Additional Phenomena of the Radiometer. 375 



a very remarkable character, announced by Mr. Crookes in 

 the < Proceedings of the Royal Society/ No. 172, p. 136. I 

 refer chiefly to the phenomena exhibited by " a small piece of 

 pith hanging down like a pendulum at the distance of about a 

 millimetre from the rotating vanes of the radiometer." It was 

 noticed that " scarcely any movement of the pendulum was 

 produced when the rotation was very rapid ; but at one parti- 

 cular velocity the pendulum set up a considerable movement." 

 At the suggestion of Professor Stokes, the candle which by its 

 light produced the rotation, was placed at the distance from 

 the radiometer for which a revolution of an arm of the fly 

 synchronized with a vibration of the pendulum. " In this 

 way the pendulum was kept for some time swinging through 

 a large arc." 



To account theoretically for these facts, I have, first, to direct 

 attention to that part of the article in the May Number where 

 it is said (p. 396) that " the incident light thus produces an 

 abnormal state of the atoms at and near the superficies of the 

 vane, analogous in some degree to the state of the superficial 

 atoms of a body electrified by friction." Since this was 

 written I have seen reasons (which will presently be adduced) 

 for concluding that the light, or heat in the radiant form, in- 

 cident on the vanes has the effect, after being transmuted into 

 heat of temperature, of so changing the relative positions of 

 the atoms in a superficial stratum as actually to induce the 

 electric state. According to the hydrodynamical theory of 

 electricity which I have proposed in the Philosophical Maga- 

 zine for October 1860, and in ' The Principles of Physics,' 

 pp. 544-546, the electrified state of a solid body is solely and 

 necessarily the result of a displacement of the atoms constitu- 

 ting a thin superficial stratum from ■ their normal positions. 

 According to the same theory such disturbance of the super- 

 ficial atoms is always accompanied by an interior gradation of 

 atomic density, in consequence of which setherial streams are 

 generated and maintained by the action of those aetherial vi- 

 brations to which are due, under normal circumstances, the 

 attractive and repulsive forces treated of in my communica- 

 tion in the September Number. It was, in fact, argued in the 

 " Theory of the Radiometer," given in the May Number, that 

 the state of the vanes is such as is here stated, although they 

 were not directly said to be electrified. I shall now assume 

 that they are in the condition of electrified bodies, and proceed 

 to inquire what consequences follow from this supposition rela- 

 tively to the new facts it was proposed to account for. 



Since the light, or heat in a radiant form, incident on the 

 vanes is converted into heat of temperature in greater degree 



