MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
283 
dotted lines, without absorption and consequently without doing work. But in 
addition to light, the candle is radiating ultra-red dark heat rays. These in great 
measure are arrested by the glass, and raise its temperature. The inner surface of the 
bulb then becomes the surface on which molecular pressure is generated, 5 ' as shown 
Eig. 13. f 
by the shading at the side next the candle. This molecular disturbance presses on 
the mica vane which is in front of it, and drives it round in the direction of the arrows, 
as if it were subjected to a bombardment of small shot. The vane, in fact, may be 
said to be blown round by what may be likened to a wind, w r hich however is not 
molar but molecular, inasmuch as there is no wind in the sense of an actual trans- 
ference of gas from one part of the bulb to the other. 
277. One of the flies of the mica radiometer is much thinner than the other (273), 
and moves less readily to a candle. The reason is this : — 
Mica' is not perfectly transparent to light and heat ; a small portion is arrested, 
and is converted into heat of temperature. This causes molecular disturbance on the 
surface of the mica, and thereby assists the motion, as the pressure from the glass 
and from the mica acts in the same direction. The thick mica naturally obstructs 
more of the incident rays than does the thin mica, and it also holds more heat. More 
molecular pressure, therefore, comes from the thick than from the thin mica, and the 
thick will move better to a candle. A reference to fig. 13 will show why the mica 
radiometers will not rotate unless the candle is moved round to drive the vane before 
it. Imagine the fly moving slowly round in the direction of the arrows. When it 
has moved a few degrees further than is here represented, the advancing vane comes 
within the layer of pressure, and the retreating vane is emerging from it. Both will 
therefore be receiving the push, but in opposite directions. The more favourable presen- 
tation of the retreating over that of the advancing vane, will not carry the fly round, 
but it will only cause it to come to rest at such a point that the retreating vane is 
further from the candle than the advancing vane. If a certain amount of impetus 
* This may for shortness he called the driving surface. 
f This fig. and figs. 14, 20, 21, 24, 25, 26, 27, are simply explanatory, and are not to scale. The 
instrnment is shown in plan, bnt the candle is in elevation and much diminished in size. 
2 o 2 
