— 
caused by the B and y Rays of Radium. 673 
59 sheets of paper. But the secondary radiation from paper 
alone comes from a depth ranging to 2 or 3 millimetres. 
Scele Orwisions 
1S 20 24. 
Sheets of Paper 
Similar experiments with glass and aluminium gave an 
effective depth of 3 mm., but here again lead or brick could 
be detected through a greater thickness. A plate of lead 
produced a measurable effect from behind 2°8 mm. of brass ; 
but if a brass plate were placed behind, there was no increase 
of radiation. The state of the surface, whether wet or dry, 
dirty or clean, rough or polished, was immaterial to the 
results. A layer of stout filter-paper well soaked in water, 
placed upon a lead plate, reduced the radiation from 141 to 
120 scale-divisions. A screen of barium platino-cyanide, as 
used for showing the shadows thrown by Réntgen rays, gave 
rise to secondary radiation nearly four times as great as that 
from an uncoated cardboard of equal thickness. But the 
coefficient of absorption, 23°6, was only a little greater than 
from paper or millboard, so that these secondary rays are 
readily absorbed. 
Tron filings gave very little less radiation than solid wrought 
iron, and about the same amount as cast iron. 
Some experiments were made with y rays alone, cutting 
off the 8 rays with layers of lead placed between the radium 
and the radiating plates. The secondary radiation was 
decreased thus :— 
Lead interceptor. Secondary radiation from lead. 
141 
6°38 mm. 27 
Ieee nim” 21 
