916 Mr. F. P. Slater on Excitation ofy Radiation 



Part II. 



Soft Radiations. 



It has been pointed out that evidence of soft radiation was 

 indicated in the preceding investigations. 



To study the nature of this radiation the method used was 

 the same* in principle as that described in Part I. of this 

 paper. ,\, 



The electroscope was somewhat smaller than the lead 

 electroscope used previously, and could be rilled with S0 2 

 by displacement to increase the effect of the soft radiations. 



The examination-tube differed from that used in Part I., 

 being specially designed to allow very soft radiation to pass 

 through its walls. To accomplish this the glass tube A 

 (fig. 1) was made as thin as possible, but even then it was 

 too thick to permit examination of the softest radiations. 

 This thin glass tube was used to determine the ionization- 

 thickness curve for total absorption thicknesses greater than 

 4' 5 mm. The softest radiations were examined by using a 

 brass tube as shown in fior. 4. This limited the effective area 



Fiar. 4. 



£/ectroscope 



-M/ca Sheet 

 Sect/'cna/ View 



Abscrbmq 

 y<ty/ss?7\ Screens 



^ 



N 



8rass Tube 



■&m 



Af/ca Sheet 



Ebonito 

 Stopper 



of the target bombarded by the particles, since the holes 

 occupy a comparatively small fraction of the total internal 

 surface area of the tube. This grid formed the essential 

 feature of the tube, and was covered by a thin sheet of 

 mica to render the whole vacuum-tight. The metal target to 

 be used was fixed on to a light carrier which slipped into 

 the tube and was held firmly against the under surface of 

 the grid by four light spring supports. When a metal target 

 was being examined, the curved internal surface of the tube 

 was lined with india paper to eliminate any radiation which 

 might be emitted by the "brass. 



