250 
MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
experiment with lead carbonate tried in a somewhat similar way. The effect was 
then found to be only 13 as against lampblacked pith=100. The much stronger 
effect now obtained (32'5) is probably due to the different physical condition of the 
two samples of lead carbonate. The one giving the lower number, was prepared 
in the wet way by double decomposition, whilst the sample now used was prepared 
in the dry way by the corrosion of metallic lead by carbonic acid and air in the 
presence of moisture and acetic acid vapour, as in the manufacture of white lead. 
It is well known that lead carbonate thus prepared possesses great density and 
opacity. 
226. Table III. — Red Powders, 
No screen. • 
Water screen 
interposed (5 millims.). 
Lampblack (standard disk ) 
o 
. . 100-0 
100-0 
Amorphous phosphorus 
. . 40.-4 
38-0 
Precipitated selenium 
. . 35-8 
69-5 
Ferric oxide 
. . 29-5 
25-1 
Mercury and copper iodide 
. . 22-4 
11-7 
The results given in this Table deserve notice. The very high action shown by 
precipitated selenium in the second column (water screen interposed) is one which, 
considered alone, might be supposed to belong to its elementary character, and this 
view would in some measure be borne out by the similar behaviour of reduced iron in 
Table I. ; but other elementary bodies— e.g., phosphorus in Table III. and tungsten in 
Table I. — -do not act in this way. The averages of columns 1 and 2 are 33 T with no 
screen interposed and 36 ‘1 with a water screen ; if selenium be left out as anomalous, 
the averages become 32 - 2 and 24‘9. In subsequent Tables (242) other bodies will be 
seen to act like selenium, although not to the same extent. 
227. Table IY. — Brown Powders. 
No screen. 
Water screen 
interposed (5 millims.). 
Lampblack ( standard disk ) . ... . 
. . 100-0 
100-0 
Thallic oxide 
. . 121-7 
m-o 
Lead peroxide 
. . 113-5 
105-0 
Platinum protochloride 
77-8 
84-0 
Copper ferrocyanide 
. . 71-2 
87-8 
Thallium vanadate ... . . 
50-4 
69-2 
Bismuth peroxide 
. . 21-5 
10-2 
The very high action exhibited by the thallium and lead peroxides is worthy of 
notice. Of all the substances I have yet examined these head the list, thallium per- 
oxide being more repelled under the influence of radiation than any other body. 
The averages of the columns, without and with a water screen, are 92 '7 and 94 - 5. 
