22 Mr. F. T. Trouton. On the Motion under [June 1, 



.velocity is probably due to the great diminution of the surface tension, 

 which for the 5 per cent, solution was less than half that of water. 



In connexion with, this, as it probably depends on the formation 

 of soap, must be mentioned a rather pretty experiment, which is 

 easily made. A bubble of sweet oil if allowed to ascend a tube (say, 

 of diameter = 1) of ordinary pure water passes up in the ordinary 

 way. But, if the tube contains a weak solution of caustic soda, as 

 the bubble ascends, the motion of the solution over the surface of the 

 oil raises a series of circular waves round the cylindrical bubble, or, 

 rather, a series of surface tension ripples. The waves almost invari- 

 ably join at once to form one continuous spiral wave round the 

 bubble, which then lends a surprisingly life-like appearance to the 

 bubble as it wriggles its way upwards through the tube.* 



The system of circular waves is evidently unstable, since the 

 formation of the spiral means the opening of a continuous channel 

 for the flow past of the solution. 



The sign of the spiral (right- or left-handed) depends on initial 

 circumstances, and can, when the tube is held in the hand, be con- 

 ditioned at will by a judicious turn of the wrist. 



The solution must be very weak, best about 1 part of strong caustic 

 soda in 50,000 parts water ; much stronger than this has too great a 

 tendency to emulsify the oil, doubtless itself a phenomenon in part 

 arising from diminution in surface tension on the more exposed parts 

 of the oil. 



When the liquid of which the column is composed is very viscous, 

 the system of waves is prevented from forming, as is the case with 

 a creasote bubble passing through glycerine, despite the fact of the 

 surface tension being so very small. 



At first sight it might appear that the ascent of bubbles through 

 tubes of different liquids would prove a convenient method for com- 

 paring their viscosities. As has been seen, it is necessary, among 

 other things, to know the surface tension in each case. This renders 

 the comparison of viscosity in this way really a more troublesome 

 process than by ordinary methods, since the determination of surface 

 tension, especially if the liquids be viscous, is often accompanied by 

 considerable uncertainty, owing to a persistent tendency to stick to 

 glass frequently exhibited. This is particularly so in finding the 

 surface tension between two liquids. 



The phenomenon, however, would appear to be suited for the 

 comparison of the surface tension between liquids. If the same 

 liquid constitute the column in each case, it need not be necessary to 

 know its viscosity, only the density being requisite. 



* On inclining the tube, especially with smaller sized tubes, the bubble is seen to 

 kave quite a caterpillar mode of progression. 



