The Recoil of Radium D from Radium C. 811 



less completely masked by the spectrum of the gas with 

 which it is mixed, may still be taking* a very important part 

 in carrying the current in the dark space. It is doubtful, 

 however, whether this conclusion can be carried further so as 

 to include the positive column. 



Until more is known definitely of the manner in which an 

 atom emits its spectrum, it will be very difficult to suggest a 

 cause for the masking effect of one spectrum upon another. 

 But it is interesting to note in this connexion the discovery 

 of multiply-charged atoms. Sir J. J. Thomson has ob- 

 served that in the positive-ray tube the heavier atoms can 

 lose more corpuscles than the lighter atoms. As the masking 

 •effect only occurs when there is an appreciable difference 

 between the atomic weights, it may be that it is connected 

 with the property of the heavier atoms to become more 

 completely ionized. If these multiply-charged atoms occur 

 in the positive column, they would form the more strongly 

 attractive centres, and recombination with subsequent spectral 

 emission would bo more frequent in their case. 



In conclusion, I desire to thank Sir J. J. Thomson for 

 suggesting this research, and for the kind interest taken in 

 its progress. 



Cavendish Laboratory, 

 Cambridge. 



LXXXVI. The Recoil of Radium I) from Radium C. 

 By A. B. Wood, M.Sc, and W. Makowkr, M.A., D.Sc* 



MANY attempts have been made to detect the photo- 

 graphic action of the recoil stream emitted by 

 radioactive surfaces, hut hitherto they have been unsuc- 

 cessful in detecting any direct action. The position at 

 which the recoil stream from radium A impinges on a photo- 

 graphic plate has, however, been fixed by allowing the 

 radium B reaching the plate to decay in situ, and obtaining 

 a photographic record by means of the a radiation sub- 

 sequently emitted by the radium C produced f. It seemed 

 possible that the failure to procure any direct photographic 

 evidence of a recoil stream might be due to the extreme 

 facility with which this type of radiation is absorbed by 

 matter ; for an ordinary photographic film contains so much 

 gelatin that most of the recoil particles striking the plate are 

 absorbed bv the gelatin before colliding with a grain of 



* Communicated by the Authors, 

 f Walmsley and Makbwer, Phil. Mag-. Feb. 1915. 

 3 G 2 



