378 Explanations of . [additional Phenomena of the Radiometer. 



over, be employed to test the truth of the a priori theory, 

 because, it the latter he true, it ought to be capable of giving 

 reasons for the experimental facts on which the empirical 

 theory is founded. The phenomena of the radiometer have 

 attracted my attention as being peculiarly adapted to be of 

 service in carrying on researches as to the laws of physical 

 force. I have, in fact, been able, by applying to them the 

 foregoing theoretical discussion, to settle an important physical 

 question. In my Theory of Electricity I had occasion to speak 

 of electricity-radiants, but had no means of determining in 

 what order they stood relatively to heat-radiants. The ten- 

 dency of the explanations given in this communication is to 

 prove their identity, at least, as to kind, if not in degree, 

 with heat-radiants, and to show that, like these, they are sub- 

 ject to the law of exchanges. On the assumption that such 

 is their character, I propose to conclude this communication 

 with giving a theory of the induction of electricity more pre- 

 cise than that contained in arts. 12-16 of the " Theory of 

 Electric Force " in the Phil. Mag. for October 1860, or that 

 in pp. 521-531 of ( The Principles of Physics.' 



This theory of electrical induction rests essentially on a cer- 

 tain state of the atoms at and near the surface of a body (first 

 recognized by Poisson), according to which through an ex- 

 tremely small thickness the density of the atoms increases from 

 the surface towards the interior. The fulfilment of this con- 

 dition is necessary for the equilibrium of the atoms so situated. 

 It will now be supposed that the undulations of the aether cor- 

 responding to the radiants we are concerned with can traverse 

 substances freely without undergoing transmutation. Such 

 undulations pertain to the electrified state of a solid body, 

 and, in conjunction with the forces of atomic repulsion, main- 

 tain by attractive action the interior gradation of density 

 which is a necessary condition of that state. When a body in 

 a neutral state is brought into the neighbourhood of an elec- 

 trified body, those radiants of the latter that are incident upon 

 it pass freely through it, and consequently traverse the whole 

 of the thin superficial stratum of varying atomic density spoken 

 of above. Now when aether in motion, whether the motion be 

 vibratory or steady, permeates a collection of atoms varying 

 as to the number in a given space from point to point, there 

 is always an increment of the velocity of the aether, and there- 

 fore a decrement of its condensation, towards the parts of 

 greater atomic density. The variation of pressure consequent 

 upon this variation of condensation acts upon the atoms and 

 displaces them from their normal positions ; and this, as we 

 have already argued, suffices to induce electricity. It is, how- 



