Page:Radio-activity.djvu/423

 activity. The activity reaches half its final value in about six days, and the value of λ is equal to ·115 (day)^{-1}. We have shown in section 203 that a rising curve of this character indicates that the β ray activity arises from a product which is supplied at a constant rate from a primary source. Before discussing in detail the explanation of these curves, showing the rise with time of the α and β ray activity, further experimental results will be considered.

232. Effect of temperature on the activity. A platinum plate, made active in the manner described, was exposed to varying temperatures in an electric furnace, and the activity tested at atmospheric temperature after exposure. Four minutes' exposure in the furnace, at first at 430° C., and afterwards at 800° C., had little, if any, effect on the activity. After four minutes at about 1000° C. the activity decreased about 20 per cent., and a further exposure of eight minutes at a temperature of about 1050° C. almost completely removed the α ray activity. On the other hand, the β ray activity, when measured immediately after removal, was not altered by the heating, but exposure to a still higher temperature caused it to decrease. These results show that the active matter consists of two kinds. The part which emits β rays is not volatile at 1000° C., but the other part, which emits α rays, is almost completely volatilized at that temperature.

It was found, however, that the β ray activity after heating to about 1000° was not permanent, but decayed according to an exponential law with the time, the activity decreasing to half value in about 4·5 days. From the recovery curve of the β ray activity already considered, it was to be expected that the activity would decay to half value in six days. This difference in the periods is possibly due to an effect of the high temperature in altering the rate of decay of radium E. The period of six days is more probably correct. The results obtained on the rise and decay of the β rays, taken together, show:—

(1) That the product giving β rays is supplied at a constant rate from some parent matter of very slow rate of change.

(2) That this parent matter is volatilized at or below 1000° C., and the β ray product is left behind. Since the parent