128 Mr. L. Simons on the Beta-Ray Emission from 



through sulphuric acid. A fresh supply of air constantly 

 passed through the chamber during the experiments. 



Throughout the first set of experiments the silver plate 

 was used as the secondary radiator. First the carbon screen 

 was placed in position, the pressure within the ionization 

 chamber adjusted, and the mean ratio obtained of the 

 ionization in it to that in a standardizing electroscope 

 12 to 14 points on the curves being obtained within the 

 range from zero up to atmospheric pressure. Another screen 

 was substituted and the process repeated. The screens were 

 not used in any definite order. The barium secondary 

 radiator (with its accessory aluminium plate for cutting out 

 the L radiation) was now substituted for the silver one, and 

 the whole process repeated, again using the eleven screens 

 iii no definite order. 



Experimental results. 



Fig. 3 (PI. III.) gives the points obtained in using the 

 eleven screens successively with silver as radiator, and fig. 4 

 with barium as radiator. No corrections whatever have 

 been made in putting down the points, the air effect and the 

 normal leak of the instruments being so small that they could 

 be neglected. 



The ordinates of the carbon curve were subtracted from 

 those of the other curves. Each resulting curve would 

 represent the ionization produced in air at various pressures 

 by the /3-particles emerging from the respective screens. 

 There is a critical pressure at which the range of the 

 /3-particle is just equal to the thickness of the ionization 

 chamber : viz., 1*45 cm. Above this pressure the curves 

 become horizontal straight lines, for the /3-particles are now 

 completely absorbed in the air. Below this pressure some 

 of the energy af the particles will be lost by their being- 

 absorbed in the carbon front of the chamber. 



The resulting ten curves, say with silver as secondary 

 radiator, differ widely from one another in their maximum 

 ordinate. In other words, the total ionization by the 

 /3-particles in these experiments shows a considerable 

 variation over the range of substances used. It must be 

 remarked at once that this variation here has but little 

 meaning. It is known that the intensity of the /3-ray 

 emission increases with increasing atomic weight of the 

 screen, but apart from this, as the films of substances 

 radiating were so exceedingly thin to a certain extent of 

 variable thickness, but by no means thick enough to absorb 

 to any extent the /3-particles produced from the side of the 



