234 report— 1877. 



recorded in the Proceedings of the Royal Society. The following is the 

 summary of this part of the research : — 



" 1st. The bubbles in certaiu fluid- cavities approach a source of heat 

 which is brought near them. 



"2nd. The bubbles in certain fluid-cavities recede from the same source 

 of heat. 



" 3rd. That a rise of 5° C. above the temperature of the specimen suffices 

 to cause the apparent attraction. 



" 4th. Tbat a rise of only ^° C. will in some cases cause the apparent re- 

 pulsion. 



" 5th. That in certain cases a bubble which receded from the source of 

 heat at ordinary temperatures approached it when raised to 60° C, the 

 source of heat always being from J° C. to 5° C. warmer than the specimen. 



" 6th. That this could occur in cavities containing liquid carbonic acid as 

 well as water, but that it made no difference whether the carbonic acid was 

 raised above its critical point or not." « 



This latter fact affords a means of controlling to some extent the conditions 

 of the experiment, since we know that the tension of liquid carbonic acid 

 when it has just passed the critical point amounts to 109 atmospheres. 



Hence gas-bubbles enclosed in minute tubes containing water may be 

 caused to recede from or approach a source of heat according as their tempe- 

 rature is below or above 60° C, and even when the gas is confined under 

 enormous pressure. It was found that the warmth of the fingers is sufficient 

 to propel even in a vertical direction a plug of water contained in a capillary 

 tube open at both ends. The apparent attraction of bubbles by heat is evi- 

 dently due to the same cause which occasions this movement. Professor 

 Stokes assigns this apparent repulsion of the liquid to a diminution by heat 

 of the surface-tension at one end of a plug of liquid in a tube, or side of a, 

 bubble in a cavity. 



When attraction of the liquid takes place it may be because a slight rise 

 of temperature effects a disengagement of gas from the water on the side of 

 the bubble nearest to the source of heat, which increases the surface-tension 

 at this side : the bubble is therefore propelled in the opposite direction. 



This explanation is similar to that which Professor James Thomson gave 

 of the cause of the " tears of wine," published in the Reports of the British 

 Association (1855, Proceedings of Sections, p. 16). 



On vibrating bubbles and the Brownian movement. — Mr. Sorby was the first 

 to notice a remarkable vibration of minute bubbles in the fluid-cavities of 

 minerals precisely of the nature of tho Brownian movement. 



This motion was repeatedly seen in some sections of granites, as, for 

 instance, many specimens from Cornwall, quartzite from Snowdon, and 

 granite from Shap Fell in "Westmoreland. All the most minute cavities 

 contain bubbles incessantly vibrating. It was found that all these bubbles 

 approached a warm body, and that they ceased moving and clung for some 

 time to the warmer side of a cavity. After repeated and varied experiments 

 on these moving bubbles the following conclusion was arrived at. It is 

 impossible to imagine a body which is not gaining or losing, or, at the same 

 time, both gaining and losing heat ; it is therefore impossible to imagine it 

 entirely throughout at a uniform temperature. It is evident, then, that an 

 easily movable particle which can be set in motion by exceedingly slight 

 rises of temperature will make the transference of heat from one point to 

 another plainly visible. The minute bubbles in fluid-cavities are such par- 



