MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
265 
Class 2. 
No screen. 
Water screen. 
Difference. 
Lampblack {standard disk) . . . . 
. . 100-0 
100-0 
o-o 
Chromic oxide, pale green .... 
. . 71-5 
20-2 
+ 51-3 
Persulpho-cyanogen 
. . 43-9 
11-5 
+ 32-4 
Hydrated zinc oxide 
. . 40-5 
14-0 
+ 26-5 
Barium sulphate 
. . 37-4 
4-2 
+ 33-2 
Calcium carbonate 
. . 28-5 
3-9 
+ 14-6 
In class 1, the behaviour behind water shows that the substances experimented 
on are much more affected by light than by invisible heat ; whilst the behaviour of 
the bodies in class 2, behind water, shows that these are much more acted on by 
the ultra-red heat rays than by the luminous rays. To render these differences of 
action more comparable, the figures in the second column (water screen) must be 
divided by twelve (par. 224). Uniting the two classes together, the figures then 
become as follows : — 
243. Table XIV. 
No screen. 
Water screen 
interposed (5 millims.). 
Lampblack {standard disk ) 
ioo-o 
8-3 
Chromic oxide, pale green 
71-5 
1-7 
Copper tungstate 
51-2 
6-4 
Persulpho-cyanogen 
43-9 
1-0 
SafEranin 
41-0 
4-3 
Hydrated zinc oxide 
40-5 
1-2 
Barium sulphate 
37-4 
0-3 
Selenium, precipitated 
35-8 
5-8 
Copper oxalate 
30-1 
3-3 
Calcium carbonate 
28-5 
0-3 
EXPERIMENTAL VERIFICATION 
OF THEORY. 
244. At the end of a long investigation with complicated apparatus, it is a great 
satisfaction to find that an easy way of proving the general accuracy of the results 
offers itself ; more especially if the numerical results show many discrepancies, which 
may be due to errors of experiment, or on the other hand may be anomalies, pointing 
in the direction of further discoveries. A glance at the above table shows that the 
results can be proved by balancing one powder against another in a radiometer. A 
bulb was blown on the end of a wide tube, as shown at fig. 4. The top of the bulb was 
opened and turned over to form a lip ; this was ground smooth and polished so as to be 
readily closed by cementing on it a piece of plate glass. A glass stem supports a fine 
MDCCCLXXVIII. 2 M 
