1877.] 



Repulsion of Bubbles by Heat. 



149 



moved with a uniform motion, and were kept at the further ends of the 

 cavities until an equilibrium in temperature had been established, so 

 that sometimes the liquid had the deceptive appearance of sinking under 

 the rise of temperature. When gas-bubbles were repelled by heat, their 

 speed appeared to be accelerated after they once commenced to move. 



It may be considered an argument against the motions being due to 

 any pyroelectric conditions of the minerals, that they have been noticed 

 in crystals of fluor-spar, and that, no matter in which direction sections 

 of rock-crystal are cut, the movements are all equally well obtained. 



Regarding the repulsion of gas-bubbles two facts are striking, namely 

 the very slight rise of temperature (less than ^° C.) on one side of the 

 bubble capable of causing the movement, and the great tension existing 

 within the bubble. 



Note.— Received April 13, 1877. 



I am much indebted to Prof. Stokes for having furnished an elucidation 

 of the cause of these movements, which is perfectly consistent with all 

 the facts which I have noticed. In consequence of this I have discarded 

 my own explanations, which were originally embodied in the foregoing 

 paper, but which were never perfectly satisfactory to me. 



In notes dated April 7th and 12th Prof. Stokes says : — " It seems to 

 me, as far as I can judge without having seen the specimens, that the 

 greater part, if not the whole, of the motions you describe are referable 

 to a cause different from that suggested by Prof. Tait, and that they 

 depend on capillarity. We know that the surface-tension of a liquid is 

 diminished as the temperature is raised. The explanation, according to 

 this view, would be very similar to Professor James Thomson's beautiful 

 explanation of the tears of wine (Eeports of the British Association, 

 1855, Report 2, p. 16), only here difference of temperature takes the 

 place of difference of strength, and the surface of the liquid surrounding 

 a bubble shrinks at the cooler side." The shrinkage of the liquid on the 

 cooler side of the bubble of course propels it towards the source of heat. 

 This explanation seems quite in accordance with all phenomena of attrac- 

 tion of bubbles, whether in carbonic acid or in water, with the movement 

 of water in capillary tubes, and the vibratory movements of minute 

 bubbles described in the next paper. 



" In the case of a cavity containing water with a bubble of compressed 

 carbonic acid, the water, of course, containing gas in solution, the repul- 

 sion by heat may be accounted for by a slight evaporation of the dissolved 

 gas at the surface weakening the solution, and thereby increasing the 

 surface-tension ; and it is quite conceivable that at different temperatures 

 one or other of these opposite effects may prevail." 



Conversely, one can understand how different conditions of gaseous 

 tension in bubbles, and the extent to which the surrounding water is 

 charged with gas, may render the effect of heat either repulsion or 

 attraction.— W. N". H. 



