1877.] 



Repulsion of Bubbles by Heat. 



143 



must be a gas with a density of *745 compared with liquid water at 

 unity ; hence, in all probability, at a tension of 400 atmospheres this 

 gaseous mixture would be denser than water. I have elsewhere" pointed 

 out that carbonic-acid gas which was reduced to -Jy of its volume by a 

 pressure of 223 atmospheres, at 63° C. must have been as dense, if not 

 denser, -than water (Journal of the Ohem. Soc. vol. ii. 1876, p. 250). 



Some of those gas-bubbles which I have already mentioned as being 

 readily attracted by heat, I found were made to sink by warming to about 

 150° C. It is always necessary to rotate or at least reverse the objects 

 when under examination ; and this precaution was always strictly regarded 

 to obviate errors of observations. The importance of this is shown by 

 the following experiment. A bubble in a specimen of rock-crystal was 

 seen to descend to the lower point of the cavity when it was uniformly 

 heated from above only ; it was found to be attracted by a hot spatula 

 applied to one end of the cavity. It was thought possible that the cavity 

 might have an oblique inclination, and be attracted from the upper end 

 of the cavity, because this motion might bring it nearer the surface 

 where the source of heat was placed. This was evidently the case, for 

 on turning the slide upside down no motion was caused by uniform heating. 



The following experiments were made on some good-sized cavities in 

 rock-crystal. On presenting a heated wire to one side there was instant 

 attraction, and then the bubbles remained at the bottom of the cavities, 

 after which they settled slowly into their original positions. A hot 

 spatula was passed over the specimen ; the bubbles went to the bottom 

 and there remained, in spite of the attraction of the hot spatula to the 

 other end ; they then, after cooling slightly, ascended, but descended 

 again on removal of the spatula, as if jerked back by a spring. After a 

 time they finally ascended slowly. This is a curious effect : it seems that 

 the heat, if strong, causes the bubbles to sink, and that the heat of 

 the spatula cannot attract them up until they have cooled somewhat ; 

 that after attraction has drawn them to the upper ends of the cavities, 

 and the source of heat has been removed, they sink once more, and finally 

 take up their original positions after further cooling. 



Attraction and repulsion caused by heat in different cavities of the 

 same specimens. 



Bubbles attracted by heat and those which are repelled have generally 

 been found in separate and entirely different specimens ; and it would 

 appear most improbable that they should exist in the same piece of stone 

 side by side. 



Fig. 5 shows six cavities, which, though not in the same field of the 

 microscope, yet exist within a quarter of an inch square of the same 

 section of rock-crystal. The cavity marked A' contains water and liquid 

 carbonic acid, and cavity E contains liquid carbonic acid only; this might 

 be considered sufficient evidence of other cavities containing a highly com- 



VOL. XXVI. L 



