Miss A. Mtiszkat on the /3-Recoil. 691 



liad struck it. Experience showed, however, that the whole 

 .apparatus, • in spite of the high vacuum obtained, became 

 filled with the distilling RaB + 0, which was present pro- 

 bably in a gaseous form, and the receiver was contaminated 

 with active matter before being exposed to the recoil stream. 

 Diaphragms of different forms proved useiess. This shows 

 that at ordinary temperature a metallic surface does not 

 possess the faculty of retaining an atom of RaB or RaC after 

 a single shock. Similar facts were observed by Knudsen * 

 in the case of the condensation of mercury and other metallic 

 vapours on glass walls. Xo doubt, the results could be 

 improved by cooling the surface to a very low temperature, 

 but it would be a very troublesome operation, and so I tried 

 to avoid this difficulty by disposing the receiver in a part of 

 the vessel that could be separated hermetically from the other 

 parts of the apparatus during the distillation, and then set 

 in communication with them during the recoil experiment. 



For this purpose a device was used similar to that adopted 

 hy Mr. Wertenstein for his exposure vessel f . The part of 

 the apparatus A (ef. tig.) containing the active wire and the 

 cold disk had at its top a support on which reposed the bottom 

 of the little cylindric vessel B, containing the receiver D. 

 The two vessels had concentric openings of the same diameter 

 (12 mm.), so that by displacing the upper vessel a perfect 

 separation of A and B was obtained. The " cold disk 5 ' M 

 was fixed on a cylinder rotating on a horizontal axis by 

 means of the ground-cock S 2 - The wire Pt, 01 mm. thick, 

 was fixed on a ground-cock S^ which also fitted well in 

 the exposure vessel. It was stretched in the form of a loop 

 between two thick platinum wires which passed through an 

 ebonite plug. These wires enabled me to bring the thin 

 wire to the high potential required for activating it in the 

 exposure vessel and also to heat it by an electric current for 

 distillation purposes, when in the recoil apparatus. 



The tube r connected the apparatus to the G-aede molecular 

 pump and to a thermic manometer, described by Mr. Wer- 

 tenstein J. 



The method of conducting the experiment consisted in 

 heating at first thoroughly the wire in vacuo in order to clean 

 the platinum and to remove the occluded gases. The wire 

 was then made active by placing it for about \\ hours in the 

 exposing vessel, containing several millicuries * of radium 



* Ann. d. Physik, 1916, 1. p. 472. 

 t L. Wertenstein, These, Paris, 1913. 



X L. Wertenstein, " Sur quelqiies procedes de la technique du vide." 

 C It. de la Societe Scientifique de Varsovie, 1917. 



