=223. Analysis of the α-ray curves=. The analysis of the decay curves of the excited activity of radium, measured by the α rays, will now be discussed. The following table shows the variation of the intensity of the radiation after a long exposure in the presence of the radium emanation. A platinum plate was made active by exposure for several days in a glass tube containing a large quantity of emanation. The active platinum after removal was placed on the lower of two parallel insulated lead plates, and a saturating electromotive force of 600 volts was applied. The ionization current was sufficiently large to be measured by means of a sensitive high-resistance galvanometer, and readings were taken as quickly as possible after removal of the platinum from the emanation vessel. The initial value of the current (taken as 100) was deduced by continuing the curves backwards to meet the vertical axis (see Fig. 90), and was found to be 3 × 10⁻⁸ ampere. Time in Current minutes 0 100 2 80 4 69·5 6 62·4 8 57·6 10 52·0 15 48·4 20 45·4 30 40·4 40 35·6 50 30·4 60 25·4 80 17·4 100 11·6 120 7·6 These results are shown graphically in the upper curve of Fig. 90. The initial rapid decrease is due to the decay of the activity of the matter A. If the slope of the curve is produced backwards from a time 20 minutes after removal, it cuts the vertical axis at about 50. The difference between the ordinates of the curves _A_ + _B_ + _C_ and _LL_ at any time is shown in the curve _AA_. The curve _AA_ represents the activity at any time supplied by the change in radium _A_. The curve _LL_ starting from the vertical axis is identical with the curve already considered, representing the decay of activity measured by the β rays for a long exposure (see Fig. 88). Time in Calculated Observed minutes value of value of activity activity 0 100 100 10 96·8 97·0 20 89·4 89·2 30 78·6 80·8 40 69·2 71·2 50 59·9 60·8 60 49·2 50·1