524 
MR. W. CROOKES ON ATTRACTION AND 
Experiment has, however, shown me that, whilst the action is in one direction in 
dense air, and in the opposite direction in a vacuum, there is (as I have already pointed 
out with the balances) an intermediate pressure at which differences of temperature 
appear to exert little or no interfering action. By experimenting at this critical pres- 
sure, and at the same time taking all the precautions which experience shows are 
necessary, it would seem that such an action as was obtained by Cavendish, Reich, and 
Baily should be rendered evident. 
74. Throughout the course of these investigations I have endeavoured to keep in 
my mind the possible explanations which may be given of the actions observed, and I 
have tried, by selecting some circumstances and excluding others, to put each hypothesis 
to the test of experiment. The most obvious explanation is that the movements of the 
beam, or of the horizontal index (69, 70), are due to the currents formed in the residual 
gas, which, theoretically, must be present to some extent even in those vacua which are 
most nearly absolute. 
In favour of this explanation it may be urged that a highly rarefied gas may be much 
more mobile than when it is denser, and therefore the more rapid impingement of its 
particles, when set in ascension by warmth, would increase their mechanical action. 
Increased momentum may counterbalance diminished number. 
That the residual gas in an air-pump vacuum is capable of exerting considerable 
mechanical action, may be assumed by the phenomena attending the passage of mete- 
orites through the upper regions of the atmosphere, their friction against the air at 
an average height of 65 miles above the earth’s surface raising them to incandescence, 
although at that height the attenuation of the air probably surpasses that of most 
artificial vacua. 
On the other hand, it is most difficult to believe that the residual air in a Sprengel 
vacuum, where the gauge and barometer are appreciably level, can exert, when gently 
warmed by the finger, an upward force capable of instantly overcoming the inertia 
of a mass of matter weighing several grains, and setting it in motion. It must be 
remembered that the upward current supposed to do this is simply due to the dimi- 
nished weight of a portion of the gas, caused by its increase in volume by the heat 
applied. 
75. Another argument in favour of the air-current explanation can be drawn from 
the fact that when a light beam, having equal weights of pith and platinum suspended 
at the ends, is sealed up in a Sprengel vacuum, the application of warmth below causes 
the pith to rise more readily than the platinum ( 62) ; the pith obviously offers a much 
more extended surface than the platinum does to the impact of air-particles. 
This, moreover, is not an isolated instance. Throughout the whole of these experi- 
ments the law appears to be that the force exerted is in proportion to the extent of 
surface exposed (48, 62, 63, 64, 65, 67) rather than in proportion to the mass. Much 
