14 RADIO-ACTIVE SUBSTANCES ; [CH. 
thus obvious that the expense and labour involved in preparation 
of a minute quantity of radium are very great. 
M. and Mme Curie were indebted for their first working 
material to the Austrian government, who generously presented 
them with a ton of the treated residue of uranium materials from 
the State manufactory of Joachimstahl in Bohemia. With the 
assistance of the Academy of Sciences and other societies in France, 
funds were given to carry out the laborious work of separation. 
Later the Curies were presented with a ton of residues from the 
treatment of pitchblende by the Société Centrale de Produits 
Chimiques of Paris. The generous assistance afforded in this 
important work is a welcome sign of the active interest taken in 
these countries in the furthering of purely scientific research. 
The rough concentration and separation of the residues was 
performed in the chemical works, and there followed a large amount 
of labour in purification and concentration. In this manner, 
the Curies were able to obtain a small quantity of radium which 
was enormously active compared with uranium. No definite results 
have yet been given on the activity of pure radium but the Curies 
estimate that it is about one million times the activity of uranium, 
and may possibly be still higher. The difficulty of making a 
numerical estimate for such an intensely active body is very great. 
In the electric method, the activities are compared by noting the 
relative strength of the maximum or saturation current between 
two parallel plates, on one of which the active substance is spread. 
On account of the intense ionization of the gas between the plates, 
it is not possible to reach the saturation current unless very high 
voltages are applied. Approximate comparisons can be made by 
the use of metal screens to cut down the intensity of the radiations, 
if the proportion of the radiation transmitted by such a screen has 
been determined by direct experiment on impure material of easily 
measurable activity. The value of the activity of radium compared 
with that of uranium will however vary to some extent according to 
which of the three types of rays is taken as a basis of comparison. 
It is thus difficult to control the final stages of the purification 
of radium by measurements of its activity alone. Moreover the 
activity of radium immediately after its preparation is only about 
one-fourth of its final value; it gradually rises to a maximum after 
