310 Mr. W. Crookes on the Radiometer, [Nov. 16, 



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 condensa- 

 tion theory, as this requires that the light should shine intermittently on 

 the black surface in order to keep up continuous movement. 



By cutting a thin plate of aluminium into the form of a spiral, then 

 drawing it oat corkscrew fashion, blacking the upper surface and sus- 

 pending it on a point, a spiral radiometer is made, which rotates like a 

 screw on exposure to light. Here also the black surface need never be 

 in darkness, the pressure acting continuously between the black side of 

 the spiral and the cylindrical tube in which it is mounted. 



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 reflecting mirror, as described in my previous papers. At one 

 end of the beam is a disk 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 mth the vacuum. This 

 apparatus is very sensitive and gives good quantitative results. It has 

 proved that w^hen light falls on the black surface molecular pressure is set 

 up, whatever be the degree of exhaustion. At the atmospheric pressure 

 this disturbance can only be detected when the mica screen is brought 

 close to the black surface, and it is inappreciable when the screen is 

 moved away. As the barometer-gauge rises, the thickness of the layer of 

 disturbance increases. Thus, retaining the standard candle always the 

 same distance off, when the gauge is at 660 millims., the molecular pres- 

 sure is represented by 1 when the space separating the screen from the 

 black surface is 3 millims., by 3 when the intervening space is reduced 

 to 2 millims., and by 5 when the space is 1 millim. With the gauge 

 722 millims. high, the values of the molecular pressure for the spaces of 

 3, 2, and 1 millim. are respectively 3, 7, and 12. "When the gauge is at 

 740 millims., the corresponding values for spaces of 3, 2, and 1 millim. 

 are 11, 16, and 23. With the gauge at 745 millims., the molecular pres- 

 sures are represented by 30, 34, and 40, for spaces 3, 2, and 1 millim. 

 When the gauge and barometer are level, the action is so strong that the 

 candle has to be moved double the distance off, and the pressures when 

 the intervening spaces are 12, 6, and 3 millims. are respectively 60, 86, 



