234 



SCIENCE. 



[Vol. XVI. No. 403 



THE CAUSE OF MOTION IN THE RADIOMETER. 



It has been satisfactorily determined that the fly-wheel of the 

 radiometer will not revolve in a bulb from which all residual gas 

 has been removed, and that it will not i-evolve from the action of 

 light from which heat has been eliminated. These determinations 

 demonstrate that the motion of the fly must result in some way 

 from the action of heat on the residual gas in the bulb. But be- 

 yond this, demonstration has not yet gone. The theory generally 

 adopted by scientists is that the application of heat increases the 

 ▼ibration of the molecules of which the residual gas is constituted, 

 and that, heat being more readily absorbed by the black sides 

 than by the bright sides of the vanes of the fly-wheel, the mole- 

 »ules in contact with, or adjacent to, the black sides of the vanes 

 tiius become more heated, and vibrate with more force, than the 

 molecules on the opposite side ; and this increased vibration of the 

 ■lolecules against the black sides of the vanes pushes them around, 

 thus causing the rotation. 



Again, when the bulb has been heated and is cooling, the 

 blackened sides of the vanes cool more rapidly than the bright 

 sides, and consequently the vibration diminishes more rapidly on 

 that side, and, the vanes being pushed by the greater vibration on 

 the bright sides, the fly revolves backwards. This explanation is 

 plausible, but it has not been demonstrated; and, with our present 

 knowledge of molecule? and of their vibrations, its demonstration 

 is impossible. 



There is another explanation equally plausible, which, I think, 

 oan be demonstrated. It is that the lieat imparts to the residual 

 gas in the bulb an impetus to motion in a direction radiant or 

 tangential from the source of heat. If we suppose that the effect 

 «f heat on the tenuous matter in the bulb of the radiometer is to 

 impart to it an impetus to move in a direction away from the 

 source of beat, then all the results foUow which are supposed to 

 follow from molecular vibration. The particles of tenuous matter 

 in contact with the blackened sides of the vanes, receiving more 

 heat, would feel more intensely the impetus to radiant motion, 

 and would push the vanes around ; and again, when, in process of 

 «ooling, tlie gi'eater heat is on the bright sides of the vaues, the 

 ]»articles in contact with that side, having the greater impetus, 

 would push the vanes in the opposite direction. 



The general efl'ect of heat on matter is to increase its tenuity. 

 By expansion, commencing with the solid form, heat reduces 

 matter to the liquid, and then to the gaseous form, or, as in some 

 conditions of matter, causes it to pass directly from the solid to 

 t-he gaseous form without having become liquid; and. after reach- 

 ing the gaseous form, the further application of heat increases the 

 expansion and consequent tenuity, so far as has been observed, 

 indefinitely. But if we suppose that matter, after reaching a 

 •ertain degree of tenuity, begins to resist further expansion, the 

 effect of the application of heat to matter in that condition would 

 be necessarily to put the matter in motion in a direction radiant 

 from the source of heat; and this motion would continue until 

 the matter reached a temperature where no expansion was re- 

 quired. 



Motion, or increase of tension, which is merely resisted motion, 

 must result when heat is applied to matter; and, if the matter 

 resists expansion, it must move to a place where no expansion is 

 required, unless restrained by a countervailing force. 



After various efforts to find some means of determining whether 

 the motion imparted by the heat to the residual gas in the radi- 

 ometer was vibratory or radiant, it occurred to me that a simple 

 and satisfactory test could be found by applying radiant heat to 

 the bulb from all directions at the same time. If the motion was 

 caused by increased vibration, it could make no diS^erence from 

 ■what direction the radiation came; but if the motion was the re- 

 sult of an impetus imparted to the residual gas to move in a direc- 

 tion radiant from the source of heat, then, if the radiation came 

 from all directions at once, the impetus to motion in any given 

 direction would be counteracted, and the fly would not move. 

 Some crude experiments, such as could be made by an amateur 

 without skill or facilities, show very clearly, according to this 

 test, that the motion of the residual gas is radiant from the source 

 •of heat, and not mere vibration. 



A piece of iron pipe four inches in diameter was heated suffi- 

 ciently to cause the fly of the radiometer to revolve rapidly when 

 brought within three or four inches of it ; and the radiometer was 

 then suspended inside of the pipe, the fly being about three inches 

 from the top of the pipe. The fly at first revolved very rapidly, 

 but in a few seconds began to move slower, and at the expiration 

 of three minutes had come to a full stop. I had no thermometer 

 which could measure the temperature inside the pipe, but it was 

 sufficient to char paper. The radiometer lost some of its delicacy 

 by the heating, but the experiment was repeated with the same 

 radiometer, and with the same result. The heated pipe caused 

 the fly to revolve when the radiometer was brought neat it on the 

 outside, but stopped in a few minutes when the radiometer was 

 suspended inside of the pipe. 



To be certain that the injury to the radiometer in the first ex- 

 periment did not afiect the result, two new and very delicate 

 radiometers were obtained. These were suspended from a rod so 

 that while one hung in the pipe, the other would hang on the 

 outside two inches from the pipe. The pipe was again heated, but 

 more carefully, so as not to injure the radiometers; and they 

 were suspended, one on the inside, and the other on the outside, 

 of the pipe. The fly of the j:adiometer on the inside of the pipe 

 revolved at first very rapidly, but in a few seconds began to go 

 slower, and finally stopped ; while the one on the outside kept up 

 a steady motion, diminishing in speed very slowly as the pipe 

 cooled. The experiment was repeated; the radiometer which 

 had been on the outside of the pi,oe in the first experiment, being 

 placed this time on the inside. The result was the same : the fly 

 of the inside radiometer stopped, while the fly of the one outside 

 of the pipe continued to revolve. Unless there is something in 

 the nature of molecular vibration which has escaped my compre- 

 hension, it is impossible to account for the difference of effect in 

 these two radiometers, operated on from the same source of heat, 

 on any theory of vibratory motion in the residual gas; but it is 

 just what ought to result if the heat imparts to the residual gas 

 an impetus to motion radiant from the source of heat. 



The importance of this determination is the chief reason for 

 doubting its accuracy; but, surely, enough has been indicated to 

 induce those who have the requisite skill and facilities to con- 

 tinue the experimental work until the question is satisfactorily 

 settled. 



The proposition that heat applied to highly tenuous matter im- 

 parts an impetus to motion in a direction radiant from the source 

 of heat, explains the puzzling phenomena of comets' tails. " The 

 tail, or train," says Professor Young (Qeveral Astronomy, art. 

 737), "is a streamer of light which ordinarily accompanies a 

 bright comet, and is often found even in connection with a tele- 

 scopic comet. As the comet approaches the sun, the tail follows 

 it much as the smoke and steam from the locomotive trail after 

 it. But that the tail does not really consist of matter simply left 

 behind in that way, is obvious from the fact that, as the comet 

 recedes from the sun, the train precedes it instead of following. 

 It is always directed away from the sun, though its precise posi- 

 tion and form are to some extent determined by the comet's mo- 

 tion. There is abundant evidence that it is a material substance 

 in an exceedingly tenuous condition, which in some way is 

 driven from the comet and then repelled by some solar action." 



Professor Newcomb thus describes the phenomenon (Popular 

 Astronomy, pp. 4!3, 414): "It has long been evident that the tail 

 could not be an appendage which the comet carried along with it, 

 and this for two reasons, — first, it is impossible that there could 

 be any cohesion in a mass of matter of such tenuity that the 

 smallest stars could be seen through a million miles of it, and 

 which, besides, constantly changes in form; secondly, as a comet 

 flies around the sun in its immediate neighborhood, the tail ap- 

 pears to move from one side of the sun to another with a rapidity 

 which would tear it to pieces and send the separate parts flying 

 off in hyperbolic orbits, it the movement wei'e real. The inevita- 

 ble conclusion is, that the tail is not a fixed appendage to the 

 comet which the latter carries with it, but a stream of vapor ris- 

 ing from it like smoke from a chimney. As the line of smoke 

 which we now see coming from the chimney is not the same which 

 we saw a minute ago, because the latter has been blown away 



