264 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
powder, without complicating the results. Coloured glasses, or very thick masses 
of colourless plate glass, generally act in proportion to the amount of light they 
apparently cut off ; whilst coloured solutions act like water, with a special action 
superadded, due to the colour. A very considerable thickness of glass offers less 
obstruction to the motion-producing rays than does a very thin layer of water. A 
clear plate of alum is very similar to water in its effects, and a solution of alum 
is almost identical with the same thickness of water. Copper sulphate solution 
offers great obstruction when artificial fight is used. These facts show that glass 
is not so opaque to the ultra fed heat rays as is water ■ that the lowest visible red 
rays are likewise powerful heat rays ; and that by cutting oft’ these also, by adding 
copper sulphate to water, a considerable fraction of the rays which produce repulsion 
when artificial fight is used will be cut off.* 
Seeing the very strong absorptive action which gaseous ammonia exerts on rays of 
radiant heat,t as measured by a thermo-pile, I endeavoured to detect any diminution 
in the intensity of the repulsion oh pith, lampblacked and plain, lampblacked mica, as 
well as on other bodies, when the rays from the candle were allowed to pass through a 
layer of pure ammonia gas, 6 inches in thickness. I was, however, unable to detect 
the slightest diminution of action which could be ascribed to the absorption of heat by 
the ammonia gas (200). 
DIFFERENT ACTIONS OF RADIANT HEAT AND OF LIGHT. 
242. An examination of the preceding tables shows that the substances used in 
experiment can be divided into two classes : 1, negative, those in which the repulsion 
behind water is greater, in proportion to the standard, than when no screen is present ; 
and 2, positive, those in which the repulsion, in proportion to the standard, is less 
behind water than when no screen is interposed. From these tables a few illustrations 
are selected : — 
Table XIII. 
Class 1. 
No screen. 
Water screen. 
Difference. 
Lampblack ( standard disk) .... 
. . 100-0 
ioo-o 
o-o 
Copper tungstate 
. . 51-2 
77-0 
-25-8 
Saffranin 
. . 41-0 
52-5 
-11-5 
Precipitated selenium 
. . 35-8 
69-5 
-33-7 
Copper oxalate 
. . 30-1 
40-2 
-10-1 
* In some rooms lighted by gas, a layer of -water is interposed between tbe light and the lower part of 
the room, with tbe view to cut off tbe beat. This device answers very well, but tbe absorption of beat 
could be greatly augmented by adding to tbe water a very little copper sulphate, whilst tbe colour of the 
light would at tbe same time be improved. 
| ‘ Contributions to Molecular Physics in the Domain of Radiant Heat.’ By J. Tyndall, LL.D., F.R.S., 
p. 80. 
