ME. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
115 
and the distance between the reacting surfaces may be very small with corresponding 
advantages of increased speed. 
Spiral flies of mica are much more sensitive than those of aluminium, but are less 
easy to make. A very sensitive spiral radiometer may be made by threading flat 
mica disks on a thin aluminium wire. The blackened surfaces must in all these 
instruments make an angle with the inner surface of the glass tube, or there will be 
no tangential action of the molecular pressure. 
440. The following experiments were tried with an aluminium spiral radiometer 
exhausted to the most sensitive point. It was completely immersed in water at 
20° C., and when quite still it was suddenly plunged into water at 60° C. Rapid 
negative* movement took place, continuing for a few minutes ; the fly then became 
still. When it was quiet in the hot water the radiometer was suddenly plunged 
into cold water (20° C.). Rapid positive movement took place, stopping when 
the temperature was uniform. 
The radiometer was now heated with a lamp over the upper half of the tube ; 
the spiral immediately rotated positively, and continued so to do till quite cold. 
Heat applied in a similar manner to the lower half of the radiometer case caused 
the spiral to rotate rapidly in the negative direction. Whilst so rotating nega¬ 
tively, a lighted candle was brought near ; the fly stopped and then turned positively, 
resuming its negative rotation when the candle was removed, and continuing it till 
the tube and fly were cold. When the lower half only of the radiometer was 
immersed in hot water, the upper half being allowed to remain cool by radiation 
to the air, the negative rotation of the fly continued at an almost uniform speed, 
not coming to rest till the water was nearly cold. 
These positive and negative movements of the spiral fly under the influence of 
partially applied heat are in strict accordance with the explanation already given, 
that when the glass case is heated its inner surface becomes the generating surface 
for the molecular pressure, which then acts as if it were a molar wind, repelling 
whatever happens to be within the sphere of its action (218, postscript). 
441. In the foregoing experiments the results have in all cases testified to the 
truth of the theory that the glass case of the radiometer is essential to the move¬ 
ment. It was of interest to ascertain whether the substitution of another kind of 
surface for that of polished glass would have any effect on the motion of the fly. 
A radiometer was accordingly constructed with a four-vaned mica fly, the disks 
being blacked on one side. The case of the radiometer was made so that it could readily 
be opened and sealed again when a change of contents had to be made. Round 
the inner surface of the case an equatorial band of aluminium was fixed a little wider 
than the diameter of the disks on the fly, and coated with lampblack on the inner 
side. Fig. 22 shows the instr um ent complete. It was attached to the pump and 
* In all cases tlie positive direction is that in which the fly moves when exposed to candle or day-light; 
the reverse direction being negative. 
Q 2 
