762 Prof. J. Joly on the Distribution of Thorium 



which is neither too fast nor too slow. If too slow a large 

 part of the emanation must be transformed before it attains 

 the electroscope. If too fast it is swept through the electro- 

 scope in so highly dilute a condition that, again, its effects 

 must be enfeebled. There must be, therefore, some most 

 favourable rate at which the air-flow should take place. 

 With the arrangements described we evidently have the 

 adjustment of the current under control. To find the most 

 favourable current we work with solutions containing small, 

 known amounts of thorium ; first reading the normal rate of 

 discharge of the electroscope while the air-current is flowing, 

 and then again when the emanation is entering on the air- 

 current. It will be found that the normal rate of discharge 

 is but little affected, if at all, by small variations in the rate 

 of air-flow. We can in this manner not only determine the 

 most favourable air-current, but we can standardize the 

 readings of the electroscope. 



The electroscope I have used in the observations given 

 below has a voluminar capacity of about 450 ccs. The gold 

 leaf is about 2*5 x 0*2 cm. in dimension, and the electrostatic 

 capacity is such that the radium constant is 0*8 x 10~ 12 grams 

 radium per scale-division per hour. 



The following experiments will show the degree of sensi- 

 tiveness and consistency attained by this method of measuring- 

 small quantities of thorium. I am by no means satisfied 

 that the best rate of transfer of the emanation has yet been 

 attained. The thorium salt used is the nitrate as supplied 

 by Kahlbaum. 



Gain in Scale- 

 Divisions per hour. 



6-6 X 10-4 grams. Th in 450 ccs. distilled water 22"5 



3-3 „ ., „ 1400 „ sea-water 113 



1-7 „ „ „ 1600 „ „ 6'3 



1'7 „ „ „ 1500 „ rock-solution 7'6 



Under different and less favourable conditions of air- 

 current the following comparative observations were made 

 on (a) thorium nitrate of Kahlbaum, (b) thorium oxide of 

 Kahlbaum, and (c) a specimen of thorium oxide kindly given 

 to me by Mr. Moss ; a specimen which had been some years 

 in his possession. 



Gain in Scale- 

 Divisions per hour. 



3-3X 10-4 grams. Th (a) in 1000 ccs. water 6*9 



6-8 „ „ „ (b) „ 1000 „ „ 14-0 



88 „ „ „ (c) „ 1400 „ „ 19-0 



It w^ould seem, from the foregoing results, that the readings 

 of the electroscope are not dependent on the volume of the 

 solution containing the thorium, and consequently we must 



