McCieittanp—The Penetrating Radium Rays. 103 
the 8 rays contained in this pencil does not reach the shot-cylinder, part 
being absorbed by the cylinder a, and by the mica vessel containing the 
radium. The tin vessel @ was °3 mm. thick, and this thickness of tin 
(neglecting the paraffin and mica) would absorb four-fifths of the B rays 
(Strutt, Mature, 1900, p. 5389). The B rays contained in the pencil considered, 
therefore, carry a negative charge corresponding to more than 200 scale-divisions 
per minute. 
The same pencil of y rays is experimented with, and gives no deflection— 
certainly not as much as two divisions per minute; the pencil in this case is not 
all absorbed by the shot, because of the great penetrating power of these rays. 
We know the coefficient of absorption of lead for these rays (fig. 4); and it is easy 
to calculate from the dimensions of the apparatus given that more than one-third 
of the y rays included in the pencil considered would be absorbed by the cylinder 
of shot; and we have seen that the deflection produced is not more than two 
divisions per minute. We have, therefore, the result that the 8 rays in a given 
pencil carry a negative charge corresponding to more than 200 divisions per 
minute, while the y rays contained in the same pencil do not carry a charge 
corresponding on the same scale to more than 5 divisions, positive or negative, 
per minute. 
The absorption of 8 rays by the mica vessel and the insulating paraffin has 
been neglected in this estimate; and this absorption would be considerable, so 
that the electricity, if any, carried by the y rays coming from a sample of radium 
is certainly not more than one or two per cent. of the negative electricity carried 
by the ® rays from the same source. The y rays might, of course, consist of a 
stream of uncharged particles ; but, as the forces causing disintegration are probably 
of an electrical nature, this is not likely. 
ABSORPTION OF y Rays By Dirrerent SUBSTANCES. 
Rutherford has measured the absorption of these rays by different substances, 
and shown that the absorption is roughly proportioned to the density of the 
substance. The absorption, however, increases more rapidly than the density, 
so that the ratio of the coefficient of absorption to the density is greater for the 
denser substances, being for lead about twice what it is for water. 
On account of the importance of this law of absorption, I have made some 
further experiments which show that when the rays have passed through 
some thickness of the denser substances, the coefficient of absorption is diminished, 
so that the law that the coefficient of absorption is proportional to the density is 
then followed with remarkable closeness. 
