520 
MR. W. CROOKES ON ATTRACTION AND 
On applying the warm finger to the cool extremity of the balance-tube, either above 
or below the pith ball, the latter was repelled by it ; and by employing a lump of ice 
in the same way in each position the pith ball was attracted. The balance seemed to 
be as sensitive when one end was heated in this manner as it was when both ends were 
of the same temperature ; but the rise and fall were not to so great an extent, owing to 
the controlling action of the hot spiral. 
61. The following experiments were tried with a view of ascertaining the condi- 
tions of greatest sensitiveness. In all cases the balances were in a good Sprengel 
vacuum. 
A ball of ivory was balanced against a ball of brass on a straw beam ; the ivory was 
more sensitive to radiation than the brass. 
62. A pith ball and one of platinum of the same weight were balanced on a straw 
beam. The pith was very sensitive, being readily repelled by the finger ; but the 
platinum was sluggish, and required a spirit-flame to move it. 
63. Two pith balls of the same weight, one gilt and the other plain, were fixed to 
the ends of a straw beam ; they appeared equally sensitive to radiation. 
Two rectangular blocks of silver and bismuth, each weighing 4J grains, were 
balanced against one another on a straw beam*; they were each repelled by a warm 
body applied above or below. The bismuth was a little more sensitive than the 
silver to the action of heat, but it exposed a little more surface for the rays to im- 
pinge upon. 
64. A selenium ball was balanced against a copper ball: the selenium was more 
sensitive to radiation than the copper ; but I do not think it was more so than would 
be due to its more extended surface. When I allowed luminous rays, either from the 
sun or from artificial sources of light, to fall on the selenium, I could detect no special 
action which I could correlate with the action of light on selenium lately discovered by 
Mr. Willoughby Smith f. 
65. Two pieces of thin mica, each having a surface of half a square centimetre, were 
fastened to the end of a straw beam, one being horizontal and the other vertical. On 
applying slight warmth, each end was found to be very sensitive. The horizontal mica 
was easiest affected by the heat ; but it had not that great advantage over the vertical 
piece which might have been expected from the much greater surface exposed had the 
movement been due to air-currents. 
* These two metals were taken as representing nearly the two extremes of metallic conductors for heat 
and electricity, and also being the best (silver) and the worst (bismuth) in the list of “ vibrators ” given by 
Professor Fokbes in his “ Experimental Researches regarding certain vibrations which take place between 
metallic masses having different temperatures ” (Transactions of the Royal Society of Edinburgh, 1834, vol. xii. 
pp. 429-461). 
t Telegraphic Journal, vol. i. p. 78. 
