VIIt] RADIO-ACTIVE EMANATIONS 213 
Measurements of Emanating Power. 
140. Emanating power. The compounds of thorium in the 
solid state vary very widely in the amount of emanation they emit 
under ordinary conditions. It is convenient to use the term 
emanating power to express the amount of emanation given off per 
second by one gram of the compound. Since, however, we have 
no means of determining absolutely the amount of emanation 
present, all measurements of emanating power are of necessity 
comparative. In most cases, it is convenient to take a given weight 
of a thorium compound kept under conditions as nearly as possible 
constant, and to compare the amount of emanation of the compound 
to be examined with this standard. 
In this way comparisons of the emanating power of thorium 
compounds have been made by Rutherford and Soddy', using an 
apparatus similar to that shown in Fig. 37 on page 199. 
A known weight of the substance to be tested was spread on a 
shallow dish, placed in the glass tube C. A stream of dry dust-free 
air, kept constant during all the experiments, was passed over the 
compound and carried the emanation into the testing vessel. After 
ten minutes interval, the current due to the emanation in the 
testing vessel reached a constant value. The compound was then 
removed, and the standard comparison sample of equal weight 
substituted; the saturation current was observed when a steady 
state was again reached. The ratio of these two currents gives 
the ratio of the emanating power of the two samples. 
It was found experimentally, that, for the velocities of air 
current employed, the saturation current in the testing vessel was 
directly proportional to the weight of thorium, for weights up to 
20 grams. This is explained by the supposition that the emanation 
is removed by the current of air from the mass of the compound, 
as fast as it is formed. 
Let 7, = saturation current due to a weight @, of the standard, 
ly = ‘ i 3 »  @, of the sample to 
be tested. 
emanating power of specimen 2, @ 
Then gare eames ly ey epee 

emanating power of standard 2, ,’ 
1 Trans, Chem. Soc., p. 321, 1902, Phil. Mag. Sept. 1902. 
