Vil] RADIO-ACTIVE EMANATIONS 217 
emanation must be occluded in the compound, and there must in 
consequence be a sudden release of this emanation on solution of 
the compound. On account of the very slow decay of the activity 
of the emanation of radium, the effects should be far more marked 
in that compound than in thorium. 
From the poimt of view developed in section 124, the expo- 
nential law of decay of the emanation expresses the result that NV; 
the number of particles remaining unchanged at a time ¢ is given 
by 
N; 
N, 
where J, is the initial number of particles present. When a 
steady state is reached, the rate of production q, of fresh emanation 
particles is exactly balanced by the rate of change of the particles 
NV, already present, 2.¢. 
= eo, 
G— NN 
JV, in this case represents the amount of emanation “ occluded” in 
the compound. Substituting the value of % found for the radium 
emanation in section 136, 
NE: 
—= so 463,000. 
Jo : 
The amount of emanation stored in a non-emanating radium 
compound should therefore be nearly 500,000 times the amount 
produced per second by the compound. ‘This result was tested in 
the following way?: 
A weight of 03 gr. of radium chloride of activity 1000 times that 
of uranium was placed in a Drechsel bottle and a sufficient amount of 
water drawn in to dissolve it. The released emanation was swept 
out by a current of air into a small gas holder and then into a testing 
cylinder. The initial saturation current was proportional to V,. A 
rapid current of air was then passed through the radium solution 
for some time in order to remove any slight amount of emanation 
which had not been removed initially. The Drechsel bottle was 
closed air-tight, and allowed to stand undisturbed for a definite 
time ¢t. The accumulated emanation was then swept out as before 
into the testing vessel. The new ionization current represents 
1 Rutherford and Soddy, Phil. Mag. April, 1903. 
