a. Particles with Light Atoms. 541 



•obtain in this way a disk, coated on one side with radium C. 

 which has a gam ma-ray activity equal to 80 mg. of radium, 

 In most experiments, sources were employed of activity 

 between 5 and 80 mg. of Ha. 



The active disk after removal was washed in alcohol and 

 then heated for a minute in an exhausted tube inside an 

 electric furnace at about 300° 0. As Hatner (loc. cit.) has 

 pointed out, the treatment with alcohol reduces greatly the 

 loss of active matter by so-called volatilization, while the 

 heating tends to remove the surface gases and the emanation 

 occluded in the disk during its exposure. The quantity of 

 active matter on the disk was determined with the aid of a 

 standardized gamma-ray electroscope. The decrease of 

 intensity with time is known from the well-known curve 

 of decay. 



§ 4. Counting scintillations. 



As the systematic counting of H scintillations under varied 

 conditions is a rather difficult and trying task, it may be of 

 some value to mention the general arrangements found most 

 suitable and convenient in practice. Using the excellent 

 zinc sulphide screens, specially prepared by Mr. Glew, the 

 scintillation due to a high-speed H atom appears as a fine 

 brilliant star or point of light, very similar in appearance and 

 intensity to that produced by an alpha particle about 3 mm. 

 from the end of its range. Near the end of the range of the 

 H atom, the scintillation becomes very feeble, and can only be 

 observed on a dark background. Consequently, in a hetero- 

 geneous beam of H atoms, the actual number counted per 

 minute is to some extent dependent on the luminosity of the 

 background seen in the microscope. It is important to adjust 

 and keep the luminosity of the screen to the right amount 

 throughout the whole interval of an experiment. This is 

 most simply done by means of a small " pea "-lamp fixed in 

 a metal tube in which the current is varied. While weak 

 scintillations are readily counted on a dark background, it 

 is difficult under such conditions to keep the eye focnssed on 

 the microscope image and the eye rapidly becomes fatigued 

 and counting becomes erratic. The microscope employed 

 had a magnification of about 40 and covered a field of 2 mm. 

 diameter. This in practice was found to be a very convenient 

 magnification. In later experiments, special zinc sulphide 

 screens were prepared in which the smaller crystals were sifted 

 through a fine gauze on to a glass plate covered with a thin 

 layer of adhesive material. These fine crystals completely 

 •covered the plate several crystals deep. With such a screen, 



