1878.] 



resulting from Radiation. 



35 



vanes, the author describes experiments with metal and other cones, 

 cylinders, and cnps. An abstract of some of these resnlts was given in 

 the Proceedings of the Royal Society for November 16, 1876, No. 175, 

 page 313, but it was stated that the "molecular pressure" theory 

 seemed incapable of accounting for some of the movements there 

 observed. Further experiments have now cleared up this difficulty, 

 and all the various movements are now seen to agree with this theory. 

 The movement which appeared most anomalous was the attraction 

 observed when a candle was allowed to shine on the hollow side of 

 a cone or cup-vaned radiometer, the light being screened off the re- 

 treating side. When light falls on the hollow side of a cone-shaped 

 vane, some is reflected and lost, but some is absorbed and is converted 

 into heat of temperature. Aluminium being a good conductor of heat, 

 and the thickness of metal being insignificant, it becomes equally 

 warm throughout, and a layer of molecular pressure is formed on each 

 surface of the metal. At a low exhaustion the thickness of this layer 

 of pressure is not sufficient to reach from the metal cone to the side 

 of the glass bulb. As the exhaustion increases, this layer extends 

 further from the generating surface, until at a sufficiently high 

 exhaustion the space between the side of the glass bulb and the ad- 

 jacent portion of the metallic cone is bridged over, and pressure is 

 exerted between the two surfaces. The more favourable presentation 

 offered by the cone causes the pressure to be greatest between the 

 glass bulb and the outside of the cone; the vanes are, therefore, 

 pressed round by a force which is really acting from behind, although 

 the movement looks as if the candle were attracting them. 



A radiometer, the fly of which is furnished with four bright alumi- 

 nium cups, rotates in the light as well as a flat vaned instrument 

 blacked on one side. Experiments are described with one of these, 

 instruments attached to the mercury pump, and the observations of 

 pressure and revolutions per minute under the influence of a standard 

 candle are tabulated and drawn as a curve, taking the rarefaction of 

 the air in millionths of an atmosphere as abscissas and the number of 

 revolutions a minute as ordinates. The curve, which is traced through 

 the dots representing observations, shows a gradual increase in the 

 sensitiveness of the instrument to light, up to about 50 millionths of 

 an atmosphere ; then the action keeps with little variation to about 

 30 millionths, thence it rapidly sinks, until at about 1 millionth it 

 is less than g^th of the maximum, and at 0"2 millionth the radiometer 

 refuses to turn even when five candles are put near it. 



The concluding portion of the paper is devoted to an examination 

 of the movements produced in highly rarefied gases, when thin mica 

 vanes suspended horizontally, and sloping like the vanes of a windmill, 

 are exposed to the action of a ring of platinum, placed just below 

 them, and rendered incandescent by a current of electricity. The 



d2 



