Feb. I, 1877] 



NATURE 



301 



which constitutes the force can be reflected from a 

 plane surface in such a manner as to change its direc- 

 tion. A two-disc radiometer was made having flat opaque 

 mica discs blacked on one side. In front of the black 

 surface of the mica and about a millimetre off, is fixed a 

 larger disc of thin clear mica. On bringing a candle near, 

 the molecular pressure streaming from the black surface 

 is caught by the clear plate and thrown back again, 

 causing pressure behind instead of in front, and the re- 

 sult is rapid rotation in the negative direction, the black 

 side now moving towards the light. 



The above actions can be explained on the *' evapo- 

 ration and condensation " theory, as well as by that of 

 molecular movement. I therefore devised the following 

 test to decide between these two theories. A radiometer 

 has its four discs cut of very clear and thin plates of mica 

 and Jhese are mounted in a somewhat large bulb. At 

 the side of the bulb in a vertical plane, a plate of mica, 

 blacked on one side, is fastened in such a position that 

 each clear vane in rotating shall pass it, leaving a space 

 between of about a millimetre. If a candle is brought 

 near, and by means of a shade the light is allowed to fall 

 only on the clear vanes, no motion is produced ; but if the 

 light shines on the black plate the fly instantly rotates as 

 if a wind were issuing from this surface, and keeps on 

 moving as long as the light is near. This could not 

 happen on the evaporation and condensation theory, as 

 this requires that the light should shine intermittently on 

 the black surface in order to keep up continuous move- 

 ment. 



The experiments with the double radiometer of different 

 sizes showed that the nearer the absorbing surface was to 

 the glass the greater was the pressure produced. To test 

 this point in a more accurate manner a torsion balance 

 was fitted up with a glass suspending fibre and a reflecting 

 mirror. At one end of the beam is a disc of roasted mica 

 blacked on one side. In front of this black surface, and 

 parallel to it, is a plate of clear mica so arranged that its 

 distance from the black surface can be altered as desired 

 at any degree of exhaustion without interfering with the 

 vacuum. This apparatus has proved that when light falls 

 on the black surface molecular pressure is set up, what- 

 ever be the degree of exhaustion. A large series of obser- 

 vations have been taken with it with the result of not only 

 supplying important data for future consideration, but of 

 clearing up many anomalies which were noticed, and of 

 correcting many errors into which I was led at earlier 

 stages of the research. Among the latter may be men- 

 tioned the speculations in which I indulged as to the 

 pressure of sunlight on the earth. 



I now tried similar experiments to the above, using the 

 best conductors of heat instead of the worst. A radio- 

 meter, the fly of which is made of metallic plates perfectly 

 flat and lampblacked on one side, is much less sensitive to 

 light than one of mica or pith ; but, as I proved in a paper 

 sent to the Royal Society in January last year, it is more 

 sensitive to dark heat, which indeed causes the black face 

 of a metal radiometer rapidly to rotate in a negative direc- 

 tion, the black continuing to advance until the tempera- 

 ture has become uniform throughout ; but as soon as the 

 source of heat is removed the fly commences to revolve 

 with rapidity the positive way, the black retreating as it 

 would if light shone on it. 



Experiments with discs of aluminium, mounted diamond- 

 wise and turned up and folded at different angles, show 

 that shape has even a stronger influence than colour. 

 A convex bright surface is strongly repelled, whilst a con- 

 cave black surface is not only not repelled by radiation, 

 but is actually attracted. 



Carefully-shaped cups of gold, aluminium, and other 

 metals, have been tried, as well as cones of the same 

 materials. If a two-disc, cup-shaped radiometer, facing 

 opposite ways and both sides bright, is exposed to a 

 standard candle 3*5 inches off, the fly rotates continuously 



at the rate of one revolution in 3 '3 7 seconds. A screen 

 placed in front of the concave side so as to let the light 

 shine only on the convex surface, the latter is repelled, 

 causing continuous rotation at the rate of one revolution 

 in 7*5 seconds. When the convex side is screened off so 

 as to let the light strike only on the concave side, con- 

 tinuous rotation is produced at the rate of one revolution 

 in 6'95 seconds, the concave side being attracted. 



These experiments show that the repulsive action of 

 radiation on the convex side is about equal to the attrac- 

 tive action gra.dation on the concave side, and that the 

 double speed with which the fly moves when no screen is 

 interposed is the sum of the attractive and repulsive 

 actions. 



With a two-disc, cup-shaped aluminium radiometer as 

 above, lampblacked on the concave surfaces, the action 

 of light is reversed, rotation taking place, the bright con- 

 vex side being repelled and the black concave attracted. 

 When the light shines only on the bright convex side no 

 movement is produced, but when it shines only on the black 

 concave side this is attracted, producing rotation. 



Light shining on a cup-shaped radiometer similar to 

 the above, but having the convex side black and the con- 

 cave bright, causes it to rotate rapidly, the convex black 

 being repelled. No movement is produced on letting the 

 light shine on the bright concave surface, but good rota- 

 tion is produced when only the black convex surface is 

 illuminated. 



With a cup-shaped radiometer like the above, but 

 blacked on both sides, a candle causes rapid rotation, 

 the convex side being repelled. On shading off the light 

 from the concave side the rotation continues, but much 

 more slowly ; on shading off the convex side the concave 

 is strongly attracted, causing rotation. 



Radiometers have also been made with cups and cones 

 of plain mica, roasted mica, pith, paper, &c., and they 

 have been made either plain or blacked on one or both 

 surfaces. These have also been balanced against each 

 other, and against metal plates, cups, and cones. The 

 results are of considerable interest but too complicated to 

 explain without numerous diagrams. The broad facts 

 are contained in the above selections from my experi- 

 ments. 



Some of the phenomena produced by the action of 

 light on the cup-shaped vanes of a radiometer may be 

 explained on the assumption that the molecular pressure 

 acts chiefly in a direction normal to the surface of the 

 vanes. A convex surface would therefore cause greater 

 pressure to be exerted between itself and the inner surface 

 of glass than could a concave surface. But it is not easy 

 to see how such an hypothesis can explain the behaviour 

 of those instruments where the action of the bright convex 

 surface more than overcomes the superior absorptive and 

 radiating power of the concave black surface ; and the 

 explanation appears to fail to account for the powerful 

 attraction which a lighted candle is seen to exert on the 

 concave surfaces in other instruments. 



These experiments, interpreted by the light of the dyna- 

 mical theory given in my last communication, explain 

 very clearly how it was that I obtained such strong 

 actions in my earlier experiments when using white pith, 

 and employing the finger as a source of heat ; and 

 how it hp-ppened that I did not discover for some time 

 that dark heat and the luminous rays were essentially 

 different in their actions on black and white surfaces. 

 Rays of high intensity pass through the glass bulb with- 

 out warming it ; they then, falling on the white surface, 

 are simply reflected off again ; but, falling on the black 

 surface, they are absorbed, and raising its temperature, 

 produce the molecular disturbance which causes motion. 

 Rays of low intensity, however, do not pass through the 

 glass to any great extent, but being absorbed, raise its 

 temperature. This warmed spot of glass now becomes 

 the repelling body through the intervention of the mole- 



