MR. W. CROOKES OK REPULSION RESULTING EROM RADIATION. 
273 
“ The illuminating power of these sources of light was compared by means of the 
Bunsen photometer. 
“ The light of the ordinary Bunsen flame could scarcely be measured, but was some- 
where about gwoth part of a candle, and of the luminous Bunsen flame about 10 
candles. 
“ The heating effects of these sources were compared by means of the thermo- 
electric pile and delicate astatic galvanometer. 
“ At a distance of one foot from the face of the pile, the deflection produced by the 
ordinary Bunsen flame was 46^°, and by the luminous Bunsen flame was 52°. 
“ These experiments clearly show that very little effect is produced by the radiation 
of obscure heat, but that the effect is due almost entirely, if not entirely, to light. 
“ These experiments show that the action on the selenium is due principally, if not 
entirely, to radiations belonging to the visible part of the spectrum.” 
These passages might almost have been written by myself to describe the repulsion 
produced by obscure and luminous radiation on scarlet selenium, so closely do they 
express the actions which I have been elucidating. 
259. Professor Adams concludes his paper by suggesting two hypotheses as possible 
explanations which may help as guides in further experiments, but which he thinks 
cannot be accepted as proved without further evidence. These hypotheses are : — 
(1.) That the light, falling on the selenium, causes an electro-motive force in it, in 
the same direction as the battery-current passing through it, the effect being similar to 
the effect due to polarization in an electrolyte, but in the opposite direction. 
(2.) That the light, falling on the selenium, causes a change on its surface, akin to 
the change which it produces on the surface of a phosphorescent body, and that in 
consequence of this change the electric current is enabled to pass more readily over 
the surface of the selenium. 
I think there is little doubt that my own experiments show that the second of these 
two hypotheses is more likely to be correct. The change on the surface, which 
diminishes electrical resistance in Professor Adams’s experiments, sets the molecules 
of the selenium into such a state of vibration that this vibration is communicated to 
the adjacent molecules of residual gas, and causes them to exert molecular pressure, 
which drives back the selenium. In comparing the two series of experiments, it must 
be borne in mind that in testing plates of crystalline selenium which were in the state 
most sensitive to light from Professor Adams’s point of view (236), I failed to detect 
any special action ; whilst the variety of selenium found to be most sensitive to the 
luminous rays is a scarlet, amorphous powder, precipitated from a cold solution, and 
very different, physically, from the crystalline variety produced by long continued heat 
on the ordinary vitreous selenium. 
2 N 
MDCCCLXXVIII. 
