40 



Mr. W. Crookes on 



[Nov. 21, 



It was suggested by Professor Stokes that a disk might be made to 

 revolve on its axis, and the author describes an instrument in which 

 this suggestion is carried out. The disk is horizontal, mounted like 

 the fly of a radiometer, and for lightness' sake is of mica, blacked 

 above. Fixed to the bulb above the disk are four flat pieces of clear 

 mica ; each extends from the side of the bulb to near the centre, and 

 ends below in a straight horizontal edge, leaving just space enough for 

 the disk to revolve without risk of scraping. The edge is in a radial 

 direction, and the plane of the plates is inclined about 45° to the 

 horizon, in the same direction for them all. Exposed to the light of 

 a candle, the rotation is against the edge. By slightly modifying this 

 form, the instrument becomes much more sensitive. Fig. 1 shows the 

 complete instrument ; a, a, are six vanes of copper foil, oxidised by 

 heating to redness in the air ; they are attached to arms, and are 

 inclined at an angle of 45° to the horizon. They are firmly fixed to 

 the support. Through the centre passes a needle-point, on which is 

 balanced a glass cup, carrying a thin clear disk of mica, b, b, freely 

 rotating about 1 millim. above the top edges of the copper vanes. 

 When exposed to light, the mica disk rotates with great speed against 

 the edges. The pressure which drives the movable fly round reacts 

 equally on the driving surface : by suspending both vanes and disk 

 independently on needle-points the effect of light causes them to 

 rotate in opposite directions. 



Whilst experimenting with the otheoscope it was found that, for a 

 given exhaustion, the nearer the reacting surfaces were together the 

 greater was the speed obtained. In the " Proceedings of the Royal 

 Society" for November, 1876,* the author described an apparatus by 

 which he was able to measure the thickness of the layer of molecular 

 pressure generated when radiation impinged on a blackened surface 

 enclosed in an atmosphere the rarefaction of which could be varied at 

 will. 



It was found that in this apparatus repulsion could be obtained at 

 ordinary atmospheric pressures. Observations are given at normal 

 pressure and at various degrees of rarefaction, with the driving and 

 moving surfaces separated 1, 2, 3, 4, 6, 8, and 12 millims. ; and dia- 

 grams of the resulting curves are shown when the atmospheric tension 

 and the force of repulsion are used as abscissae and ordinates. The 

 tables and curves show that the law of increase of the force with the 

 diminution of the distance between the disks does not remain uniform 

 at all rarefactions. At the lowest exhaustions the mean path of the 

 molecules of the attenuated gas is less than 1 millim., as rendered evident 

 by the force of repulsion diminishing rapidly as the distance increases. 

 At exhaustions higher than 9 millims. this condition alters, and as the 



* " Proc. Koy. Soc," No. 175, vol. xxv, p. 310. 



