88 Mr. W. Crookes on Attraction and 



rays fall on the pith. The movement of repulsion is energetic. 

 The iodine solution, interposed, cuts off apparently none of the 

 action. The alum plate cuts off a considerable amount, but by 

 no means all. On uniting the alum and the iodine solution the 

 whole of the spectrum is obliterated, and no action is produced, 

 whatever be the ray which would, were it not for this double 

 sifting, fall on the pith. 



Throughout the course of these investigations, which have 

 occupied much of my spare time for some years, I have endea- 

 voured to keep in my mind the possible explanations which may 

 be given of the actions observed ; and I have always tried, by 

 selecting some circumstances and excluding others, to put each 

 hypothesis to the test of experiment. 



The most obvious explanation is, that the movements are due 

 to the currents formed in the residual gas which theoretically 

 must be present to some extent even in those vacua which are 

 most nearly absolute. 



Another explanation is, that the movements are due to elec- 

 tricity developed on the moving body or on the glass apparatus 

 by the incident radiation. 



A third explanation has been put forward by Professor 

 Osborne Reynolds, in a paper which was read before the Royal 

 Society on June 18th last. He considers the results to be due 

 to evaporation and condensation. 



I will discuss these explanations in order. 



First, the air-current theory. However strong may be the 

 reasons in favour of this explanation, they are, I think, answered 

 irrefragably by the phenomena themselves. It is most difficult 

 to believe that the residual air in a Sprengel vacuum, when the 

 gauge and barometer are level, can exert, when gently warmed 

 by the finger, an upward force capable of instantly overcoming 

 the inertia of a mass of matter weighing 20 or 30 grains. It 

 must be remembered that the upward current supposed to do 

 this is simply due to the diminished w 7 eight of a portion of the 

 gas caused by its increase in volume by the heat applied. 



An air-current produced by heat may possibly cause the 

 beam of a balance to rise, may drive a suspended index side- 

 ways, and by a liberal assumption of eddies and reflections, 

 may perhaps be imagined to cause these movements to take 

 place sometimes in the opposite directions ; but as rarefaction 

 proceeols these actions must certainly get less, and they will 

 cease to be appreciable some time before a vacuum is attained : 

 a point of no action or neutrality will be reached. But this 

 neutral point should certainly be nearer to a vacuum when a 

 light body like pith, exposing much surface, is under experiment, 



