The Scintillations produced by Radium. 427 



or no rotatory power is exhibited for red or yellow light, 

 though this portion of the spectrum is tilled with fine absorp- 

 tion-lines, and the blue region is also wanting in the rotation 

 spectrum, though the absorption is strongest at this point. 

 These effects are at present under investigation, and will be 

 reported in a subsequent paper. 



The investigation reported in the present paper is one of a 

 series made possible through substantial aid received from 

 the Kumford Fund, two grants having been made by the 

 American Academy for a study of the optical properties of 

 sodium vapour. 



Much assistance has also been rendered by Mr. A. H. Pf und, 

 for whose services I am indebted to the Carnegie Institute. 



L. The Scintillations produced by Radium. 

 By Prof. R. W. Wood *. 



[Plate VI.] 



THE scintillations of phosphorescent zinc sulphide, when 

 subjected to the bombardment of the radium corpuscles, 

 does not appear to have been satisfactorily explained up to 

 the present time. Crookes regarded each tlash as due to the 

 impact of a positive electron or alpha particle, while Becquerel 

 regards the production of light as a secondary effect resulting 

 from cleavages of the crystals brought about by the action 

 of the rays. Neither hypothesis, as it stands, seems quite 

 convincing, I do not know whether any estimate has ever 

 been made of the actual number of alpha particles emitted in 

 unit time from a given amount of radium, but it certainly 

 seems as if the number must far surpass the number of 

 flashes of light as seen in the spinthariscope. As to the 

 second hypothesis, the only argument advanced in support of 

 it appears to be the circumstance that zinc sulphide crystals 

 when rubbed or crushed show similar flashes of light. This 

 argument is not very convincing, for it amounts to saying 

 that similar effects must result from similar causes. On the 

 other hand, the cleavage hypothesis removes the trouble 

 regarding the number of flashes as compared with the 

 probable number of electrons, as well as the difficulty in the 

 conception of an appreciable flash being produced by a single 

 electron. The experiments described in this paper were 

 completed just a year ago, and were undertaken in the hope 

 of settling the question of the cause of the flash. As they 

 did not lead to sufficiently definite conclusions to satisfy me, 

 * Communicated by the Author. 



