Fig. 100.
The heating effect of the radium in radio-active equilibrium was first accurately determined. The radium tube was heated to drive off the emanation, which was rapidly condensed in a small glass tube 3 cms. long and 3 mms. internal diameter. After allowing a short time for temperature conditions to become steady, the heating effect of the radium tube was measured. The results are shown in [Fig. 100]. An observation could not be taken until about 12 minutes after the removal of the emanation, and the heating effect was then found to have fallen to about 55 per cent. of the maximum value. It steadily diminished with the time, finally reaching a minimum value of 25 per cent. several hours later.
It is not possible in experiments of this character to separate the heating effect of the emanation from that supplied by radium A. Since A is half transformed in three minutes, its heating effect will have largely disappeared after 10 minutes, and the decrease is then mainly due to changes in radium B and C.
The variation with time of the heating effect of the active deposit is still more clearly brought out by an examination of the rise of the heating effect when the emanation is introduced into a small tube, and of the decrease of the heating effect after the emanation is removed. The curve of rise is shown in the upper curve of [Fig. 101]. 40 minutes after the introduction of the emanation, the heating effect had risen to 75 per cent. of the maximum value which was reached after an interval of about 3 hours.
Fig. 101.
After the heating effect of the emanation tube had attained a maximum, the emanation was removed, and the decay with time observed as soon as possible afterwards. The results are shown in the lower curve of [Fig. 101]. It is seen that the two curves of rise and decay are complementary to one another. The first observation was made 10 minutes after removal, and the heating effect had then dropped to 47 per cent. of the original value. This sudden drop is due partly to the removal of the emanation, and partly to the rapid transformation of radium A. The lower curve is almost identical in shape with the corresponding α ray curve for the decay of the excited activity after a long exposure (see [Fig. 86]) and clearly shows that the heating effect is directly proportional to the activity measured by the α rays over the whole range examined. The heating effect decreases according to the same law and at the same rate as the activity measured by the α rays.
Twenty minutes after the removal of the emanation, radium A has been almost completely transformed, and the activity is then proportional to the amount of radium C present, since the intermediate product B does not give out rays. The close agreement of the activity and heat emission curves shows that the heating effect is proportional also to the amount of radium C. We may thus conclude that the rayless product B supplies little if any of the heat emission observed. If radium B supplied the same amount as radium C, the curve of decrease of heating effect with time would differ considerably from the activity curve.
The conclusion that the transformation of radium B is not accompanied by the release of as much heat as the other changes is to be expected if the heating effect is mainly due to the energy of motion of the expelled α particles.