Lord Rayleigh : Investigations in Capillarity. 329 



issuing at moderate velocities and meeting at high obliquities, 

 is the more instructive ; but it is liable to be troublesome in 

 consequence of the tendency of the jets to unite spontaneously. 

 It is important to avoid dust both in the water and in the 

 atmosphere where the collision occurs. An electromotive 

 force of one volt suffices to determine union ; but so long as 

 the jets rebound there is complete electrical insulation between 

 them. 



As to the manner in which electricity acts, two views were 

 suggested. It was thought probable that union was the result 

 of actual discharge across the thin layer of intervening insu- 

 lation ; but it was also pointed out that the result might be 

 due to the augmented pressure to be expected from the 

 electrical charges upon the opposed surfaces. From obser- 

 vations upon the colours of thin plates exhibited at the region 

 of contact, which he found to be undisturbed by such elec- 

 trical forces as would not produce union, Mr. Newall* con- 

 cluded that the second of the above-mentioned explanations 

 must be discarded. 



On the other hand, as has been pointed out by Kaiser t, the 

 progress of knowledge concerning electrical discharge has 

 rendered the first explanation more difficult of acceptance. It 

 would appear that some hundreds of volts are needed in order 

 to start a spark, and that mere diminution of the interval to 

 be crossed would not compensate for want of electromotive 

 force. 



A more attentive examination of the conditions of the expe- 

 riment may perhaps remove some of the difficulties which 

 seem to stand in the way of the second explanation. As the 

 liquid masses approach one another, the intervening air has to 

 be squeezed out. In tho earlier stages of approximation the 

 obstacle thus arising may not be important : but when the 

 thickness of the layer of air is reduced to the point at which 

 the colours of thin plates are visible, the approximation must 

 be sensibly resisted by the viscosity of the air which still 

 remains to be got rid of. No change in the capillary con- 

 ditions can arise until the interval is reduced to a small 

 fraction of a wave-length of light ; but such a reduction, 

 unless extremely local, is strongly opposed by the remaining 

 air. It is of course true that this opposition is temporary. 

 The question is whether the air can be anywhere squeezed out 

 during the short time over which the collision extends. 



* Phil. Mag. toI. xx. p. 33 (1885). 



t Wied. Ann. liii. p. 667 (18D4). Kaiser's own experiments were 

 made upon the modification of the phenomenon observed by Boys, where 

 the contact takes place between two soap-hlms. 



