Miss A. Muszkat on the fi-Recoil. 693 



I think, that even at these temperatures a slight volatilization 

 of platinum takes place, being quite sufficient to cover the 

 RaB atoms with an impenetrable layer. 



After the current is cut off, the disk M is turned to the 

 receiver by a rotation of 180° and the upper vessel is placed 

 in the central position. The exposure to the recoil lasts for 

 rive minutes. The air is then introduced into the apparatus 

 and the two disks M and D are removed. Their activities 

 are measured, the measurements being continued at regular 

 intervals during an hour. From the analysis of the dis- 

 integration curves I obtained the amount of RaB present on 

 the cold disk at the beginning of the recoil experiment and 

 the quantity of RaC on the receiver at the end of it. If B 

 be the former quantity, C the latter *, c5 the solid angle 

 subtended by the receiver (equal to 0336 . 4-7r), then the 

 efficiency E is given by 



4tt C ■ c-b 



- 5 'B'b(j?-™-e-> c )' 



It must be noticed that I obtained in all cnses on the 

 receiver almost pure RaC. The cold disk contained RaB and 

 RaC in proportions approaching those of the radioactive 

 equilibrium between the two products, its activity amounted 

 only to a small percentage of the activity of the active wire, 

 thus showing the great dispersion, already mentioned, of the 

 distilling matter. 



I found that the efficiency depended on the nature of the 

 cold disk. It was equal to 0*2 for an ordinary brass disk, to 

 G'3 for the same disk after it was thoroughly polished, to 0'4 

 for an aluminium disk, and to 0'5 for a platinum one. We 

 see that in all cases the efficiency is of the order of unity, i. e., 

 that the conditions of my work are far more favourable to 

 the study of the /5-recoil than the conditions used in previous 

 work on this subject. The role of the nature of the surface 

 can easily be understood if one considers that the main 

 obstacle to the expulsion of RaC is due to the penetration of 

 the radioactive matter into the metal and probably also to 

 chemical actions between this matter and the substance of 

 the disk. That RaB penetrates into the metal can be proved 

 by the fact that the efficiency of the recoil diminishes witli 

 time. In one case, when the recoil experiment was conducted 

 20 minutes after the distillation, the efficiency was reduced 

 to half its value. 



The electric held, when established between the disk M 



* The quantities calculated from the curves are not B and C, but, as 

 usual, B6 and Cc. 



