MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
259 
explain it by the fact that the disk is a very bad conductor of heat ; consequently 
the molecular disturbance is retained on the front surface, and does not sink so rapidly 
as it would were the heat rapidly conducted through to the other side of the disk. In 
the case of the crystalline selenium this action is very strongly marked, and without 
denying that some of the sluggishness to return to zero may be due to the bad con- 
ducting power of selenium for heat, the greater portion of this action is explained by 
the fact of the sensitiveness to light of this form of selenium. It would seem as if the 
impact of light on the surface produced a persistent disturbance, which keeps up a 
similar disturbance in the adjacent gaseous molecules, and takes some time to subside. 
It is known that the change in electric conducting power of crystalline selenium, when 
it is transferred from light to darkness, takes some time to effect. 
The vitreous selenium does not show this strongly marked sluggishness to return to 
zero, and it is decidedly less acted on by radiation. This may be due to its non- 
sensitiveness to light, or to its shining surface reflecting more of the candle rays than 
does the crystalline surface. 
237. Table XII. — Miscellaneous Substances — Pith, Mica, Charcoal, Glass, &c. 
Water screen 
o screen. interposed (5 millims.). 
Lampblack ( standard disk ) lOO’O lOO’O 
Pith, plain white 17' 7 0’4 
Pith, lampblacked on one side, white side exposed . . O’O O’O 
Amadon, lampblacked on one side, black side exposed . 57’5 53’6 
Amadou, lampblacked on one side, white side exposed . 21 - 0 12’2 
Amadou, not blacked 20’5 7’6 
Mica, silver flake, lampblacked on one side, black side 
exposed 151 - 0 125’5 
Mica, roasted, blacked on the opaque side, black side 
exposed . 1117 89’3 
Mica, roasted, blacked on the clear side, black side 
exposed 87’5 78’5 
Mica, silver flake 12’5 2’1 
Mica, roasted, clear side exposed 67 O’O 
Mica, roasted, blacked on clear side, black turned away 
from light 6’8 3’5 
Mica, roasted, blacked on opaque side, black turned 
away from light 7’6 8’9 
Mica, silver flake, blacked on one side, black turned 
away from light 9’8 8'8 
Charcoal, wood, parallel to the grain 18’4 147 
Charcoal, wood, across the grain 15’2 107 
Charcoal, cocoa-nut shell 11 • 6 37 
Microscopic glass, quite clear 6'5 0’5 
Microscopic glass, having the portion of the glass tube 
behind it coated internally with lampblack ... 6’0 O’O 
2 l 2 
