﻿206 Prof. J. Jo 1 .}* on the Radioactivity of 



were, with two exceptions, made by the less variable in- 

 strument. 



It is known that when radium emanation is admitted to an 

 electroscope there is at first a rapid increase of the ionization 

 due to the initial formation of the active deposit of short 

 period. A marked slowing arises after about 10 minutes, 

 and a temporary maximum is reached which persists till 

 about the twentieth minute *. I have found that the rate of 

 collapse of the leaf as observed during the period of tem- 

 porary maximum is ultimately increased about 33 per cent. 

 at the expiration of 3 hours. This observation applies both 

 to the calibration tests and to the rock measurements. The 

 labour of making the observations may be lessened by either 

 calibrating the instrument for the effect in 15 or 20 minutes, 

 and correspondingly reading the leak obtaining in 15 

 minutes, when rock experiments are being made. Or, again, 

 we mav use the calibration reading observed in three hours 

 provided the reading obtained for the rock in 15-20 minutes 

 is increased 33 per cent. It is true that the 33 per cent, is 

 occasionally not fully attained and occasionally it is exceeded. 

 Soddy has pointed out one source to which variations may be 

 referred |. In a long series of measurements it is very 

 improbable that any serious error can arise by the adoption 

 of this method of reading the electroscope, and the gain in 

 doing so is considerable, not only in the saving of time but 

 in the better preservation of the electroscope. In the recent 

 experiments herein recorded this method has been used for 

 the greater number of the low results. In the higher and 

 therefore more important and questionable readings the 

 instrument was in every case but one allowed to stand for 

 the full three hours, and in some cases for an additional 

 hour. 



The melts derived from the previous experiments were 

 preserved in stoppered tubes and subsequently leached in 

 water, filtered, and the residue acidified with thorium-free 

 hydrochloric acid. The resulting solutions were examined 

 by Mr. A. L. Fletcher for thorium, using the method de- 

 scribed in the 'Philosophical Magazine,' May 1909, p. 760. 

 I have to thank Mr. Fletcher for carrying out this work. 

 The acid solutions alone were examined ; as it has been 

 repeatedly found that there is no appreciable amount of 

 thorium removed in the first filtrate. 



* Soddv, Phil. Mag-. Aug. 1907. p. 280. 

 t Soddy Phil Mag-. Dec. 1909, p. 848. - . 



