426 Mr. G. N. Antonoff on the 



known that UrX emits only /3 rays, and no appreciable 

 quantity of uranium was present in the film. The new pro- 

 duct, UrY, and UrX, were separated with as little material 

 as possible, then spread on a small area, and placed imme- 

 diately under a zinc sulphide screen. After the eye had 

 been thoroughly rested in a dark room, the scintillations 

 were counted by a low-power microscope. Immediately 

 after separation, 2-3 scintillations per minute were observed. 

 The average number could be measured with sufficient 

 accuracy by taking a large number of readings. The 

 counting was continued during three days, and the average 

 number of scintillations were found to diminish. The period 

 could not be determined with accuracy ; but the results 

 indicated that the u rays were emitted by a product whose 

 half period of decay was about 1*5 days. Finally, the 

 number of scintillations became too small in number to be 

 counted. 



These experiments indicate that UrY emits some a rays ; 

 but at the same time it should be pointed out that the number 

 of a particles observed is small compared with that to be 

 expected from the /3 ray activity of the product. Experi- 

 ments of this character, where only one or two scintillations 

 appear per minute in the microscope, are very difficult, but 

 every precaution was taken to avoid error. 



Further evidence in support of the view that UrY emits 

 a particles can be obtained from an examination of the initial 

 part of the absorption curve. 



A preparation, in which the activity of the product was 

 about 30 per cent, of that of the UrX mixed with it, was 

 examined and its absorption curve was taken immediately 

 after the separation. To determine the absorption of the 

 product it is necessary to subtract the absorption of pure 

 UrX. This can be accomplished either by taking a barium 

 preparation or a ferric preparation after the short-lived 

 product has completely decayed. The absorption curves are 

 shown in fig. 4 (curves 1 and 3). 



Curve 1 is the absorption curve of UrX and the product 

 and curve 2 is that of pure UrX. The two curves were re- 

 duced to the same scale by equating the values of the activities 

 through 0*01 cm. of aluminium. It is seen that for hard rays 

 the absorption curves are identical. 



Curve 4 was obtained by taking the difference between 1 

 and 2, and it represents the absorption curve of the product. 

 This curve shows a rapid initial drop, which corresponds to 

 the en, ray effect diminished by 1*3 cm. of air. The remainder 



