The number of a-particles expelled per second from one gram of radium is an important constant which requires to be known with accuracy. At the present time, there is considerable difference of opinion as to its exact magnitude. Rutherford and Geiger (1908) found a number 3.57 Hess and Lawson (1918) 3.72X mu', Geiger and Werner (1924) 3.4X 'oil', Jedrzejouski (1927) 3.50X ow. If the heating effect of radium and its products is entirely due to the kinetic energy of the expelled a-particles, the value should be about 3.7X Ie. The rate of production of helium by radium leads to a value about 3.6X Ie. The property of radium of producing an emanation has been utilized as a very delicate and certain method, not only of detection but of estimation of small quantities of radium. This "emanation method" depends upon the introduction of the emanation, liberated from a sub stance by boiling or heating, into a suitable electroscope. The rate of discharge of the electroscope due to the emanation affords a quantitative measure of the amount of radium present. In this way, it is not difficult to determine with certainty the presence of radium in a body which contains only 1 gram of radium. With care, gram can just be detected. This emanation method has been employed with great success in measuring the quantity of radium in minerals and in rocks. A very simple method has been devised of determining the quantity of radium present when it is not less than T- milligram. The tube con taining the radium is placed some distance from an electroscope which is surrounded by a lead screen about 3 mms. thick. This cuts off the a- andg-rays and the effect in the electroscope is then due to the penetrating 7-rays. By comparison of the rate of dis charge with that of a standard preparation of radium at the same distance, the quantity of radium can at once be deduced, provided the radium is in equilibrium. This is usually the case if the radium preparation is one month old. This method is simple and
direct, and has the great advantage that the radium tube under test need not be opened, nor its contents weighed.
An international radium standard consisting of about 22 milli grammes of pure radium chloride has been prepared by Mme. Curie and is preserved in the Bureau International des Poids et Mesures at Sevres, near Paris. Secondary radium standards have been issued to all governments who wished to purchase them. These have been calibrated by 7-ray methods both in Vienna and Paris, and are supposed to be correct within 1 in 200. The pur chase and sale of radium have generally been conducted on cer tificates given in terms of this international standard.
The wide use of radium for therapeutic purposes, and its high cost—from L12 to L30 per milligram element—have led to close search for uranium deposits. The amount of radium in an old mineral is always proportional to its content of uranium in the ratio of 3.3 parts of radium by weight to io,000,000 parts of uranium. Consequently, an old mineral containing i,000 kgm. of uranium should contain 33o milligrams of pure radium. Initially several grams of radium were separated from the uraninite de posits in Joachimsthal, Bohemia, and some of the material, which was the property of the Austrian Government, was generously loaned to representative workers in radioactivity in England. A part of this radium is in the charge of the Radium Institute of Vienna, which is specially devoted to radioactive investigations.
The increasing demand for radium led to the working of low grade ores, containing on an average only about 2% of uranium oxide, of which there are extensive deposits in Colorado and other parts of the United States. Until 1922 the greater part of the world supply of radium was produced in America, but in that year very rich deposits of uranium ore, containing about 20 times as much radium as the American ore, were found in the Belgian Congo, and the American production practically ceased. Large quantities of radium were employed by the Allies during the World War for night compasses, gun-sights, etc. The radium is mixed with phosphorescent zinc sulphide to form a paint which becomes continuously luminous but, owing to the destruction of the zinc sulphide by the rays, this luminosity gradually decays. Active preparations of mesothorium and radiothorium have also been used for a similar purpose.