SYMPOSIUM ON" RADIOACTIVITY. 47 



with it; and in this manner succeecied in preparing a tube which 

 showed no trace of helium when traversed by a spark discharge. 

 But on allowing this tube to stand for several days, distinct evi- 

 dence of helium appeared. The well known D3 line appeared, 

 followed by other less conspicuous lines. Later Dr. Giesel was 

 able to photograph this spectrum which is now shown on the 

 screen. The helium and hydrogen spectra are given on the out- 

 side for comparison. Note that D3, which is very bright in the 

 visual spectrum, is scarcely seen in the photographic spectrum; 

 also that H^ and Hy are present, probably in consequence of 

 moisture. 



It had been already noted by Ramsay that there was evidence 

 of a mineralogical nature for thinking that helium might be a 

 product of uranium and radium; but I believe it is generally 

 conceded that the evidence of Ramsay's spectrum-tube and the 

 photographs of Giesel first furnished the "impersonal validity" 

 required for a satisfactory demonstration of this fact. 



This instabiHty of the radium emanation which will doubtless 

 be discussed by Professor Noyes, is hardly surpassed in interest 

 by any recent discovery in science. Considering the emanation 

 as a disintegration product of radium, one might well apply to 

 it that line of Macbeth which has already been applied to 

 Charles I, "nothing in his hfe became him so much as his leaving 

 of it." 



SPEED OF a AND j8 RAYS. 



From a mechanical point of view there is perhaps no property 

 of radium which is of more interest than the deflection which its 

 alpha and beta radiations suffer when placed in an electric or 

 magnetic field. For it is this very deviation which, when once 

 measured, enables one to determine, in a very simple way, the 

 enormous speed with which the radium atom can expel those 

 minute material bodies which Rutherford has called a-particles. 

 For the ^-rays, this deflection was simultaneously discovered by 

 three investigators in the autumn of 1899 ; for the a-rays by 

 Rutherford in 1902. It may, therefore, be worth while to con- 

 sider the nature of the evidence for thinking that these a-particles 

 — which are in all probability helium atoms — are projected with a 

 speed no less than 25 million meters per second — a speed 

 nearly one-tenth that of light, — a speed so stupendous that, in 

 comparison with it, the projectile leaving a 13-inch rifle would 

 appear to be standing absolutely still. 



