694 Prof. R. "Whiddington on the 



and the receiver, does not influence the phenomenon, proving 

 that the /3-recoil atoms are uncharged, as was already stated 

 by Makower and Russ. 



It seems to result from my experiments that there is a 

 limit to /3-recoil efficiency, this limit being approximately 

 equal to 0'5. The probable explanation of the existence of 

 such a limit may be given by considering tbe thermal 

 velocities of the expelled RaC atoms in connexion with their 

 recoil velocity, as calculated from the mean velocity of the 

 /3-rays of RaB. If v-r be the recoil velocity, v t the mean 

 velocity of thermal agitation of RaC at ordinary temperature,, 

 then 



v- R = 5 . 10 4 cm. /sec. ; v < =l , 64 . lOcm./sec. ; v^ = Sv t . 



The two components being quite independent of one another,, 

 it results that the effective velocity of the recoiling atoms 

 oscillates between largely differing limits, being respec- 

 tively "equal to 4?y and 2v t . We can speak of the 

 equivalent temperature of the recoiling RaC-atom, meaning 

 by it a temperature at which this atom would possess a mean 

 thermal velocity equal to its effective velocity. Taking 

 290° abs. as the temperature of the experiment, we find that 

 the equivalent temperature oscillates between : 



1 6.290° abs. = 4 640 c abs. and 4.290° abs. = 1160°abs. = S87°G\ 



The lower limit being of the same order of magnitude as the 

 temperature of volatilization of radioactive matter, one can 

 easily imagine that the corresponding kinetic energy may 

 not be sufficient to remove the RaC from tbe "cold disk," 

 We see in this way a reason for the /3-rec il impulse being 

 in some unfavourable eases not strong enough to tear off the 

 radioactive matter from the surface of: the activated body. 



Warsaw, June 1919. 

 The Radiological Laboratory of the 

 Scientific Society of Warsaw. 



LXXII. Note on the X-ray Spectra of the Elements. By 

 R, Whiddington, M.A., JD.Sc, Cavendish Professor of 

 Physics in the University of Leeds *. 



SOON after Barkla's discovery of the L series of cha- 

 racteristic X-radiations, I put forward an empirical 

 formula of the type v = G .iv-\-D which approximately repre- 

 sented the K and L series of radiations of the elements t- 



* Coixrnunicatecl by the Author. 



t 'Nature,' Nov. 30th, 1911 ; see also Proc. Roy. Soc. 1912, yol.lxxxvi. 

 ser. A, p. 378. 



