716 Professor C. G. Barkia [May 2(), 



AVEEKLY EVENING MEETING, 

 Friday, May 20, lOlO. 



Sir William Phipson Bj:ale, Bart,, K.C. M.P. F.C.S., 



Vice-President, in the Chair. 



Professor C. G. Barkla, M.A. D.Sc. F.R.S. 

 X-Rays, Atomic Structure, Electromagnetic Radiation. 



[Abstract.] 



I. PROPOSE to give an outline, not of the properties of Rontgen 

 radiation in general — that would be too large a task — but only of 

 some of those properties which have an obvious bearing on two of 

 the most important and the most fascinating problems in Physical 

 Science — the problems of atomic structure and of electromagnetic 

 I'adiation, particularly of X-radiation. 



Before Sir Joseph Thomson's discovery of the electron, that is, of 

 small particles of electricity freed from ordinary matter, it had been 

 generally assumed that all bodies contained equal quantities of 

 positive and negative electricity, which just neutralised each other. 

 The process of electrification of any body was simply a process by 

 which the body was given an excess of one kind of electricity or the 

 other. Thomson's experiments showed that in a space relatively 

 free from ordinary matter (the space inside a highly exhausted 

 discharge tube) these particles could be obtained free, whatever the 

 gas inside the tube and whatever the nature of the electrodes con- 

 veying electricity to and from the gas. 



Since these early experiments, electrons have been obtained from 

 matter by very different methods — by (1) heating ; (2) bombardment 

 by swiftly moving small particles as a and ^ rays; (8) exposure to 

 light, especially of short wave-length ; (4) exposure to X-rays ; and 

 (5) they are spontaneously emitted by substances we call radioactive 

 substances. 



In some of these cases the supply of electrons is largely dependent 

 on the presence of other substances, or is influenced by surface con- 

 ditions, while radioactive processes have only been observed in a 

 limited number of elements. Perhaps the most convincing proof 

 that electrons are not^ obtained from an impurity common to all 

 substances, or that they do not reside simply on the surfaces of 

 atoms, is that the number emitted from a substance exposed to 

 X-rays is an atomic property unaffected by physical conditions or 



