750 Prof. C. G. Barkla and Mr. G. Shearer on the 



emission of a particular characteristic X-radiation from the 

 substance ionized. 



Further, — this indeed follows in part from the last ex- 

 perimental fact — the electronic radiation emitted by a 

 substance may be regarded as divided into several distinct 

 groups, each group being definitely associated with a 

 particular fluorescent characteristic X-radiation from that 

 substance. When a fluorescent characteristic X-radiation is 

 emitted, there is emitted also a particular group of electrons. 

 Each of these groups we shall denote by the letter assigned 

 to the associated fluorescent (characteristic) X-radiation. 

 Thus there is the K corpuscular radiation emitted in asso- 

 ciation with the K fluorescent X-radiation, and producing 

 the K ionization in a substance exposed to the primary 

 radiation. All these phenomena are associated with and 

 consequent upon the K absorption of the primary radiation. 



Similarly we have the associated L absorption, L cor- 

 puscular radiation, L fluorescent radiation, L ionization, &c. 



As K and L corpuscular radiations have distinct origins, 

 it seemed possible that the velocities of iheir emission might 

 differ appreciably. Experiments were therefore made in 

 order to compare the velocities of the K and L electrons 

 emitted by a given substance. To do this, the ionization 

 pressure curves were obtained as before, when the radiating 

 screen of silver placed at the back of the ionization -chamber 

 was exposed to the X-radiations (series K) from Ag, Sn, Sb, 

 and I. From these curves by subtraction of the ordinates 

 corresponding to the line OR parallel to PQ we get curves 

 of the form in fig. 3, representing the ionization-pressure 

 relation for the ionization produced by the balance of cor- 

 puscular radiation from the ends alone. Under the action 

 of Ag X-rays, Ag emits only L and some M and possibly N 

 electrons, but not K electrons; Sn X-radiation ejects a few 

 K electrons in addition ; Sb and I X-radiations eject a large 

 number of K electrons, so that there are in the compound 

 corpuscular (electronic) radiation from Ag roughly as many 

 K electrons as L and M electrons. 



Four ionization-pressure curves were obtained for the 

 ionization by the electrons ejected from the silver sheet when 

 exposed to Ag, Sn, Sb and I radiations (series K). As these 

 radiations are arranged in ascending order of frequency, 

 the velocities of ejection of the electrons, and consequently 

 the minimum pressures of air necessary to completely absorb 

 the electronic radiations, are in ascending order of magnitude. 

 By adjusting the scales of abscissae, we can, however, compare 

 the shapes of the curves and thus the velocity distribution 



