Rutherford and McClung[[325]] made an estimate of the energy of the rays, emitted by a thin layer of active matter, by determining the total number of ions produced by the complete absorption of the α rays. The energy required to produce an ion was determined experimentally by observations of the heating effect of X rays, and of the total number of ions produced when the rays were completely absorbed in air. The energy required to produce an ion in air was found to be 1·90 × 10-10 ergs. This, as will be shown in [Appendix A], is probably an over-estimate, but was of the right order of magnitude. From this it was calculated that one gram of uranium oxide spread over a plate in the form of a thin powdered layer emitted energy into the air at the rate of 0·032 gram calories per year. This is a very small emission of energy, but in the case of an intensely radio-active substance like radium, whose activity is about two million times that of uranium, the corresponding emission of energy is 69000 gram calories per year. This is obviously an under-estimate, for it includes only the energy radiated into the air. The actual amount of energy released in the form of α rays is evidently much greater than this on account of the absorption of the α rays by the active matter itself.
It will be shown later that the heating effect of radium and of its products is a measure of the energy of the expelled α particles.
244. Heat emission of radium. P. Curie and Laborde[[326]] first drew attention to the striking result that a radium compound kept itself continuously at a temperature several degrees higher than that of the surrounding atmosphere. Thus the energy emitted from radium can be demonstrated by its direct heating effect, as well as by photographic and electric means. Curie and Laborde determined the rate of the emission of heat in two different ways. In one method the difference of temperature was observed by means of an iron-constantine thermo-couple between a tube containing one gram of radiferous chloride of barium, of activity about ⅙ of pure radium, and an exactly similar tube containing one gram of pure barium chloride. The difference of temperature observed was 1·5° C. In order to measure the rate of emission of heat, a coil of wire of known resistance was placed in the pure barium chloride, and the strength of the electric current required to raise the barium to the same temperature as the radiferous barium was observed. In the other method, the active barium, enclosed in a glass tube, was placed inside a Bunsen calorimeter. Before the radium was introduced, it was observed that the level of the mercury in the stem remained steady. As soon as the radium, which had previously been cooled in melting ice, was placed in the calorimeter, the mercury column began to move at a regular rate. If the radium tube was removed, the movement of the mercury ceased. It was found from these experiments that the heat emission from the 1 gram of radiferous barium, containing about ⅙ of its weight of pure radium chloride, was 14 gram-calories per hour. Measurements were also made with 0·08 gram of pure radium chloride. Curie and Laborde deduced from these results that 1 gram of pure radium emits a quantity of heat equal to about 100 gram-calories per hour. This result was confirmed by the experiments of Runge and Precht[[327]] and others. As far as observation has gone at present, this rate of emission of heat is continuous and unchanged with lapse of time. Therefore, 1 gram of radium emits in the course of a day 2400, and in the course of a year 876,000 gram-calories. The amount of heat evolved in the union of hydrogen and oxygen to form 1 gram of water is 3900 gram-calories. It is thus seen that 1 gram of radium emits per day nearly as much energy as is required to dissociate 1 gram of water.
In some later experiments using 0·7 gram of pure radium bromide, P. Curie[[328]] found that the temperature of the radium indicated by a mercury thermometer was 3° C. above that of the surrounding air. This result was confirmed by Giesel, who obtained a difference of temperature of 5° C. with 1 gram of radium bromide. The actual rise of temperature observed will obviously depend upon the size and nature of the vessel containing the radium.
During their visit to England in 1903 to lecture at the Royal Institution, M. and Mme Curie performed some experiments with Professor Dewar, to test by another method the rate of emission of heat from radium at very low temperatures. This method depended on the measurement of the amount of gas volatilized when a radium preparation was placed inside a tube immersed in a liquefied gas at its boiling point. The arrangement of the calorimeter is shown in [Fig. 97].
Fig. 97.
The small closed Dewar flask A contains the radium in a glass tube R, immersed in the liquid to be employed. The flask A is surrounded by another Dewar bulb B, containing the same liquid, so that no heat is communicated to A from the outside. The gas liberated in the tube A is collected in the usual way over water or mercury, and its volume determined. By this method, the rate of heat emission of the radium was found to be about the same in boiling carbon dioxide and oxygen, and also in liquid hydrogen. Especial interest attaches to the result obtained with liquid hydrogen, for at such a low temperature ordinary chemical activity is suspended. The fact that the heat emission of radium is unaltered over such a wide range of temperature indirectly shows that the rate of expulsion of α particles from radium is independent of temperature, for it will be shown later that the heating effect observed is due to the bombardment of the radium by the α particles.
The use of liquid hydrogen is very convenient for demonstrating the rate of heat emission from a small amount of radium. From 0·7 gram of radium bromide (which had been prepared only 10 days previously) 73 c.c. of gas were given off per minute.
In later experiments P. Curie (loc. cit.) found that the rate of emission of heat from a given quantity of radium depended upon the time which had elapsed since its preparation. The emission of heat was at first small, but after a month’s interval practically attained a maximum. If a radium compound is dissolved and placed in a sealed tube, the rate of heat emission rises to the same maximum as that of an equal quantity of radium in the solid state.