About 25 minutes after removal, the activity decays approximately at the same rate in each case. For convenience of representation, the ordinates of the curves were adjusted so that they all passed through a common point. We shall see later (chapter XI) 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 Danne8 made an investigation of the curves of decay of excited activity for different times of exposure to the radium emanation, but apparently did not take into account the fact that measurements made by the α and β rays give quite different curves of decay. Some of the family of curves, given in their paper, refer to the α rays and others to the β rays. They showed, however, the important fact that the curve of decay obtained by them for a long exposure (which is identical with the β ray curve) could be empirically expressed by an equation of the form
