connected with the Boiling of Liquids. 445 



rid of and bubbles of nearly pure vapour escape. The vapour 

 rises into the bell just as gas does into a jar of water at the 

 pneumatic trough, and would remain but for the heated bottom 

 of the flask near to the source of heat, which is hotter than any 

 other part of the arrangement. This superior heat causes the 

 vapour in the little bell constantly to expand and discharge 

 itself; but it does not do so continuously, it is an oscillation 

 between vapour escaping and vapour charged with water enter- 

 ing — an oscillatory motion which, as I have said, is only possible 

 at the bottom of the vessel (at least in this form of the experi- 

 ment), where the heat is most intense. If the little bell be raised 

 some way above the heated bottom it ceases to act, no vapour 

 escapes ; and hence I say that this experiment does not show 

 that air has the exclusive function assigned to it. The heated 

 bottom of the vessel is the chief source of the bubbles of steam. 

 They may be seen flashing from this hot surface, rising. upwards 

 and rapidly vanishing if the liquid has not yet attained its boiling- 

 point, or bursting on the surface when that point has been 

 attained. There are also certain specks in the bottom of glass 

 flasks, retorts, tubes, &c. which act as powerful nuclei; they 

 discharge streams of bubbles with unceasing vigour, which does 

 not decline although the action be continued during many hours. 

 At the same time 1 do not deny, never have denied, that the air 

 in solution has a useful part to perform in lessening the forces 

 of cohesion and adhesion in the liquid, and in facilitating the 

 formation of bubbles of vapour. All 1 contend for is that 

 air is not the only nucleus, but that there are other modes of 

 separating gas or vapour from their solutions, as already pointed 

 out. 



I am quite prepared to admit that, seeing how vast a number 

 of important vital functions depend on the solubility of air in 

 water, a portion of air should cling obstinately to water even 

 when raised to high temperatures, or, by virtue of this property, 

 even when expelled by boiling, air may be, as Mr. Grove sug- 

 gested, continually reabsorbed at or near the boiling temperature; 

 and I should prefer to adopt this view rather than the idea that 

 a bubble of air one millim. in diameter is an efficient cause in 

 liberating half a million of bubbles of steam, each five millims. 

 in diameter. 



I arranged M. Marco's experiment on a larger scale by em- 

 ploying a short tube nearly 2 inches in length and about f inch 

 in diameter. A few coils of thin binding wire were passed round 

 it near the bottom and also near the mouth of the tube ; each 

 coil terminated in a loop ; and to each loop was attached, also by 

 a loop, a longer and stouter wire extending beyond the neck of 

 the flask. In this way, without removing the tube from the 



