For the α rays,
226. Effect of temperature. We have so far not considered the evidence on which the 28-minute rather than the 21-minute change is supposed to take place in the matter C. This evidence has been supplied by some recent important experiments of P. Curie and Danne[[316]] on the volatilization of the active matter deposited by the emanation. Miss Gates[[317]] showed that this active matter was volatilized from a platinum wire above a red heat and deposited on the surface of a cold cylinder surrounding the wire. Curie and Danne extended these results by subjecting an active platinum wire for a short time to the action of temperatures varying between 15° C. and 1350° C., and then examining at room temperatures the decay curves not only for the active matter remaining on the wire, but also for the volatilized part. They found that the activity of the distilled part always increased after removal, passed through a maximum, and finally decayed according to an exponential law to half value in 28 minutes. At a temperature of about 630° C. the active matter left behind on the wire decayed at once according to an exponential law, falling to half value in 28 minutes. P. Curie and Danne showed that the matter B is much more volatile than C. The former is completely volatilized at about 600° C., while the latter is not completely volatilized even at a temperature of 1300° C. The fact that the matter C, left behind when B is completely volatilized, decays at once to half value in 28 minutes shows that the matter C itself and not B is half transformed in 28 minutes.
Curie and Danne also found that the rate of decay of the active matter varied with the temperature to which the platinum wire had been subjected. At 630° C. the rate of decay was normal, at 1100° C. the activity fell to half value in about 20 minutes, while at 1300° C. it fell to about half value in about 25 minutes.
I have repeated the experiments of Curie and Danne and obtained very similar results. It was thought possible that the measured rate of decay observed after heating might be due to a permanent increase in the rate of volatilization of C at ordinary temperatures. This explanation, however, is not tenable, for it was found that the activity decreased at the same rate whether the activity of the wire was tested in a closed tube or in the open with a current of air passed over it.
These results are of great importance, for they indicate that the rate of change of the product C is not a constant, but is affected by differences of temperature. This is the first case where temperature has been shown to exert an appreciable influence on the rate of change of any radio-active product.
227. Volatility of radium B at ordinary temperature. Miss Brooks[[318]] has observed that a body, made active by exposure to the radium emanation, possesses the power of exciting secondary activity on the walls of a vessel in which it is placed. This activity was usually about ¹⁄₁₀₀₀ of the whole, but the amount was increased to about ¹⁄₂₀₀ if the active wire was washed in water and dried over a gas flame—the method often adopted to free the wire of any trace of the radium emanation. This effect of producing activity was most marked immediately after removal of the wire from the emanation, and was almost inappreciable ten minutes afterwards.
The effect was particularly noticeable in some experiments with a copper plate, which was made active by leaving it a short time in a solution of the active deposit from radium. This active solution was obtained by placing an active platinum wire in dilute hydrochloric acid. On placing the copper plate in a testing vessel for a few minutes, and then removing it, activity was observed on the walls of the vessel amounting to about one per cent. of the activity of the copper plate.
It was found that this effect was not due to the emission of an emanation from the active body, but must be ascribed to a slight volatility of radium B at ordinary temperatures. This was proved by observations on the variation of the activity of the matter deposited on the walls of the vessel. The activity was small at first, but rose to a maximum after about 30 minutes, and then decayed with the time. The curve of rise was very similar to that shown in Fig. 87, and shows that the inactive matter radium B was carried to the walls and there changed into C, which gave rise to the radiation observed.
The product B only escapes from the body for a short time after removal. This is a strong indication that its apparent volatility is connected with the presence of the rapidly changing product radium A. Since A breaks up with an expulsion of an α particle, some of the residual atoms constituting radium B may acquire sufficient velocity to escape into the gas, and are then transferred by diffusion to the walls of the vessel.