1] RADIO-ACTIVE SUBSTANCES 13 
this respect the discovery of these bodies is quite analogous to the 
discovery of rare elements by the methods of spectrum analysis. 
The method employed in the separation consisted in examining 
the relative activity of the products after chemical treatment. In 
this way it was seen whether the radio-activity was confined to one 
or another of the products, or divided between both, and in what 
ratio such division occurred. 
The activity of the specimens thus served as a basis of rough 
qualitative and quantitative analysis, analogous in some respects 
to the indication of the spectroscope. To obtaim comparative 
data it was necessary to test all the products in the dry state. 
The chief difficulty lay in the fact that pitchblende is a very 
complex mineral, and contains in varying quantities nearly all the 
known metals. 
12. Radium. The analysis of pitchblende by chemical 
methods, using the procedure sketched above, led to the discovery 
of two very active bodies, polonium and radium. The name polo- 
nium was given to the first substance discovered by Mme Curie 
in honour of the country of her birth. The name radium was 
a very happy inspiration of the discoverers, for this substance in 
the pure state possesses the property of radio-activity to an 
astonishing degree. 
Radium is extracted from pitchblende by the same process 
necessary to separate barium, to which it is very closely allied in 
chemical properties'. After the removal of other substances, the 
radium remains behind mixed with barium. It can, however, be 
partially separated from the latter by the difference in solubility of 
the chlorides in water, alcohol, or hydrochloric acid. The chloride 
of radium is less soluble than that of barium, and can be separated 
from it by the method of fractional crystallization. After a large 
number of precipitations the radium can be almost completely freed 
from the barium. 
Both polonium and radium exist in infinitesimal quantities im 
pitchblende. In order to obtain a few decigrammes of very active 
radium, it is necessary to use several tons of pitchblende, or the 
residues obtained from the treatment of uranium minerals. It is 
1 M. and Mme Curie and G. Bemont, C. R. 127, p. 1215, 1898. 
