190 PROFESSOR E. RUTHERFORD OX THE 



For a very long exposure 



I,/I = 1-038 e~^ - -038 -**, 



where I n is the initial value after removal, and 



X, = 2-80 X 10-+, X, = 77 X 10~ s . 



For the first 10 minutes after removal, the activity in consequence decays more 

 slowly than is to be expected on a simple exponential law. This result has been 

 observed experimentally. The variation of the activity for any time of exposure 

 to the emanation is expressed by an equation of the same form as for thorium, 

 equation (12), with the values X l5 X 2 found above. 



There is some evidence that there is a product actinium X in actinium, corresponding 

 to thorium X in thorium. This point is at present under investigation, and the 

 results will be given in a later paper. If this is the case, actinium and thorium are 

 very closely allied in the number and nature of their products. Both give rise to an 

 emanation, and this is transformed into an active deposit which undergoes two further 

 transformations, the first change being a ' rayless ' one. 



16. Changes in the Active Deposit from Radium. In the case of the active 

 deposit from radium, we are dealing with matter that undergoes at least four successive 

 changes. For convenience, the matter initially produced from the emanation will be 

 called the matter A and the succeeding products B, C, D, E respectively. The 

 equations expressing the quantities of A, B, C, D present at any time are complicated, 

 but the comparison of theory with experiment may be simplified by temporarily 

 disregarding some unimportant terms. For example, the activity of the matter D is 

 generally negligible compared with that of A or C, being as a rule less than 1/100,000 

 of the initial activity observed for the matter A or C. A still further simplification 

 can be made by disregarding the first 3-minute change. In the course of 6 minutes 

 after removal, three-quarters of the matter A has been transformed into B, and 

 20 minutes after removal all but about 1 per cent, has been transformed. The 

 variation of the amount of matter B or C present at any time agrees more closely 

 with the theory, if the first change is disregarded altogether. A discussion of this 

 important point is given later in section 21. 



17. ft-Ray Curv es. The explanation of the /3-ray curves (see figs. 5 and G), obtained 

 for different times of exposure, will be first considered. For a very short exposure, 

 the activity measured by the /8 rays is small at first, passes through a maximum 

 about 36 minutes later, and then decays steadily with the time. 



The curve shown in fig. 6 is very similar in general shape to the corresponding 

 thorium and actinium curves. It is thus necessary to suppose that the change of the 

 matter B into C does not give rise to /8 rays, while the change of C into D does. In 

 sucli a case the activity (measured by the ft rays) is proportional to the amount of C 

 present. Disregarding the first rapid change, the activity I, at any time f should be 



