Mass of Long-range Particles from Thorium C. 571 



would be emitted in number from the radioactive source 

 instead of from the volume of the gas bombarded by 

 a rays. 



In the original experiments of Dr. Wood and the writer, 

 the active deposit of thorium was used as a source of radiation, 

 and with the active material available, the 7-ray activity of 

 •the source of radiation was about 2/100 of a milligram of 

 radium in equilibrium. In order to obtain about 20 scin- 

 tillations a minute, due to the long-range particles, it was 

 necessary in these experiments to place the mica within a 

 few millimetres of the zinc-sulphide screen In order, 

 however, to determine the mass of the particles by observing 

 the amount of their deflexion in a magnetic field, it was 

 imperative to work at a distance of at least 5 centimetres, 

 and to employ a source at least 100 times stronger. 



By the generosity of Dr. Herbert McCoy of Chicago, well 

 known for his contributions to our knowledge of the radio- 

 activity of thorium, I was presented with a quantity of radio- 

 thorium of 7-ray activity equal to 24 milligr^us of radium. In 

 order to obtain powerful sources of thorium C, this material, 

 after suitable chemical treatment, was obtained in a dilute 

 solution of small volume. I am indebted to Mr. Chadwick 

 for his kindness in preparing this solution in a form to yield 

 the maximum amount of thorium C. This was obtained by 

 exposing one side of a nickel plate of area about one square 

 centimetre in the hot solution for one hour. During the 

 exposure the plate was kept in slow rotation by a small motor. 

 By this method it was possible to obtain an amount of thorium C 

 on the surface of the nickel plate equal in y-ray activity in- 

 itially to about 8 milligrams of radium. The activity of this 

 source decayed with the time according to the period of 

 thorium C, viz. to half value in one hour. 



This source gave a sufficient number of long-range particles 

 of range 11*3 cm. to determine their bending in a magnetic 

 field by the scintillation method. Two different arrangements 

 were employed. The first was similar to that described in 

 the Bakerian Lecture (Proc. Roy. Soc. A, 1912), and illus- 

 trated in fig. 1 of that paper. The a rays from the nickel 

 plate passed between two parallel plates, I cm. long and 

 2 cm. apart, placed in an exhausted rectangular box between 

 the poles of a large electromagnet. The plates were distant 

 1*2 cm. from an opening in the end of the box, which was 

 covered by an aluminium plate of stopping power for 'a par- 

 ticles equal to 5*4 cm. of air. To examine the deflexion oi' 

 the long-range particles, additional absorbers were added to 

 stop completely the a rays of range 8*6 cm. For this purpose 



