The implications of the bare statement that radium is a mem ber of the uranium-radium-lead disintegration series are remark able. The statement implies, firstly, that the only sources of radium in Nature are minerals containing uranium, for without the ultimate parent of the series there can be no series, and there fore no radium. Secondly, since radium is produced by the dis integration of its parent ionium and disappears on disintegrating into its product radon, the weight of radium in a mineral con taining a given amount of uranium is fixed at the weight of radium which has the same number of atoms breaking up to form radon in a given interval of time as is produced in the same time by the disintegration of ionium. There is, thus, a definite ratio be tween the weight of uranium in a mineral and the weight of radium ; experimentally this is found to be I to 0.00000033. To extract a gram of radium from a mineral which is half uranium, therefore, no less than 5.9 tons of the mineral must be worked up, or, to extract a pound, no less than 2,630 tons. In actual practice the minerals are rarely as rich as this, and one containing 1 gm. of radium in 10 tons of ore is considered rich; even minerals which contain only I gm. in 200 tons are worked. It is not surprising, therefore, that the price of radium is about ii5,000 per gram.
Industrial Production.—The principal minerals containing uranium, and therefore radium, are pitchblende, carnotite and autunite. The first of these consists of the oxide of uranium, more or less pure, and is found principally in Czechoslovakia and in Belgian Congo; the second is a vanadate of uranium and potassium mined in Colorado, Utah, and Australia; the third is a phosphate of uranium and calcium mined in Portugal and in the U.S.A. The method of extracting radium varies with the nature and quality of the ore, but in broad outline, all processes consist of five main steps first worked out by Mme. Curie. These consist in getting the uranium mineral into solution, separating from the solution all metallic sulphates which are insoluble in water (these include the whole of the barium and the radium constituents of the mineral), the conversion of the sulphates into double salts, the purification of the compounds of radium and barium, and finally the separation of the radium from the accompanying barium by a process of fractional crystallization.
At St. Joachimstal in Czechoslovakia the broken-up pitch blende is treated with a mixture of nitric and sulphuric acids of such concentrations that, whereas the whole of the uranium goes into solution, the radium remains behind as insoluble sulphate along with impurities of calcium, barium and lead. These sul phates are then boiled several times with a solution of sodium carbonate and so converted into carbonates, and thence into chlorides. A repetition of this process separates the radium and barium from the impurities calcium and lead. Finally, owing to the fact that the chloride of radium is much more insoluble than that of barium in a solution of hydrochloric acid, it is possible to enrich a mixture of radium and barium chlorides in the former until finally radium is quite free from barium.
At Denver, Colorado, the carnotite is heated with a solution of concentrated nitric acid containing hydrochloric acid which dis solves the radium as well as the uranium and the vanadium. To
this a solution of barium chloride is added. When the acidity of the solution is reduced by the addition of alkali the barium, and with it the radium and lead, are precipitated as sulphates and removed from the solution by filtration. These sulphates are then heated in a crucible of graphite with wood charcoal and so con verted into the sulphides, which unlike the sulphates are easily soluble in hydrochloric acid. From this solution the lead is pre cipitated by one chemical reagent and the radium and barium by another. The enrichment of the radium in the radium-barium mixture is based on the comparative insolubility of the bromide of radium, and not, as at St. Joachimstal, on the comparative insolubility of the chloride.
Chemical Properties.—The principal compounds of radium are the sulphate the ..:hloride the hydrated chlo ride the bromide the hydrated bromide the sulphide RaS, the nitrate Ra(NO3)2, the carbo nate and the hydroxide Like the corresponding compounds of barium, strontium and calcium, these are all colour less compounds and their chemical properties may be summar ized in the remark that they are exactly what would be expected from a study of the compounds of these three elements. It is not surprising therefore that a purely chemical study of radium compounds has not occasioned much interest. At 25° C the solu bility of strontium sulphate is that of calcium sulphate, while that of barium sulphate is A. that of strontium ; it is consistent that the solubility of radium sulphate is - that of barium sulphate. As with sulphates, so with chlorides and bromides. On the other hand the lightest of the alkaline-earth metals, calcium, has the most insoluble hydroxide in water; it is to be expected therefore (and confirmed experimentally) that the heaviest, radium, has the most soluble hydroxide.
As with such a property as the solubility of typical chemical compounds, so with most other properties. The optical spectrum of radium, for example, is composed of a comparatively small number of lines of great intensity. The strongest line in the visible part of the spectrum is in the violet, and this line is a very sensi tive test for the presence of radium. In this, radium resembles barium, strontium and calcium. They too have a comparatively small number of spectral lines of great intensity, the most promi nent of which are used in chemical analysis as tests for their presence. Again, the method in use for the preparation of metal lic radium is identical with that used for preparing barium, namely, first the metal is deposited by an electric current on to mercury, and then the volatile mercury is removed by distillation in a current of hydrogen. The metal which remains behind is white, melts at about 700° C, and greatly resembles barium in its chemical reactions. It must be preserved out of contact with the air; it dissolves in water and in all acids with the evolution of hydrogen, forming the hydroxide in the first case and the corre sponding salt in the other cases. Radium is generally sold in the form of the chloride or the bromide, mixed, when impure, with the corresponding compound of barium. For the physiological effeots of radium see RADIUMTHERAPY.