The interesting point in connection with these results is that some specimens of pitchblende have four times the activity of the metal uranium; chalcolite, the crystallized phosphate of copper and uranium, is twice as active as uranium; and autunite, a phosphate of calcium and uranium, is as active as uranium. From the previous considerations, none of the substances should have shown as much activity as uranium or thorium. In order to be sure that the large activity was not due to the particular chemical combination, Mme Curie prepared chalcolite artificially, starting with pure products. This artificial chalcolite had the activity to be expected from its composition, viz. about 0·4 of the activity of the uranium. The natural mineral chalcolite is thus five times as active as the artificial mineral.
It thus seemed probable that the large activity of some of these minerals, compared with uranium and thorium, was due to the presence of small quantities of some very active substance, which was different from the known bodies thorium and uranium.
This supposition was completely verified by the work of M. and Mme Curie, who were able to separate from pitchblende by purely chemical methods two active bodies, one of which in the pure state is over a million times more active than the metal uranium.
This important discovery was due entirely to the property of radio-activity possessed by the new bodies. The only guide in their separation was the activity of the products obtained. In 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 obtain 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 polonium 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 process used to separate barium, to which radium is very closely allied in chemical properties[[17]]. 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 freed almost completely from the barium.
Both polonium and radium exist in infinitesimal quantities in 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 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 Joachimsthal in Bohemia. With the assistance of the Academy of Science 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 that 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.