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192 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 
example, the escape of the radio-active emanations from thorium 
and radium is readily affected by heat, moisture and solution. 
A more thorough investigation, however, shows that the excep- 
tion is only apparent and not real. These cases will be discussed 
more in detail in chapter VIN, but it may be stated here that 
the differences observed are due to differences in the rate of escape 
of the emanations into the surrounding gas, and not to differences 
in the rate of production. For this reason it is difficult to test the 
question at issue in the case of the thorium compounds, which 
in most cases readily allow the emanation produced by them to 
escape into the air. 
In order to show that the rate of production is independent 
of molecular state, temperature, etc., 1t 1s necessary in such a 
case to undertake a long series of measurements extending 
over the whole time of recovery. It is impossible to make accu- 
rate relative comparisons to see if the activity is altered by the 
conversion of one compound into another. The relative activity 
in such a case, when measured by spreading a definite weight of 
material uniformly on a metal plate, varies greatly with the physical 
conditions of the precipitate, although the total activity of two 
compounds may be the same. 
The followimg method? offers an accurate and simple means 
of studying whether the rate of production of active matter is 
influenced by molecular state, The substance is chemically con- 
verted into any compound required, care bemg taken that active 
products are recovered during the process. The new compound is © 
then spread on a metal plate and compared with a standard sample 
of uranium for several days or weeks as required. If the rate of 
production of active matter is altered by the conversion, there 
should be an increase or decrease of activity to a new steady value, 
where the production of active matter is again balanced by the 
rate of decay. This method has the great advantage of being in- 
dependent of the physical condition of the precipitate. It can be 
applied satisfactorily to a compound of thorium lke the nitrate 
and the oxide which has been heated to a white heat, after which 
treatment only a slight amount of emanation escapes. The nitrate 
was converted into the oxide in a platinum crucible by treatment 
1 Rutherford and Soddy, Phil. Mag. Sept. 1902. 
