1876.] Repulsion resulting from Radiation. 



137 



formulag necessary for the redaction o£ the results. Being prevented by 

 other work from completing the experunents sufficiently to bring them 

 before the Eoyal Society prior to the close of the session, I hare thought 

 that it might be of interest were I to publish a short abstract of the 

 principal results I have obtained, reserving the details until they are 

 ready to be brought forward in a more complete form. 



In the early days of this research, when it was found that no move- 

 ment took place until the vacuum was so good as to be almost beyond 

 the powers of an ordinary air-pump to produce, and that as the vacuum 

 got more and more nearly absolute, so the force increased in power, it 

 was justifiable to assume that the action would still take place when 

 the minute trace of residual gas which theoretical reasoning proved to be 

 present was removed. The first and most obvious explanation therefore 

 was that the repulsive force was directly due to radiation. Further 

 consideration, ho^-ever, showed that the very best vacuum which I had 

 succeeded in producing might contain enough matter to offer considerable 

 resistance to motion. I have already pointed out that in some experi- 

 ments, where the rarefaction was pushed to a very high point, the torsion- 

 beam appeared to be swinging in a viscous fluid (194) ; and this at once 

 led me to think that the repulsion caused by radiation was indirectly due 

 to a difference of thermometric heat between the black and white sur- 

 faces of the mo^^ng body (195), and that it might be due to a secondary 

 action on the residual gas. 



On April 5, 1876, I exhibited at the Soire'e of the Eoyal Society an 

 instrument which proved the presence of residual gas in a radiometer 

 which had been exhausted to a very high point of sensitiveness. A small 

 piece of pith was suspended to one end of a cocoon fibre, the other end 

 being attached to a fragment of steel. An external magnet held the 

 steel to the inner side of the glass globe, the pith then hanging do-^n 

 like a pendulum, about a millimetre from the rotatiiag vanes of the 

 radiometer. By placing a candle at different distances off, any desired 

 velocity, up to several hundreds per minute, could be imparted to the fly 

 of the radiometer. Scarcely any movement of the pendulum was pro- 

 duced when the rotation was very rapid ; but on removing the candle, and 

 letting the rotation die out, at one particular velocity the pendulum set 

 up a considerable movement. Professor Stokes suggested (and, in fact, 

 tried the experiment at the time) that the distance of the candle should 

 be so adjusted that the permanent rate of rotation should be the critical 

 one for synchronism corresponding to the rate at which one arm of the 

 fly passed for each complete oscillation. In this way the pendulum was 

 kept for some time swinging with regularity through a large arc. 



This instrument proved that, at a rarefaction so high that the residual 

 gas was a non-conductor of an induction-current, there was enough matter 

 present to produce motion, and therefore to offer resistance to motion. 

 That this residual gas was something more than an accidental accom- 



YOL. XXV. L 



