PHYSICS. 109 



The vanes are fastened immovably to the glass support, and 

 near them moves a disk of mica suspended from its centre. 

 Exposed to light, the disk is caused to rotate rapidly by the 

 molecules projected from the black surface. 



Garbe has discussed the radiometer from the standpoint 

 of theory, assuming that in this case the sum of the moments 

 of different points relatively to a given axis is constant. 

 From this three conclusions follow : 1st, the containing en- 

 velope being free to move, and the vanes starting from rest, 

 when equilibrium is attained the rotations of the two will be 

 in opposite directions with a velocity inversely as their mo- 

 ments of inertia ; 2d, the vanes having a certain initial ve- 

 locity, if the apparatus be left free, either (a) the globe will 

 revolve in the opposite direction (in case the vanes revolve 

 more rapidly), (b) it will remain stationary (in case the veloc- 

 ity of the vanes remains constant), or (c) it will revolve in 

 the same direction (in case this velocity lessens) ; 3d, the ap- 

 paratus being inverted and its parts thus becoming fixed, 

 the action and reaction are equal, and no motion takes place 

 under any conditions. 



In a paper read to the Royal Society, Stoney has discussed 

 the method by which heat is transferred across the vacuous 

 spaces in Crookes's radiometers. He considered the laws un- 

 der which this transfer of heat takes place, and showed that 

 they are different from the already known laws of radiation, 

 convection, conduction, and contact. Hence he suggests that 

 this newly discovered mode of conveying heat should be call- 

 ed penetration. Numerous observations made more than thir- 

 ty years ago by De la Provostaye and Desains, but not then 

 understood, as well as more recent ones of Dulong and Petit, 

 and of Grove, are readily interpreted by means of these new- 

 ly discovered laws of heat. 



Stoney and Moss have experimented to determine the rela- 

 tion of the force which moves the radiometer and which they 

 call "Crookes's force" to the tension of the residual gas, 

 and the influence of variations in the distance between the 

 reacting surfaces. Thev find that with a residual tension of 

 five millimeters there is a reaction through a space of at least 

 ten millimeters ; that at distances of twenty to eighty mil- 

 limeters the force seemed to vary inversely as the tension ; 

 and that it appeared to be nearly independent of the dis- 



