Repulsion accompanying Radiation. 91 



The inside of the tube ab is lined with a cylinder of copper 

 gauze, having holes cut in the centre for the passage of the sup- 

 porting thread dc and the index ray of light falling on the 

 mirror c, and holes at each end to admit of the plates /and g 

 being experimented with. A wire attached to the copper gauze 

 passes through a hole in the plate b, so as to give me electrical 

 access to the copper gauze lining. Under the most diverse elec- 

 trical conditions, whether insulated or connected with " earth/' 

 this apparatus behaves normally when exhausted. 



A further reason why electricity is not the cause of the move- 

 ments I have described is, that they are not only produced by 

 heat, but also by ice and cold bodies. Moreover 1 shall pre- 

 sently show that any ray of the spectrum, besides those red and 

 ultra-red rays which produce dilatation of mercury in a thermo- 

 meter, excite an electric current between antimony and bismuth 

 couples, and cause a sensation of warmth when falling on the 

 skin, will produce the effect of repulsion in a vacuum. It is 

 therefore to my mind abundantly proved that electricity, such 

 as we at present know this force^ is not a chief agent in these 

 attractions and repulsions, however much it may sometimes in- 

 terfere with and complicate the phenomena. 



I will now discuss Professor Osborne Reynolds's theory, that 

 the effects are the results of evaporation and condensation. 

 In my exhausted tubes he assumes the presence of aqueous 

 vapour, and then argues as follows : — " When the radiated heat 

 from the lamp falls on the pith, its temperature will rise, and 

 any moisture on it will begin to evaporate and to drive the pith 

 from the lamp. The evaporation will be greatest on that ball 

 which is nearest to the lamp ; therefore this ball will be driven 

 away until the force on the other becomes equal, after which 

 the balls will come to rest, unless momentum carries them 

 further. On the other hand, when a piece of ice is brought 

 near, the temperature of the pith will be reduced, and it will 

 condense the vapour and be drawn towards the ice." 



Professor Reynolds has tried an experiment with pith-balls at- 

 tached to a light stem of glass and suspended by a silk thread in a 

 glass flask. The exhaustion was obtained by boiling water in the 

 flask and then corking it up and allowing it to cool. The gauge 

 showed an exhaustion of from \ to f of an inch. The pith-balls 

 behaved exactly as I have already shown they do at that degree of 

 exhaustion, heat repelling and ice attracting. He found that the 

 neutral point varied according to whether air was present with 

 the aqueous vapour, or whether the vapour was pure water-gas. 

 Professor Reynolds states: — "From these last two facts it ap- 

 pears as though a certain amount of moisture on the balls was 

 necessary to render them sensitive to the heat. .... These ex- 



