Colliding Water-jets, 33 



trically with different points on a fine wire, which carries the 

 current between the poles of a bichromate cell. I have ex- 

 amined the cases when the colours are of tolerably low order, 

 that is when the separating film is thin ; and I have not been 

 able to detect any change of colours, pointing to the gradual 

 approach of th'e jets. The colours remain constant, until the 

 electromotive force is raised to such a point that coalescence 

 takes place ; and then they disappear quite suddenly. I had 

 determined to give the matter one more trial, wishing to 

 observe the effects when the jets collide at an extremely small 

 angle. In this case it may be arranged that the colours pro- 

 duced are not of lower order than the fourth in the thinnest 

 part of the film. This would be the most delicate means of 

 testing. But I have been completely foiled in my attempts ; 

 for the jets persistently refuse to rebound from one another 

 for longer periods than a small fraction of a second. I attri- 

 bute this to the large amount of pollen in the air, blown from 

 fir-trees near the house ; for I find quantities of golden powder 

 collected on my table and elsewhere in the house. 



My observations then, so far as they have gone, eliminate 

 conclusively the explanation, which Lord Rayleigh has re- 

 ferred to as possible though not probable, namely that the 

 action of electricity in promoting union may be ascribed to 

 the u additional pressure called into play by electrical attrac- 

 tion of the opposed water-surfaces, acting as plates of a con- 

 denser" (Proc. Roy. Soc. vol. xxxiv. p. 145). 



The extraordinary capriciousness of union — the jets some- 

 times coalescing with very much smaller electromotive forces 

 than at other times — made me for a time think that dust 

 might still be at work ; that the dust, which in the ordinary 

 state of the jets is not able to cause coalescence, might, under 

 the possibly directive influence of the electromotive force 

 (much as in an experiment described by Faraday), be turned 

 across the separating surfaces of the jets, and so rupture them. 

 If this were the case, one would expect that a larger electro- 

 motive force would be required to bring about coalescence, 

 when the water supplying the jets had been allowed to settle. 

 I left the water for five days to settle and found no appreciable 

 difference between the electromotive forces required to pro- 

 duce union, when this " settled " water was used, and when 

 dusty water was used immediately afterwards. Nor did 

 diminution in the velocity of the jets affect the magnitude of 

 the requisite electromotive force. 



Lord Rayleigh regards the union as most probably due to 

 perforation of the separating skin, brought about by disrup- 

 tive discharge. Let us consider the case when the colour of. 



Phil. Mag. S. 5. Vol. 20. No. 122. July 1885. D 



