protected from 

 atmospheric dust. For this purpose the jets were enclosed in a beaker 

 glass, the end of which was stopped by a plug of boxwood, fitted air- 

 tight. Through the plug passed horizontally the two inclined glass 

 nozzles, and underneath a bent tube serving as a drain. The results, 

 observed under these circumstances, were such as to render it almost 

 certain that dust is the sole cause of the capricious unions. The pro- 

 tected jets of settled water were observed for a total period of 246 

 minutes, during which the unions were at the average rate of one in 

 ten minutes. The longest internals without unions were thirty-four 

 minutes and twenty-nine minutes. Comparative experiments were 

 made upon the behaviour of jets from the same nozzles under other 

 conditions. Thus jets of unsettled water, but protected from atmo- 

 spheric dust, united on an average twenty-four times in ten minutes. 

 With unsettled water the protection from atmospheric dust is not of 

 much use, as unprotected jets of the same water did not unite more 

 than twenty-six times in ten minutes. On the other hand, jets of 

 settled water, not protected from the atmosphere, united only twelve 

 times in ten minutes. Although, no doubt, somewhat different 

 numbers might be obtained on repetition of these experiments, they 

 show clearly that the dust in the water is the more frequent cause of 

 union under ordinary circumstances, but that when this is removed 

 the atmospheric dust still exerts a powerful influence. The difficulty 

 of getting water free from dust is well known from Tyndall's experi- 

 ments, so that the residual tendency to unite under the most favour- 

 able conditions will not occasion surprise. 



Although there is no reason to suppose that any other cause than 

 dust was operative in the above experiments, it remains true that very 

 little impurity of a greasy character will cause immediate union of 

 colliding jets. For this purpose the addition of milk at the rate of 

 one drop of milk to a pint of water is sufficient. It may be noticed 

 too that the effect of milk is not readily neutralised by caustic potash. 



With respect to the action of electricity, further experiments have 

 been made to determine the minimum electromotive force competent 

 to cause union. The current from a Daniell cell was led through a 



