REPULSION RESULTING- FROM RADIATION. 
525 
surface and extreme lightness are the requisites in selecting materials for the beam, 
index, or gravitating mass ; and when the masses have the same specific gravity and 
extent of surface, their position in respect to the source of heat determines the extent 
of movement. Thus a cylinder of pith is more sensitive when arranged for the heat to 
act on its side than on its end ; and the film of mica in experiment 65 was more affected 
when the heat struck its flat surface than its edge, although the difference was not so 
great as might have been expected had air-currents been the cause of motion. 
76. But these facts can equally well be used on the opposite side; for assuming that 
the movement is due to a repulsive action of radiation, it is reasonable to suppose that 
extended surface, weights being equal, would have an advantage. The repulsion by 
radiation only acts on the surface of bodies, and does not seem to act on the molecules 
which constitute thickness. When radiant heat gets below the surface of a body, it 
spends itself in doing mechanical work of another kind, viz. dilatation or expansion. 
77. However strong may be the reasons in favour of the air-current explanation, 
they are, I think, answered irrefragably by the phenomena themselves. An air-current 
produced by heat can cause the beam of a balance to rise, can drive a suspended index 
sideways, and, by a liberal assumption of eddies and reflections, can perhaps be imagined 
to cause these movements to take place sometimes in the opposite directions ; but as 
rarefaction proceeds these actions will certainly get less, and they will cease to be 
appreciable some time before a vacuum is attained : a point of no action or neutrality 
will be reached. But this neutral point should certainly be nearer a vacuum when a 
light body exposing much surface, such as pith, is under experiment than when the 
mass acted on is heavy like brass ; whereas in practice the contrary obtains. Pith and 
thin glass balances, which should be sensitive to highly attenuated air-currents, cease to 
respond to heat at a rarefaction of 7 millims. (30) and 45 millims. (66), whilst brass only 
ceases to be affected when the gauge and the barometer are appreciably level (43). 
But even could the phenomena up to neutrality be explained by air-currents, these 
are manifestly powerless to act after this critical point is passed. If a current of air 
within 7 millims. of a vacuum cannot move a piece of pith, certainly the residual air in 
a Sprengel vacuum should not do so ; and, a fortiori , the residual air in a chemical 
vacuum could not move a piece of platinum (55). 
It is, however, abundantly demonstrated that in all cases, after this critical point is 
reached, the repulsion by radiation is most apparent, and it increases in energy as the 
vacuum approaches perfection. 
78. Again, the movements not only reappear on passing a particular point of atmo- 
spheric density, but they take place in the opposite direction (31, 32, 33, 44, 46, 66). 
Thus in all cases, when the atmospheric density is between the neutral point and a 
vacuum, the action of a body hotter than the moving beam or index is to repel it, 
whilst the action of a colder body is to attract. Now it is very probable that were it 
not for the interference of air-currents the action in air of greater density would always 
be for the hot body to attract. This is actually the case in many experiments (27, 37, 
MDCCCLXXIY. 4 A 
