19 Kl] oti X-Rays and Atomic Structure 717 



even by chemical com Iji nation, and it can be foretold for an element 

 if we know its atomic weight, or, more accurately, its atomic number. 



Further, we can by X-rays get electrons from the interior without 

 permanently affecting the stability of an atom. 



We can, as I shall show, find the number of electrons in the 

 atom ; we can divide these into groups at different depths within 

 the atom, and compare their properties — properties of course depend- 

 ing on their position and state within the atom. 



Let us indicate methods of studying the electrons emitted by 

 substances exposed to Rontgen radiation in the form of the secondary 

 corpuscular radiation. 



Precise measurements upon the corpuscular (electronic) radiation 

 emitted ]>y substances exposed to X-rays may be made by various 

 methods : — 



{(() By exposing a substance in an evacuated chamber to X-rays 

 the magnetic deflection of the emitted particles may be observed 

 much as in Thomson's experiment on Kathode Rays. Such measure- 

 ments were made by Innes, and, assuming a value for e/m, the velocity 

 was obtained. 



{¥) Observations, as by Townsend, of the variation of ionisation 

 between two parallel plates with their distance apart, when one or 

 both plates is exposed too X-rays, show the maximum distance 

 traversed by the particles in air or any other gas. (The plates may 

 be kept fixed and the pressure of the gas varied.) From Whiddington's 

 Law, connecting length of path d and initial velocity v, i.e. v"^ = ad 

 when a is a known constant, the maximum velocity of emission may 

 be determined. 



(c) By C. T. R.Wilson's condensation experiments the tracks may 

 be seen and measured, and the velocity of the electrons deduced. 



(d) As all X-ray ionisation is by the swiftly moving electrons 

 (secondary corpuscular radiation), determinations of the number of 

 electrons emitted by a gas may be made from observations on the 

 ionisation in the gas. From relative ionisations we can get relative 

 numbers of ejected electrons (Barkla, Phil. Trans. A, v. 217, pp. .315- 

 860), and from measurement of the saturation current and a know- 

 ledge of the number of ions produced per electron ejected it is 

 possible to get the absolute number of electrons produced by a 

 given beam. 



Such experiments give us the number and velocity of electrons 

 expelled by X-rays, and show us how few these are in comparison 

 with the whole number of atoms traversed by the radiation. They 

 show us that the negative electron is a constituent of all matter, but 

 give us no idea of the total number of electrons in an atom. 



On the other hand, positive electricity, which must also be a 

 constituent of all matter, has never been obtained in a form which 

 shows it to be free or entirely distinct from matter as ordinarily 

 known. Thus in an evacuated tube the least massive positive 



