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 Fig. 68.

all passed through a common point. We shall see later (chapter ) that the rates of decay are not identically the same until several hours after removal; but, in the above figure, it is difficult to represent the slight variations. It will be observed that for the short exposure of 10 minutes the activity measured by the β rays is small at first but rises to a maximum in about 22 minutes, and then dies away with the time. The curve of decay of activity, measured by the β rays for a long exposure, does not show the rapid initial drop which occurs in all the α ray curves. Curie and Danne made an investigation of the curves of decay of excited activity for different times of exposure to the radium emanation,
 * sentation, the ordinates of the curves were adjusted so that they