1878.] 



Repulsion resulting from Radiation. 



41 



gauge approaches barometric height, the molecular pressure tends to 

 become uniform through considerable distances, the mean path of the 

 molecules now being comparable with the greatest distance separating 

 the surfaces between which they act. 



A similar apparatus to the one in which the last experiments were 

 tried was used to measure the action at pressures at and approaching 

 atmospheric. At pressures between atmospheric and 210 millims., the 

 first action is very faint repulsion, immediately followed by strong 

 attraction. The attraction then begins to decline, until at 15 millims. 

 pressure it disappears. At the same time the repulsion, which begins 

 to be apparent at 250 millims., increases as the attraction diminishes. 

 The author considers that the attraction is the result of air- currents, 

 caused by the permanent heating of the surface in front of the move- 

 able disk. 



The paper concludes with experiments undertaken to measure the 

 amount of repulsion, using a horizontal torsion balance,* on the prin- 

 ciple of Ritchie's, in which the force of repulsion is balanced by the 

 torsion of a fine glass fibre. The pan of the balance is a clear mica disk, 

 and a similar disk is fastened to the tube in which the beam oscillates. 

 This fixed disk is lampblacked on the upper side, and beneath is a spiral 

 of platinum wire, connected with terminals sealed through the side of 

 the tube. When the spiral is ignited by a constant electric current, the 

 blacked mica disk fixed above it becomes heated, and the molecular 

 pressure thereby generated between it and the mica pan causes the 

 latter to rise. The glass thread attached to the beam is thus twisted, 

 and by means of a graduated circle the number of degrees through 

 which the thread has to be turned in order to bring the beam back to 

 equilibrium is noted. This gives a measurement of the pressure exerted, 

 in torsional degrees, and these are converted into grains by ascertaining 

 how many torsional degrees correspond to a known weight. A ray of 

 light reflected from a mirror in the centre of the beam is used as an 

 index, being brought back to zero at each experiment. The author 

 gives in a table, and also shows in the form of a curve, the results ob- 

 tained with this apparatus, giving the force of molecular pressure in 

 grains weight at exhaustions varying between 2,237 and 0'7 millionths 

 of an atmosphere. 



* For a description of this form of torsion balance, see the author's paper, " Phil. 

 Trans.," 1876, vol. clxvi, p. 371. 



