MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
327 
dark or slightly luminous heat-rays — the fingers, a hot glass rod, hot copper, or a candle- 
flame being used as the source of radiation. I quote the following sentencefrom par. 94 : — 
“ Although I most frequently speak of repulsion by heat, and in illustrating any of the 
results obtained I generally use either the fingers or the flame of a spirit-lamp as a 
convenient source of radiation, it must be clearly understood that these results are not 
confined to the heating-rays of the spectrum, but that any ray, from the ultra-red to the 
ultra-violet, will produce repulsion in a vacuum. I have already mentioned this fact in 
my first paper (58, 68).” 
A few experiments were tried on the effect of radiation on surfaces the reflecting or 
radiating power of which was modified by coating them with various substances. In 
par. 102, after describing a torsion-apparatus for quantitative work, I mention that the 
surfaces of pith, as thin as possible, may be coated with lampblack or silver, or may retain 
their natural surface ; in par. 108 I state, as the result of a long series of experiments with 
this apparatus, that “ the conducting-power for heat and condition of the surface (whether 
coated with lampblack or consisting of polished metal) of the body on which radiation 
falls materially influence the movements.” In par. 112 I again refer to the effect caused 
by the physical condition of the surface ; and further on, in par. 116, 1 say, “ A series of 
experiments have been tried with a view to ascertain what influence the state of the surface 
of the substance submitted to radiation has on the amount or the direction of its move- 
ment.” After describing one in which white ivory was compared with lampblacked 
ivory without giving very striking results, I continue : — “ These experiments were, how- 
ever, tried in 1873, when I had not succeeded in getting any thing like the delicacy I 
now obtain in the apparatus ; and I propose to repeat them under varied conditions, 
before employing the results to found any arguments upon.” 
The present paper contains an account of these experiments on the action of radiation 
on bodies the surfaces of which have their radiating and absorbing powers modified by 
various coatings. The surfaces examined in this way are of the most diverse character, 
the incident rays have been selected of all refrangibilities from ultra-red to ultra-violet, 
the radiation has been sifted through liquid, solid, and gaseous screens, the degree 
of exhaustion and the sensitiveness of the apparatus have been brought to a state 
of perfection undreamed of in my earlier experiments, and the results, I venture to state, 
are of a correspondingly striking character. 
127. The results which I obtained on comparing the action of radiation on thin sub- 
stances, plain and lampblacked, were at first very anomalous. As already stated (116) 
the movement of lampblacked ivory under the influence of radiation was only a little more 
than that of plain white ivory. On the other hand, coating platinum with lampblack 
produced a very marked effect on its movement (114). 
Pith coated with lampblack was generally found to have its sensitiveness heightened ; 
but this was not always the case, and the following experiments were tried for the purpose 
of clearing up these discrepancies. 
128. An instrument was made similar to the one described and figured in pars. 84, 85, 
2 z 2 
