Penetrating Radium Rays. 71 
8 rays could be stopped when desired by placing a sheet of 
lead between the radium and the vessel a. 
In this way the charge got by the shot-cylinder could be 
measured when it is absorbing both @ andy rays, and also 
when absorbing ¥ rays only. 
When the shot-cylinder absorbs both 8 and y¥ rays, it 
rapidly gets a negative charge; when it absorbs only y rays 
no charge could be detected. 
This experiment shows that the y rays do not carry any 
charge sufficient to be detected by the above apparatus. 
The sensitiveness of the apparatus, and therefore the value 
of the experiment, can best be estimated by considering the 
deflexion obtained with the 6 rays. <A pencil of @ 
diverges from R wide enough to include practically the whole 
end of the cylinder b; this pencil produces a deflexion of 
38 scale-divisions per ‘minute when absorbed by the shot- 
cylinder. The whole charge carried by 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 a was *3 mm. thick, and this 
thickness of tin (neglecting the paraffin and mica) would 
absorb # of the @ rays (Strutt, ‘ Nature, p.539, 1900). The 
8 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 deflexion, certainly not as much as 2 divisions per minute ; : 
this pencil is not all absorbed by the shot because of the oreat 
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 + of the y rays included in the pencil considered 
would be absorbed by the cylinder of shot; and we have 
seen that the deflexion produced is not more than 2 divisions 
per minute. We have therefore the result that the @ 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 insu- 
lating 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 fromasample of radium is 
certainly not more than 1 or 2 per cent. of the negative 
electricity carried by the @ rays from the same radium. 
