y-Rays of Thorium and Actinium. 133 



For measurement of /3-rays a cylindrical brass electroscope 

 of the ordinary type, 13 cm. high and 10'8 cm. diameter, was 

 used. The thickness of the walls was 0"32 cm. The base 

 consisted of a layer of aluminium foil, 0095 mm. thick. In 

 the first series of measurements the preparation was placed 

 at a distance of 60 cm. below the base, but later various 

 shorter distances were also used. The apparatus was set np 

 so as to reduce secondary /3-radiation to a minimum. The 

 electroscope was flush with the table, which had a large hole 

 cut in it, and was supported by light steel brackets from the 

 wall. The preparation was supported centrally below the 

 electroscope on a light framework of brass rods, made in 

 sections screwed together, and attached to the under side of 

 the table, so that any distance between the preparation and 

 base of the electroscope could be employed. For mea- 

 surements of 7-rays three different dispositions were used, as 

 follows ; — 



(7i) The active preparation was placed at a distance of 

 8*6 cm. below an electroscope oflead of the usual type having 

 wall and base thickness 0*3 cm. of lead ; 



(72) at a distance of 3'3 cm. below a lead electroscope 

 whose wall was 0*65 cm. and whose base 0*U75 cm. thick. 



(73) At a distance of 8*7 cm. below the same electroscope 

 as in 7 2 . 



The radium C was prepared by exposing one side of a 

 negatively charged brass disk to a quantity of radium 

 emanation for 20 hours. After the exposure the disk was 

 placed in a brass cell and a piece of 0'095 mm. thick 

 aluminium foil was cemented over the cell to prevent any 

 possible escape of adhering emanation. One hour after the 

 preparation of the film of radium C, alternative measurements 

 of the (3- and 7-rays, in the four different dispositions detailed 

 above, were taken over a period of three hours, and from the 

 four decay curves obtained from these measurements the /3- 

 and the three different 7-activities could be readily deduced. 

 The four decay curves obtained were found to be very nearly 

 exponential with values of X very approximately the same, 

 (1*39 (hour) _1 ). The relative values therefore of the intensity 

 of the preparations measured under the four different dispo- 

 sitions could be accurately obtained. 



For thorium D the procedure of preparation of the disk 

 was exactly the same, the disk being exposed negatively 

 charged to the thorium emanation from a preparation of 

 radiothorium spread out on a shallow dish for 20 hours under 

 conditions, such that only the one side of the disk was coated. 

 The disk was then mounted and covered with the same 



