vul] RADIO-ACTIVE EMANATIONS 229 
in 1871, in his investigations of the coefficient of inter-diffusion 
of gases. 
Fig. 42 shows the general arrangement. A long brass cylinder 
AB, of length 73 ems., and diameter 6 cms., was divided into two 



Electrometer Battery 2 
ll A | Electrometer 
oes Platinum Tube . 
N 
YY XXX XX1 oi) 

Re 
asometer 
Radi um! 
Fig. 42. 
equal parts by a moveable metal slide S. The ends of the cylinder 
were closed with ebonite stoppers. ‘Two insulated brass rods, a 
and 6, each half the length of the tube, passed through the ebonite 
stoppers and were supported centrally in the tube. The cylinder 
was insulated and connected with one pole of a battery of 300 
volts, the other pole of which was earthed. The central rods couid 
be connected with a sensitive quadrant electrometer. The cylinder 
was covered with a thick layer of felt, and placed inside a metal 
box filled with cotton-wool in order to keep temperature con- 
ditions as steady as possible. 
In order to convey a sufficient quantity of emanation into 
the half-cylinder A, it was necessary to heat the radium slightly. 
The slide S was closed and the side tubes opened. A slow 
current of dry air from a gasometer was passed through a platinum 
tube, in which a small quantity of a radium compound was placed. 
The emanation was carried with the air into the cylinder A. When 
a sufficient quantity had been introduced, the stream of air was 
stopped. The side tubes were closed by fine capillary tubes. 
These prevented any appreciable loss of gas due to the diffusion, 
but served to keep the pressure of the gas inside A at the pressure 
of the outside air. The three entrance tubes into the cylinder, 
shown in the figure, were for the purpose of initially mixing the 
emanation and gas as uniformly as possible. 
After standing several hours to make temperature conditions 
steady, the slide was opened, and the emanation began to diffuse 
