This curve is similar in shape to the corresponding curve obtained for the active deposit from thorium, and is explained in a similar way. The activity It at any time t is given by
where λ1 and λ2 are two constants, and IT the maximum activity reached after an interval T. After 20 minutes the activity decreased exponentially with the time, falling to half value in 35·7 minutes. This gives the value λ1 = ·0194 (min.)-1. By comparison with the curve, the value of λ2 was found to be ·317 (min.)-1. This corresponds to a change in which half the matter is transformed in 2·15 minutes. Exactly as in the analogous curve for thorium, it can be shown that the matter initially deposited undergoes two changes, the first of which is a rayless one. The same difficulty arises in fixing which of the values of λ refers to the first change. An experiment made by Miss Brooks (loc. cit.) shows that the rayless product has the slower period of transformation. The active deposit of actinium was dissolved off a platinum wire and then electrolysed. The anode was found to be active, and the activity fell off exponentially with the time, decreasing to half value in about 1·5 minutes. Allowing for the difficulty of accurately measuring such a rapid rate of decay, this result indicates that the product which gives out rays has the rapid period of 2·15 minutes. The analysis of the active deposit of actinium thus leads to the following conclusions:
(1) The matter initially deposited from the emanation, called actinium A, does not give out rays, and is half transformed in 35·7 minutes.
(2) A changes into B, which is half transformed in 2·15 minutes, and gives out both α and β (and probably γ) rays.