246 
[April, 
Mechanical Action of Light. 
so that it feels decidedly warm to the hand. I cover the 
radiometer with it, and the rotation first stops, and then re- 
commences the reverse way. On removing the hot shade the 
reverse movement ceases, and normal rotation recommences. 
If, however, I place a hot glass shade over a pith radio- 
meter the arms at once revolve the normal way, as if I had 
exposed the instrument to light. The diametrically opposite 
behaviour of a pith and a metal instrument when exposed to 
the dark heat radiated from a hot glass shade is very striking. 
The explanation of the adtion is not easy, but it depends on 
the fadb that the metal is one of the best conductors of heat, 
whilst pith is one of the worst. 
One more experiment with this metallic radiometer. I 
heat it strongly with a spirit-lamp, and the arms spin round 
rapidly. Now the whole bulb is hot, and I remove the lamp : 
see what happens. The rotation quickly diminishes. Now 
it is at rest; and nowit is spinning round just as fast the 
reverse way. I can produce this reverse movement only 
with difficulty with a pith instrument. The adtion is due 
to the metal being a good conductor of heat. As it absorbs 
heat it moves one way ; as it radiates heat it moves the 
opposite way. 
At first I made these instruments of the very lightest mate- 
rial possible, some of them not weighing more than half a 
grain ; and where extreme sensitiveness is required lightness 
is essential. But the force which carries them round is quite 
strong enough to move a much greater weight. Thus the 
metallic instrument I have just experimented with weighs 
over 13 grains, and here is one still heavier, made of four 
pieces of looking-glass blacked on the silvered side, which are 
quickly sent round by the impadt of this imponderable 
agent, and flash the rays of light all round the room when 
the eledtric lamp is turned on the instrument. 
Before dismissing this instrument let me show one more 
experiment. I place the looking-glass and the metal radio- 
meter side by side, and, screening the light from them, they 
come almost to rest. Their temperature is the same as that 
of the room. What will happen if I suddenly chill them ? 
I pour a few drops of ether on each of the bulbs. Both 
instruments begin to revolve. But notice the difference. 
Whilst the movement in the case of the metal radiometer 
is diredt, that of the looking-glass instrument is reverse. 
And yet to a candle they both rotate the same way, the 
black being repelled. ■ 
Now, having found that this force would carry round a 
comparatively heavy weight, another useful application 
