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 of rays always occur together and in the same proportion. The curves measured by the β rays are very different, especially for the case of a short exposure to the emanation. This is clearly shown in Fig. 68, which gives the β and γ ray curves for exposures of 10 minutes, 40 minutes, and 1 hour, and also the limiting case of an exposure of 24 hours.

Fig. 67.

About 25 minutes after removal, the activity decays approximately at the same rate in each case. For convenience of repre