1878.] 



resulting from Radiation. 



37 



At a pressure of 706 millionths of an atmosphere the direction 

 keeps the same as at 1 millim. in each case, but the speed is greater. 



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 positively as before, but with a little 

 diminished velocity. It is probable that some of the speed of the 

 disk has been quenched by the rapid movement of the vanes in the 

 opposite direction, as the author has previously shown* that the 

 viscosity of air at a rarefaction of 129 millionths of an atmosphere is 

 only a little less than its viscosity at the normal density, and hence 

 the vanes, at a speed of 100 revolutions a minute, must exert a con- 

 siderable drag on the opposite rotation of the disk. 



As the rarefaction increases above this point, the speed of both the 

 disk and vanes increases, till they exceed 600 revolutions a minute. 

 At the highest rarefaction attained (0"4 millionth of an atmosphere), 

 there is no apparent diminution in speed. 



These experiments have been repeated in a more elaborate series 

 with an apparatus of great complexity. It is impossible without 

 drawings to give an idea of the various arrangements by which data 

 are secured, but it may be mentioned that at each pressure observa- 

 tions can be taken on the velocity of rotation of the disk and vanes, 

 the viscosity of the residual gas, the repulsion exerted by a standard 

 candle on a black mica plate, and the appearance of an inductive 

 spark through a tube furnished with platinum wires. Different gases 

 can be experimented with, and a series of observations are given with 

 hydrogen gas as well as with air. 



The author concludes his paper by showing that the ordinary ideas 

 of a " vacuum " are very erroneous. Formerly an air-pump which 

 would diminish the volume of air in the receiver 1,000 times was said 

 to produce a vacuum. Later a " perfect vacuum " was said to be pro- 

 duced by chemical absorption and by the Sprengel pump, the test 

 being that electricity would not pass ; this point being reached when 

 the air is rarefied 100,000 times. According to Mr. Johnstone Stoney, 

 the number of molecules in a cubic centimetre of air at the ordinary 

 pressure is probably something like 1,000,000000,000000,000000 (one 

 thousand trillions). Now when this number is divided by 0*4 million, 

 there are still left 250,000000,000000 molecules in every cubic centi- 



* " Proceedings of the Koyal Society," November 16, 1876, No. 175, p. 305. 



