1884.I On Electricity and its Present Applications. 407 
angles from a slender shaft of the same metal, and kept in 
position within a glass vessel from which the air is ex- 
hausted. The plates are bright on one side and blackened 
on the other. When exposed to light or to heat the shaft 
moves with a constant rotary motion, the most probable 
cause of which seems to be the impetus given by the undu- 
lations of light or heat impinging upon the bright sides of 
the metallic discs, while their dark sides attract and absorb 
its material (?) constituents, and are thus drawn towards it, 
so that the bright and the dark sides, thus differently acted 
on by the light, co-operate together in producing the constant 
rotation. The friction or resistance of the atmosphere being 
removed, and the pivots on which the axis moves being very 
fine, the slightest possible force is sufficient to cause the 
motion. When we see the tremendous force exerted by so 
rare and delicate a substance as atmospheric air, when put 
in rapid motion, we need not be surprised at the effect pro- 
duced in this instrument by the momentum caused by the 
immeasurably greater velocity even of what are so impon- 
derable as the waves of light. But strange to say, when the 
vessel is made as empty as it is possible to be made, by 
chemical means, the motion becomes turned in the opposite 
direction — a phenomenon for which, though it may be diffi- 
cult to account, I would venture to give the following 
explanation : — As long as any matter remained diffused, 
probably in the form of aqueous vapour in the Torricellian 
vacuum of the glass vessel, its affinity or attraction for the 
light would draw the motion in the direction that has been 
described; but when this element in the case had been 
withdrawn, the omnipresence of Electron, with his more 
rapid motion and his preference for the bright side of the 
metallic discs, would associate the light with himself and 
attract the bright side of the disc, and thus draw them in 
the opposite direction from what the light itself did. By 
entering into the vacuum in the glass vessel, and being 
released from the compression of the atmosphere, whatever 
material atoms might be in union with Electron would have 
their mutual repulsion greatly augmented, and their undu- 
lations being thus driven against the bright sides of the 
discs, so as to cause their movement. 
There may be other ways of explaining the action of this 
curious and suggestive piece of apparatus, but they all tend 
to confirm the idea of the existence of matter in a radiant 
and inconceivably minute condition, diffused through space, 
— in fact, the old and never yet abandoned idea of a uni- 
versal ether. 
