244 RADIO-ACTIVE EMANATIONS [CH. 
Now in the case of the radium emanation, the rate of decay 
is about 5000 times slower than that of the thorium emanation, 
and consequently the actual number of particles that must be 
present to produce the same number of rays per second in the two 
cases must be about 5000 times greater in the case of radium 
than in the case of thorium. This conclusion involves only the 
assumption that the same number of rays is produced by a 
particle of emanation in each case, and that the expelled particles 
produce in their passage through the gas the same number of 
ions. The number of particles present, in order to be detected 
by the electrometer, in this experiment, must therefore have 
been about 5000 x 3500, 27.e. about 2x 107. The difference of 
behaviour in the two cases is well explained by the view 
that, for equal electrical effects, the number of radium emana- 
tion particles must be far larger than the number of thorium 
emanation particles. It is to be expected that the probability 
of the particles coming into each other’s sphere of influence will 
increase very rapidly as the concentration of the particles in- 
creases, and that, in the case of the radium emanation, once the 
temperature of condensation is attained, all but a small proportion 
of the total number of particles present will condense in a very 
short time. In the case of the thorium emanation, however, the 
temperature might be far below that of condensation, and yet 
a considerable portion remain uncondensed for comparatively long 
intervals. On this view the experimental results obtained are 
exactly what is to be expected. A greater proportion condenses, - 
the longer the time allowed for condensation under the same con- 
ditions. The condensation occurs more rapidly in hydrogen than 
in oxygen, as the diffusion is greater in the former gas. For the 
same reason the condensation occurs faster the lower the pressure 
of the gas present. Finally, when the emanation is carried by 
a steady gas stream, a smaller proportion condenses than in the 
other cases, because the concentration of emanation particles per 
unit volume of gas is less in these conditions. 
It is possible that the condensation of the emanations may not 
occur in the gas itself but at the surface of the containing vessel. 
Accurate observations of the temperature of condensation have so 
far only been made in a copper spiral, but condensation certainly 
