Radioactivity

Origin of Radium.

According to the transformation theory, radium, like all other radioactive products, must be regarded as a changing element. Preliminary calculations showed that radium must have a period of transformation of several thousand years. Consequently in order that any radium could exist in old minerals, the supply must be kept up by the transformation of some other substance. Since radium is always found associated with uranium minerals, it seemed probable from the beginning that uranium must be the primary element from which radium is derived. If this were the case, in old minerals which have not been altered by the action of percolating waters, the ratio of the amount of radium to uranium in a mineral must be a constant. This must evidently be the case, for in a state of equilibrium the rate of breaking up of radium must equal the rate of supply of radium from uranium. If P, Q be the number of atoms of uranium and radium respectively in equilibrium, and their constants of change, then where T2 and are the half-periods of transformation of ura nium and radium respectively. The work of Boltwood, Strutt and McCoy has conclusively shown that the ratio of radium to ura nium in old minerals is a constant. Boltwood and Strutt deter mined the quantity of radium present in a mineral by the emana tion method, and the amount of uranium by analysis. In order, however, to obtain a direct proof of the genetic relation between uranium and radium, it is necessary to show that radium appears after some time in a uranium compound from which all trace of radium has been initially removed. It can readily be calculated that the growth of radium should be easily observed by the emanation method in the course of one week, using a kilogram of uranium nitrate. Experiments of this kind were first made by Soddy, but initially no definite evidence was obtained that radium grew in the solution at all. The rate of production of radium, if it took place at all, was certainly less than part of the amount to be expected if uranium were transformed directly into radium. It thus appeared probable that another product of slow period of transformation existed between uranium and radium. Since uranium must be transformed through this intermediate stage before radium appears, it is evident that the initial rate of production of radium under these conditions might be ex tremely small. This conclusion has been confirmed by Soddy, who has shown that radium does ultimately appear in the solution which has been placed aside for several years and as theory pre dicts the amount increases as the square of the time.

Since the direct parent of radium must be present in radio active minerals, one of the constituents separated from the min eral must grow radium. This was shown to be the case by Bolt wood who found that actinium preparations produced radium at a fairly rapid rate. By the work of Rutherford and Boltwood, it was found that the growth of radium was not due to actinium itself, but to a new substance separated in some cases with the actinium. This new substance, which emits a-rays, was sepa rated by Boltwood and called by him "Ionium." It has chemical properties very similar to thorium and as we now know is an iso tope of thorium of atomic weight 23o. Ionium is always separated mixed with the thorium which is present in the mineral. The period of ionium is probably about 76,00o years, indicating that the amount of ionium in uranium minerals is about fifty times the quantity of radium. It has not yet been directly shown that ura

nium produces ionium but there can be no doubt that it does do so. Since ionium produces radium, Boltwood and later Miss Gleditsch have determined by direct experiment the period of transforma tion of radium and have found a number in good agreement with other data on that subject. The constant relation between uranium and radium will only hold for old minerals where there has been no opportunity for chemical alteration or removal of its constituents by the action of percolating water or other agencies. It is quite possible that altered minerals of no great age will not show this constant relation. It seems probable that this is the explanation of some results of Miss Gleditsch, where the relation between uranium and radium has been found not to be constant for some mineral specimens.

Polonium and Radium D.—Polonium (radium F) has proved of much importance in experimental work as a source of a-rays entirely free from j3- and 7-rays. The weight of polonium in equilibrium with one gram of radium is only about one-fifth of a milligram, so that it is difficult to obtain a sufficient quantity of this material for an atomic weight determination.

Mme. Curie separated the polonium from several tons of pitch blende and obtained an exceedingly active preparation of a few milligrammes, but was unable to obtain it in a pure state, although several of its spectrum lines were detected. It was hoped by this experiment to decide whether polonium was transformed directly into lead, but this was found difficult to establish owing to the presence of impurities with the very small quantity of polonium. The most convenient source for obtaining strong sources of polonium is the radium D-1-E-FF which accumulates in old radium preparations. These substances can be separated from the radium solution by chemical or electrolytic methods. A solution of the radium D thus obtained grows polonium and is thus a very con venient source for obtaining active plates coated with polonium. In this way, also, strong sources of radium D and radium E are obtainable.

Radon (Radium Emanation).—Radon is to be regarded as a typical radioactive product or transition element which exists in a gaseous form. It is produced from radium at a constant rate, and is transformed into radium A and helium. Its half period of transformation is 3.83 days. Radon has been purified by condensing it in liquid air, and pumping out the residual gases. The volume of radon at normal pressure and temperature to be derived from one gram of radium in equilibrium is about o.6 cubic millimetres. This small quantity of gas contains initially more than three-quarters of the total activity of the radium be fore its separation. In a pure state, radon is ioo,000 times as active weight for weight as pure radium. Pure radon in a spectrum tube gives a characteristic spectrum of bright lines. The dis charge in the gas is bluish in colour. Notwithstanding the minute volume of radon available, its boiling-point has been determined at various pressures. At atmospheric pressure Rutherford found the boiling-point to be —67° C, and Gray and Ramsay — 7I° C. Liquid radon appears colourless when first condensed ; when the temperature is lowered, the liquid freezes, and at the temperature of liquid air glows with a bright rose colour. The density of liquid radon has been estimated at 5 or 6.