53^ 



NATURE 



[April lo, 1879 



At a pressure of 80 millims. the disk does not rotate. 

 The vanes rotate positively but slowly. 

 ■ At 19 millims. no movement whatever takes place. 

 The disk and vanes are as still when the wire is heated 

 as when it is cold. 



At 14 millims. the disk remains stationary. The vanes 

 move slowly in the negative direction. 



At I millim, the disk rotates in the positive direction 

 slowly, whilst the vanes rotate negatively rzxhtr fast ; the 



Fig. 13. 



disk commences to rotate in the same direction as" the 

 vanes at a speed of three revolutions a minute. 



(At low exhaustions I speak of millimetres of pressure, 

 but at high exhaustions I prefer to count in^millionths 01 

 an atmosphere.) 



At a pressure of 706 millionths of an atmosphere the 

 direction keeps the same as at i millim. in each case, but 

 the disk makes ten revolutions and the vanes forty revo- 

 lutions a minute. 



At 294 millionths, the speed of the disk and vanes is 



exactly alike, both rotating together in the same direction* 

 Up to this pressure and at some distance beyond, the 

 vanes have been gradually diminishing whilst the disk 

 has been increasing in speed. At a pressure of 141 

 millionths the disk rotates rapidly, positively, but the 

 vanes do not rotate at all. At a little higher exhaustion 

 than the last, viz., at 129 millionths, a great change is 

 observed. The vanes which were still now rotate in the 

 positive direction at a speed of 100 revolutions a minute, 

 whilst the disk rotates as before, but with a little 

 diminished velocity. I have previously shown, in a paper 

 to the Royal Society, that the viscosity of air at a rarefac- 

 tion of 129 millionths of an atmosphere is only a little less 

 than its viscosity at the normal density, and hence it is 

 certain that the vanes at a speed of 100 revolutions a 

 minute exerts a considerable drag upon the disk when it 

 rotates in the opposite direction. 



As the rarefaction increases above this point, the speed 

 of both the disk and vanes increases till those of the 

 latter exceed- 600 revolutions a minute. 



To carry these experiments to a much higher exhaustion 

 it was necessary to modify the apparatus. The complex 

 apparatus I now employed is shown at Fig. 15. Only 

 the upper part of the pump ab\s shown. It has five fall 

 tubes and is fitted with a small radiometer, c, and a 

 McLcod measuring apparatus, de, to enable the degree 



Fig. 14. 



of exhaustion in the apparatus to be ascertained. The 

 phosphoric anhydride, for absorbing aqueous vapour, is 

 contained in the horizontal tube f. In order as far as 

 possible to prevent the passage of mercury vapour, three 

 long narrow tubes gg are introduced between the pump 

 and the apparatus to be exhausted ; the one nearest the 

 pump is filled with precipitated sulphur, the centre tube 

 contains metallic copper reduced from its oxide, and the 

 third tube phosphoric anhydride. At h is a vacuum-tube 

 containing aluminium wires, and having a capillary bore 

 for examining the spectra of the residual gas. An induc- 

 tion coil and battery are connected with the tube by wires. 

 From the tube h two tubes branch off, one of them, /, 

 leads to the "viscosity" apparatus contained in the 

 case k, and the other, /, goes to the apparatus to be 

 exhausted. 



The apparatus s, containing the rotating disk and 

 vanes, is sealed to the tube j. The platinum ring is 

 ignited by the battery /. On the top of the ring rests a 

 disk of mica, H, lampblacked on the upper surface ; this 

 cuts off direct radiation from the hot ring, and diffuses 

 the heat somewhat over the surface of the black mica. 

 Instead, therefore, of the molecular pressure starting from 



