168 



HEAT. 



Fig. 92. Fig. 93 represents the curves raised a distance equal to the 

 atmospheric pressure above the line of zero pressure. 



Superposing the two figures, so that ox is the same for each, the 

 points where the curve of Fig. 93 cuts those of Fig. 92 represent positions 

 of equilibrium for bubbles containing different quantities of gas. As long 

 as the vapour-pressure is less than the atmospheric pressure, the curve of 

 Fig. 93 cuts the curves of Fig. 92, where they have a greater slope than it, 

 and any increase of volume, therefore, means an excess of external over 

 internal pressure, and, hence, equilibrium is stable. The bubbles, there- 

 fore, grow only with rise of temperature and addition of gas. But when 

 the vapour- pressure exceeds the atmospheric it is possible for the surface 

 tension curve of Fig. 93 to cut a gas curve of Fig. 92 at a less slope, and 

 equilibrium is then unstable, as in boiling with " bumping." We leave 



., 

 A if* 



TO pump 



FlG. 94. Apparatus for obtaining a Dust-Free Space. 



the reader to consider how the diagrams would lead us to expect that 

 after the first " singing " discharge of bubbles no more large bubbles will 

 be found till the boiling-point is reached. 



Condensation on Nuclei. We have seen that the change from 

 the liquid to the gaseous condition is essentially a surface phenomenon. 

 It takes place at the top level surface in ordinary quiet evaporation, and 

 in all probability at the surface of already existing bubbles or cavities 

 in ebullition. The converse change in condensation is also generally 

 a surface phenomenon, the condensation occurring on nuclei of dust or 

 suspended matter other than air or vapour. This was first discovered 

 by Coulier (Journal de Pharmacie et de Cliemie, xxii., pp. 165 and 254, 

 1875), and later again by Aitken (Trans. E.S.E., xxx., pt. 1, p. 337), 

 to whom we owe extensive investigations on the subject. 



When a beam of sunlight crosses a room, the visibility of its track is 

 due to particles of " dust," many perhaps large enough to be seen, others 

 quite beyond our range of sight. If the air be filtered through cotton- 

 wool this dust is filtered out. The clean air can then be quite appre- 



