290 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
(276 to 280), I showed that the glass heated by the ultra-red rays became hot, and 
acted as the driving surface, generating molecular pressure and causing the sloping 
vanes to turn in the positive direction. I also showed that another action took place 
at the same time, the vanes got warm and became themselves sources of molecular 
pressure. Now the amount of molecular pressure thus generated depends on the 
capacity of the material of the vanes to absorb heat. Thin mica, owing to its thinness, 
will hold very little ; thick mica will hold more ; and aluminium, on account of its 
superior mass and good conducting power, will hold most. This extra capacity for heat 
causes more molecular pressure to proceed from the aluminium and thick mica than from 
the thin mica, and generates a proportionate amount of driving power on the surfaces 
of the vanes, turning them, as shown in paragraph 278 and fig. 14, in the positive 
direction, and supplementing the action of the equatorial ring of hot glass (302). 
306. When these radiometers are heated at the poles they go negatively, and when 
heated at the equator they rotate positively. On allowing them to cool, the flies 
being inside must lag behind the glass bulb in losing heat, the vanes will therefore 
be hotter than the outer bulb ; two forces are now at work tending to move the flies 
in opposite directions. These forces are — 
(a.) The increased temperatures of the vanes and consequent generation of molecular 
pressure from their surfaces. That from the inner surface need not be taken into 
account, but that from the outer surface produces pressure between it and the glass 
bulb, and tends to produce positive rotation. 
( b .) The radiation of heat from the inner fixed parts of the radiometer, such as the 
supporting stem, the needle, the upper glass tube, &c. This causes a stream of 
molecular pressure to strike against the inner surfaces of the vanes, and gives them 
a tendency to negative rotation. 
30 7. Of these two forces, the latter (6), will be practically constant in intensity, 
whatever be the material constituting the vanes. The former force (a) will, however, 
vary considerably, being very slight in thin mica and pith, and strong in aluminium. 
With thin mica and pith, therefore, force b will preponderate, and the fly will rotate 
negatively ; whilst with aluminium the force a will preponderate, and the fly will 
rotate positively. 
308. As the thickness of the mica increases, force a will gain strength, and the 
phenomena exhibited by the thick mica radiometer will be produced, namely, tendency 
to rotation in an undecided manner, first one way and then the other, according as 
one force or the other gains temporary ascendancy. 
In fig. 19, A and B, I have attempted to represent an aluminium and a mica 
radiometer, cooling, from a high temperature, the aluminium fly, under the pre- 
ponderating influence of force a, going positively, and the mica fly under the 
preponderating influence of force b, going negatively. 
