164 SUMMARY OF ELECTRON THEORY [CH. xvm. 



semi-atoms, consisting of something like 

 helium apparently, likewise shot off with 

 great energy : called a rays ; 

 4th, as a consequence of all this radiation, 

 detached portions of the residue of the 

 substance drift away, not charged with 

 electricity, but emanating something after 

 the fashion of an odour. This gaseous 

 emanation is found itself to possess a 

 very high intrinsic radiating power, and 

 to be capable of attaching itself to, or 

 causing a deposit on, other materials in 

 the neighbourhood, so that they too acquire 

 temporary radiating power : a deposit at 

 one time spoken of as induced or excited 

 activity.* 



The substances which possess any noteworthy 

 amount of this radiating power are substances with 

 very high atomic weight ; and their emitting power 

 would appear to be probably due to an internal 

 commotion or convulsion, of sufficient violence to 

 detach and expel, every now and then, some particle 

 or fragment ; and also, by the shock of the expulsion, 

 to generate some feeble but exceedingly penetrating 

 Eontgen rays. 



witn quantitative determinations concerning it. Also in mil. 

 Mag., July and November, 1902. Other references are M. and 

 Mme. Curie, Comptes Rendus, November, 1899 ; Hon. R. J. Strutt, 

 Phil. Trans., A 1901, vol. 196, p. 525; Sir W. Crookes, Proc. Roy. 

 Soc., vol. 66, p. 409 (1900), vol. 69, p. 413 (1902), also "Electrical 

 Evaporation," 1891, Proc. Roy. Soc., vol. 50, p. 88; and many other 

 workers. References to them are now conveniently collected in 

 Professor Rutherford's excellent treatise. 



Madame Curie's original Thesis on Radio-activity for her Doctorate, 

 of date 1903, is a masterly production. 



