370 
MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
and glass, although the glass shell of the apparatus was as thin as was consistent with 
strength to resist the atmospheric pressure. 
208. The action of radiation on surfaces of pith coated with thin layers of different 
substances is deserving of considerable attention. I have had an apparatus at work for 
several months past, in which six disks can’ be experimented with on the same beam 
and during the same exhaustion (similar to the arrangement described in par. 198, which 
held two disks). With this I have tried many hundred experiments, using the flame of 
a candle direct, or shaded by screens of water, alum, &c. The results are of much 
value, as showing that there is no definite connexion between the colour of a body and 
the mechanical action of radiation upon it. For instance, taking the movement of 
the lampblacked pith, under the influence of a standard candle, as 100°, 1 find that 
under the same conditions 
Precipitated silver 
O 
56 
Amorphous phosphorus 
40 
Sulphate of baryta 
37 
Red oxide of iron 
■ • , , 
28 
Scarlet iodide of mercury and copper 
> • „ 
22 
Lampblacked silver 
18 
White pith 
18 
Rock-salt 
6-5 
Glass 
6-5 
These are only a few of the results I have obtained. The experiments will occupy 
some time to carry out with the completeness which they deserve, and I therefore 
propose to defer any further mention of them to a subsequent paper. 
The Measurement of the Force. 
209. I have long endeavoured to devise some means of measuring the amount of 
force exerted by radiation, from a standard candle for instance at a foot off, on a 
measured surface of matter. The data given in pars. 199 and 200 are sufficient to 
enable the amount of force acting on the pith disks to be calculated indirectly; but I 
wished to have a means of measuring the force by as direct a method as we have of getting 
the weight of a ponderable body. I wanted an apparatus which will give me, at once, 
the pressure in grains which a ray of light exerts on a surface on which it may fall. 
Such an apparatus is shown in fig. 17. 
The principle of the construction is that of the torsion-balance first described by 
W. Ritchie, F.R.S., in 1830*. a b is a glass tube 13 - 5 inches long, 1 inch wide as far 
as a c, and T2 inch wide at c h. It is closed at the end and ground flat and polished 
at the end k, so as to be capable of being closed by a plate of glass cemented on. d e and 
* Phil. Trans, vol. 120 (for 1830), p. 215. 
