• Ramsay' and Soddy succeeded in isolating the emanation of radium This gas is brilliantly luminous and rapidly blackens the tube contain ing it. The emanation has a definite spectrum of .bright lines, similar in general character to that observed for the inert gases argon or helium.
Curie and Laborde first observed the striking fact that radium kept itself continuously at a temperature several degrees higher than the surrounding air. The radium is continuously emitting heat at the rate of 132 gram calories per hour per gram of radium in equilibrium with its products ; that is, it would heat its own weight of water through more than C. in one hour. Rutherford and Barnes showed that the greater part of the heating effect of radium was not due directly to the radium itself, but to the emanation continuously produced from it. This emanation was separated from the radium, and condensed in a small glass tube immersed in liquid air. The heating effect of the minute quantity of emana tion thus obtained was found to be three quar ters of that originally possessed by the radium. Compared with its weight, the radium emana tion emits a quantity of energy more than a million times greater than that released in the most violent chemical reaction. The heating effect of the emanation is not, however, permanent, but dies away with the time at the same rate as it loses its activity; that is, it falls to half value in about four days.
If a cubic centimeter of the emanation were collected in a glass tube, it would probably give off sufficient heat to melt down the walls of the tube. It can readily be deduced that one pound weight of the emanation, after its separa tion from radium, would emit energy at the rate of 10,000 horse power and, during the time that its activity lasts, would emit an amount of energy corresponding to 60,000 horsepower days. The heating effects of radium have been found to be directly due to the expulsion of a particles from its mass. Each of these particles is projected with such a great velocity that it has considerable energy of motion, which is transformed into heat when it strikes a body. These a particles are easily stopped by matter and, in a mass of radium, most of the a par ticles projected throughout its volume are not able to escape, but are stopped by the radium it self. Radium is thus heated by the unceasing and vigorous self-bombardment by the a par ticles shot out from its own mats. The heat ing,effect of the emanation in a similar way, is due to the energy of motion possessed by the a particles expelled from it.
The emanations all show another very strik ing property. If the emanation is drawn through a tube, the surface of the tube becomes coated with an invisible active deposit. This deposit can be dissolved in acids, and after evaporation of the solvent, remains behind on the dish. After removal of the emanation, the deposit gradually loses its activity but at a dif ferent rate from the emanation itself. This
"excited" or "induced" activity, as it is termed, is due to radioactive matter which arises from the decomposition of the emanation. The ex cited activity of thorium falls to half value in 11 hours, that of radium in •about 30 minutes, and that of ..ctinitim in .36 minutes.
Considerable light was thrown on radio active phenomena by some experiments of Rutherford and Soddy that will now be con sidered. It was found that an intensely active substance called thorium X could be separated by a single chemical process from thorium. If some thorium nitrate is dissolved in water and ammonia added, the thorium is precipitated. If the ammonia solution is then evaporated and ignited, a very active residue is obtained, sev eral thousand times more active, weight for weight, than thorium itself. The thorium itself is at the same time deprived of more than half of its original activity. A very interesting re sult is then observed. The activity of this resi due thorium X is found not to be permanent but to decay according to a geometrical pro gression with the time, falling to half value in four days. At the end of a month, the activity is less than 1 per cent of the initial value. At the same time, the precipitated thorium regains its lost activity, such that the sum of the activ ity of the thorium and thorium X together is always the same, and equal to that of the orig inal thorium before chemical treatment. At the end of a month's interval, the thorium has nearly recovered its old value, and a new quan tity of thorium X can then be obtained by the same chemical operation. This process can be continued indefinitely provided sufficient ' in terval is allowed to elapse between each pre cipitation for the thorium to regain its activity. These results are very simply and fully ex plained on the following hypothesis. A small fraction of the thorium atoms every second be comes unstable and breaks up with explosive violence, an a particle being projected during the process. After the expulsion of an a tide, the the residue of the thorium atom is lighter than before and becomes the atom of the new substance, thorium X. This substance is again unstable and breaks up with the expulsion of a particles, half of the matter breaking up in four days. In ordinary thorium, there are thus two opposing processes at work. The substance thorium X is produced at a constant rate by the disintegration of some of the thorium atoms, and at the same time this thorium X breaks up and changes into another substance A steady state will obviously be reached when the number of atoms of thorium X, which are •pro duced per second, is equal to the number of atoms of thorium X which break up per second. The constant radioactivity of thorium may thus be likened to the constant population of a Cl D un try, where the number of births is equal to the number of deaths.