454 Prof. Challis on the Actions of the Radiometer, 



of the body, in order that it may be in equilibrium there must 

 be (as Poisson first indicated) a diminution of density through 

 a small superficial stratum in the direction towards the surface. 

 "Within this stratum the resultant atomic repulsion from within 

 to without is controlled by molecular attraction from without 

 to within. The latter force is due to sethereal currents extend- 

 ing beyond the boundary of the substance, being, in fact, the 

 force which gives rise to the phenomena of capillary attraction. 

 But when the state of the atoms in the superficial stratum 

 is violently disturbed, as by friction, the positions of equili- 

 brium of all the atoms, both superficial and interior, are in 

 some degree changed ; and in order that they may take new 

 positions of equilibrium it is necessary that there should be a 

 gradation of atomic density, and that the resultant atomic re- 

 pulsion on any atom in the direction of decreasing density 

 should be counteracted by the resultant molecular attraction 

 on the same in the contrary direction. According to the par- 

 ticular constitution of the substance, the new state of equili- 

 brium of the atoms is more or less persistent, and the substance 

 is more or less electric. Now, by the above-stated general 

 theorems, such actions, whether repulsive or attractive, are 

 accompanied by sethereal currents which in their aggregate 

 extend beyond the boundaries of the substance (much further 

 than those above mentioned to which capillary attraction was 

 attributed), and operate, in fact, in producing the mutual 

 attractions or repulsions which are observed to take place be- 

 tween electrified bodies. These views are treated of in detail 

 in ' The Principles of Physics,' pp. 505-555, and in the Philo- 

 sophical Magazine for October 1860. I have referred to them 

 here for the purpose of stating that the Theoretical Explana- 

 tion of the action of the Radiometer which I have given in the 

 Numbers of the Philosophical Magazine for May and Novem- 

 ber 1876, rests on the same principles — with, however, the 

 difference that the disturbance of the superficial stratum, in- 

 stead of being produced by friction, is considered to be due to 

 the difference of action of light, or radiant heat, on the black- 

 ened and bright surfaces of the vanes. It is proved that, under 

 these circumstances, sethereal currents are generated, the action 

 of which on the component atoms of the vanes is such as to 

 produce rotation in the direction in which the vanes of a freely 

 suspended radiometer are observed to move. For these rea- 

 sons I regard the motion of the radiometer as clue to a force 

 which is of the same kind as that which is induced in a body 

 electrified by friction. 



The explanation of the action of the Otheoscope is a simple 

 corollary from the above theory of the action of the radiometer. 



