MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
375 
218. But the result of my observations of the sun on Dec. 13th being only equal to 
10'2 candles 6 inches off, is of course far below the real power of the unclouded mid- 
summer sun. I propose trying this experiment under more favourable circumstances. 
Meanwhile I have endeavoured to find what results in this direction have been obtained 
by other observers. The data given by different authorities vary considerably, as will 
be seen by the following Table : — 
Baugee* found that sun= 62,280 candles 1 metre off. 
WoLLASTonf „ ,, = 59,830 „ „ „ 
BecquerelJ „ „ = 50,000 „ „ „ 
Zollner§ „ „ =154,500 „ „ „ 
In these cases the sun was measured when at the highest point in a clear sky, and 
the optical difference alone was taken. In my case the light was very faint and hazy, 
and the total radiation, both of the sun and of the candle, was measured. I do not 
give my results as attaching the least importance to the actual figures, but simply as an 
illustration of the marvellous sensitiveness of the instrumental means at my disposal. 
I hope during the forthcoming summer to work out the subject more fully, and to be 
in a position to communicate to the Society many observations obtained with the torsion- 
balance, not only in photometry and the repulsion caused by radiation, but in other 
branches of science in which the possession of a balance of such incredible delicacy is 
likely to give valuable results. 
Postscript. — January 17, 1877. 
In the foregoing paper, pars. 126, 128, 155, 156, 157, 158, 159, 170, and 171, expe- 
riments are referred to which show that the repelling force appears to be different on a 
white or a black surface according as the radiation causing the movement is dark heat 
(from the fingers, hot water, hot glass, or copper below 250° C.) or the luminous rays. 
In commenting on these results, I gave what appeared at the time to be the most 
reasonable explanation. Twelve months’ research, however, has thrown much light upon 
these actions ; and the explanation afforded by the dynamical theory of gases makes what 
was a year ago obscure and contradictory, now reasonable and intelligible. 
In a preliminary notice submitted to the Royal Society, Nov. 16, 1876, and published 
in No. 175 of the ‘Proceedings,’ I gave the explanation of the movements of repulsion 
under the influence of radiation according to the dynamical theory of gases, first, I 
believe, used in this connexion by Mr. Johnstone Stoney. In this preliminary notice 
I brought forward experimental proof that the presence of residual gas is the cause of 
* Essai d’optique sur la gradation de la lumiere, 1729, p. 30. 
t Phil. Trans, vol. 89, 1799. 
+ Ann. de Chim. et de Phys. 3 serie, t. lxii. p. 34, 1861. 
§ Private letter to the author. 
