245. Connection of the heat emission with the radiations. The observation of Curie that the rate of heat emission depended upon the age of the radium preparation pointed to the conclusion that the phenomenon of heat emission of radium was connected with the radio-activity of that element. It had long been known that radium compounds increased in activity for about a month after their preparation, when they reached a steady state. It has been shown ([section 215]), that this increase of activity is due to the continuous production by the radium of the radio-active emanation, which is occluded in the radium compound and adds its radiation to that of the radium proper. It thus seemed probable that the heating effect was in some way connected with the presence of the emanation. Some experiments upon this point were made by Rutherford and Barnes[[329]]. In order to measure the small amounts of heat emitted, a form of differential air calorimeter shown in Fig. 98 was employed. Two equal glass flasks of about 500 c.c. were filled with dry air at atmospheric pressure. These flasks were connected through a glass U-tube filled with xylene, which served as a manometer to determine any variation of pressure of the air in the flasks. A small glass tube, closed at the lower end, was introduced into the middle of each of the flasks. When a continuous source of heat was introduced into the glass tube, the air surrounding it was heated and the pressure was increased. The difference of pressure, when a steady state was reached, was observed on the manometer by means of a microscope with a micrometer scale in the eye-piece. On placing the source of heat in the similar tube in the other flask, the difference in pressure was reversed. In order to keep the apparatus at a constant temperature, the two flasks were immersed in a water-bath, which was kept well stirred.
Fig. 98.
Observations were first made on the heat emission from 30 milligrams of radium bromide. The difference in pressure observed on the manometer was standardized by placing a small coil of wire of known resistance in the place of the radium. The strength of the current through the wire was adjusted to give the same difference of pressure on the manometer. In this way it was found that the heat emission per gram of radium bromide corresponded to 65 gram-calories per hour. Taking the atomic weight of radium as 225, this is equivalent to a rate of emission of heat from one gram of metallic radium of 110 gram-calories per hour.
The emanation from the 30 milligrams of radium bromide was then removed by heating the radium ([section 215]). By passing the emanation through a small glass tube immersed in liquid air, the emanation was condensed. The tube was sealed off while the emanation was still condensed in the tube. In this way the emanation was concentrated in a small glass tube about 4 cms. long. The heating effects of the “de-emanated” radium and of the emanation tube were then determined at intervals. It was found that, after removal of the emanation, the heating effect of the radium decayed in the course of a few hours to a minimum, corresponding to about 25 per cent. of the original heat emission, and then gradually increased again, reaching its original value after about a month’s interval. The heating effect of the emanation tube was found to increase for the first few hours after separation to a maximum, and then to decay regularly with the time according to an exponential law, falling to half its maximum value in about four days. The actual heat emission of the emanation tube was determined by sending a current through a coil of wire occupying the same length and position as the emanation tube.
The variation with time of the heating effect from 30 milligrams of radium and the emanation from it is shown in [Fig. 99].
Fig. 99.
Curve A shows the variation with time of the heat emission of the radium and curve B of the emanation. The sum total of the rate of heat emission of the radium and the emanation together, was at any time found to be equal to that of the original radium. The maximum heating effect of the tube containing the emanation from 30 milligrams of radium bromide was 1·26 gram-calories per hour. The emanation together with the secondary products which arise from it, obtained from one gram of radium, would thus give out 42 gram-calories per hour. The emanation stored up in the radium is thus responsible for more than two-thirds of the total heat emission from radium. It will be seen later that the decrease to a minimum of the heating effect of radium, after removal of the emanation, is connected with the decay of the excited activity. In a similar way, the increase of the heating effect of the emanation to a maximum some hours after removal is also a result of the excited activity produced by the emanation on the walls of the containing vessel. Disregarding for the moment these rapid initial changes in heat emission, it is seen that the heating effect of the emanation and its further products, after reaching a maximum, decreases at the same rate as that at which the emanation loses its activity, that is, it falls to half value in four days. If Qmax. is the maximum heating effect and Qt the heating effect at any time t later, then
