538 
MR. W. CROOKES ON REPULSION RESULTING- FROM RADIATION. 
condition of the surface (whether coated with lampblack or consisting of polished 
metal) of the body on which radiation falls materially influence the movements. 
109. The accompanying Table gives the results of numerous experiments as to the 
effect of screens, tried with an exceedingly delicate apparatus, constructed as above 
lagnesium 
wire, 
burnt for 
•5 seconds, 
distant 
140 
millims. 
Standard 
candle, 
distant 
140 
millims. 
Standard 
candle, 
distant 
280 
millims. 
Copper 
ball, 
400’ C., 
distant 
140 
millims. 
Copper 
ball, 
400° C., 
distant 
280 
millims. 
Copper 
ball, 
100° C., 
distant 
140 
millims. 
o 
o 
54 
52 
o 
185 
9 
6 
148 
220 
88 
100 
32 
28 
115 
90 
1-5 
2 
3-25 
110 
76 
0 
1-75 
— 
72 
24 
23 
o 
0-62 
0 
17 
8 
3 
20 
0 
0 
18-5 
0 
0 
0 
0 
30 
0 
0 
0 
off the scale. 
l o 
8 
72 
7 
— 
0 
0 
0 
29 
3 
— 
0 
0 
0 
Interposed sceen. 
None 
Rock-salt, 20 millims. thick, not very clear . . 
Rock-crystal, in two pieces, 42 millims. 1 
thick altogether J 
Talc, clear hut very dark, 1*25 millim. 1 
thick } 
Plate glass, white, 2 millims. thick, one \ 
piece J 
Ditto, two pieces 
Ditto, three pieces 
Ditto, two pieces, enclosing 8 millims. 1 
water J 
Plate glass, of a greenish colour, 10 - 5 | 
millims. thick J 
Ditto, 20 millims. thick 
Alum, a clear plate, 5 millims. thick 
Plate glass, slightly greenish, 40^ millims., I 
and clear alum plate, 8| millims. thick. J 
Calc spar, 27 millims. thick 
Yery thin film of mica 
Ammonio-sulphate of copper, 8 millims. 
thickness of solution, opaque to rays 
less refrangible than line F 
Ditto, stronger solution, opaque below G. 
described, the window, c' (fig. 7), being of quartz. The candle used was the kind 
employed in gas photometry, and defined by Act of Parliament as a “ sperm candle 
of 6 to the pound, burning at the rate of 120 grains per hour.” The distances were 
taken from the front surface of the pith when the luminous index stood at zero. 
They were in the proportion of 1 to 2 (140 to 280 millims.), to enable me to see if 
the action would follow the law of inverse squares and be four times as great at the 
half distance. No such proportion can, however, be seen in the results, the radiant 
source possibly being too close to allow the rays to fall as if from a point. The figures 
given are the means of a great many fairly concordant observations. Where a dash rule 
is put I have tried no experiment. The cipher 0° shows that experiments were actually 
tried, but with no result. 
The sensitiveness of my apparatus to heat-rays appears to be greater than that of any 
ordinary thermopile and galvanometer. Thus I can detect no current in the thermo- 
pile when obscure rays from copper at 100° C. fall on it through glass ; and Melloni 
gives a similar result. 
