344 
ME. W. CEOOKES ON EEPULSION RESULTING EEOM EADIATION. 
expressed. Brought into a uniformly heated space, the instrument comes to rest as 
soon as it has acquired the temperature of the space ; but brought into a uniformly 
lighted space (151) it continues revolving as long as the light lasts. — Received January 
10, 1876.] 
152. The following experiments were tried with a very sensitive radiometer in a 2-inch 
bulb. The moving part (the “ fly ”), consisting of glass arms, pith disks, and steel point, 
weighed only 08 grain. It was exhausted with a charcoal reservoir attached (131). 
A standard candle, placed 2 inches from the centre, made the arms spin with a velocity 
of 4 revolutions per second ; with the candle 4 inches off the velocity was 10 revolutions 
in 11 seconds. In the full sunshine of a November day the speed was too great to 
count. Nothing was visible but an undefined nebulous ring, which became more or less 
distinct as the sunlight increased and diminished owing to passing clouds. This speed 
was kept up for more than an hour ; indeed there appears no reason why it should ever 
diminish as long as light of uniform intensity shines on it. 
153. The same radiometer was tried by the light of a candle, 4 inches off, behind 
different screens, with the following results : — 
1 candle, 4 inches off, naked flame 
behind thin white glass 
„ thick plate glass 
„ purple glass . 
„ dark red glass . 
„ pink glass . . 
„ light yellow glass 
„ blue glass . . 
,, orange glass . . 
r“ eclipse” glasses (blue and' 
■j orange, almost opaque 
l to daylight) ' 
7-| millims. of water in cell 6*0 
(solution chromate of pot-) g.Q 
1 ash, 7^ millims. thick J 
(solution bichromate of pot- ) . q 
1 ash, 7-§ millims. thick J 
(clear plate of alum, 
l millims. thick . . 
IT sec. for 1 revolution. 
1-3 
1- 4 
15 ,, ,, 
1’5 ,, ,, 
1-6 
2T 
2- 5 
^ ° 55 55 
2- 7 
^ • 55 55 
3- 0 
4- 0 
5 } 
9-0 
(solution chloride of cobalt,) q 
1 7-J millims. thick . J 
sol. ferrocyanide of potas-)^g,Q 
sium, 7£ millims. thick 
