Mr. AV r . Crookes on the Radiometer. 473 



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 suspend] ng- fibre and re fleeting-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 with the vacuum. This apparatus 

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

 proved that when light falls on the black surface molecular pressure 

 is set up, whatever be the degree of exhaustion. At the atmo- 

 spheric pressure this disturbance can only be detected when the 

 mica screen is brought close to the black surface, and it is in- 

 appreciable 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 pressure is re- 

 presented 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 pressures are represented by 30, 34, 

 and 40, for spaces 3, 2, and 1 millim. When the gauge and ba- 

 rometer are level, the action is so strong that the candle has to be 

 moved double the distance off, and the pressures when the inter- 

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

 107. A large series of observations have been taken with this 

 apparatus, with the result not only of 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 this research. Among the latter may be men- 

 tioned the speculations in which I indulged as to the pressure of 

 sunlight on the earth. 



Hitherto most of my experiments had been carried on with bad 

 conductors of heat. To get the maximum action of a radiometer it 

 appeared necessary that no heat should pass through to the back 

 surface, but that all should be kept as much as possible on the 

 surface on which the light fell *. At first I used pith ; but since 



* I have already shown, that when a ray of light from any part of the spec- 

 trum falls on a black surface the ray is absorbed and degraded in refrangibility, 

 warming the black surface and being emitted as radiant heat. In this sense 

 only can the repulsion resulting from radiation be called an effect of heat. 



