642 Miss J. M. W. Slater on the 



on a lead cylinder, as explained above, and its rate of decay 

 was also tested. The curves obtained are shown in diagram 5, 

 and they are in complete agreement with the above deductions. 

 The activity of the lead is at first very small, showing that 

 there is very little thorium B present. It rises for about 

 3 hours, when it reaches a maximum many times greater 

 than the value given by the first readings, and then begins 

 to fall off. The activity of the wire as before falls rapidly 

 at first ; after some 4 hours the rates of decay of the two 

 fractions become the same, both having the normal value of 

 half in 11 hours. The activity of the lead is only a part 

 of that lost by the wire ; it is plotted on a larger scale in the 

 •diagram. In this case no thorium B has been lost by the 

 wire during the heating ; the active matter on the lead is at 

 first entirely thorium A, and the curve it gives is similar to 

 that of a surface exposed for a short time to thorium. 



Temperature of Separation. 



The temperature at which volatilization of each constituent 

 occurs was then examined in more detail. The percentage of 

 each substance removed was calculated in the same way as in 

 the previous series of experiments. The loss of thorium B 

 was measured by the immediate reduction of activity, that of 

 thorium A by the difference between the value observed for 

 the activity after 3 or 4 hours, and the value it would have 

 had if the rate of decay had been normal. 



It was found that thorium A begins to distil at a con- 

 siderably lower temperature than had previously been noticed. 

 At quite a dull red heat it begins to volatilize, and below 

 700° some thorium B is also removed. (I minutes' heating 

 at 1050° is enough to remove all the thorium A and the 

 greater part of the thorium B, while 2 or 3 minutes' heating 

 at 1100° or 1200° leaves the wire practically inactive. Some 

 typical curves are shown in diagram 6, where the scales 

 have been so arranged that the activity immediately before 

 heating has the same value (50) in each case. The radio- 

 active constants for these curves agree well with those given 

 by Prof. Rutherford. At the higher temperatures, where 

 the thorium A has been almost entirely driven off, the 

 activity falls to half value in very little more than the 55 

 minutes required by pure thorium B. 



The following table gives a summary of the chief results 

 obtained by heating to different temperatures : — 



