caused by the B and y Rays of Radium. 677 
electroscope. The results are shown in fig 6 for a number 
of sheets ranging up to 100 with a total thickness of 6 mm. 
Sco/e Driv/sion s. 
Sheers of RPw@wPer 
The first few sheets increased the radiation, until a maximum 
was reached for about 12 sheets. The values were almost 
steady between 9 and 20 sheets. After that the curve fell, 
partly because the primary rays were cut down, and partly 
because the secondary radiation cannot penetrate more than 
about 36 sheets (fig. 4). 
It will be seen later that theoretical considerations would 
lead one to expect an equation to this curve having the form 
of the difference of two simple exponentials, and the curve 
appears to resemble this well-known type. 
In the case of zinc, the rise and fall to a fairly steady 
value took place for layers ranging up to a thickness of 
1°6 mm. 
Results. 
The secondary radiation from various substances follows 
approximately the same order as their densities (Table [.). 
The coefficients of absorption of secondary radiation by various 
substances also follow roughly that order (Table IV.). Good 
radiators are therefore good absorbers. Neither the secondary 
radiations nor the coefficients of absorption are proportional 
to the densities. The secondary radiation does not come 
from the surface merely, but trom a total depth varying from 
about 1°5 mm. in the case of lead to about 3 mm. for glass, 
aluminium, or paper. It is mainly independent of the state 
of the surface. Almost the same amount of radiation is 
obtained from solid iron as from iron filings; from liquid 
and from solid paraffin; from ice and water; from paper, 
millboard, papier-mAché, mahogany, pine, and basswood. | 
On reference to Table I. it will be seen that the most 
