1905.] The Structure of the Atom. 49 



WEEKLY EVENING MEETING, 

 Friday, March 10, 1905. 



His Grace the Duke of Northumberland, K.G. D.C.L. F.R.S., 



President, in the Chair. 



Professor J. J. Thomson, LL.D. D.Sc. F.R.S., Cavendish Professor 

 of Experimental Physics, University of Cambridge. 



The Structure of the Atom. 



In 1897 I had the pleasnre of bringing before the Royal Institution 

 experiments showing the existence of corpuscles^ i. e. negatively elec- 

 trified bodies having a mass exceedingly small compared with that 

 of an atom of hydrogen, until then the smallest mass recognised in 

 physics. A suggestive and striking property of these corpuscles is 

 that they are always the same from whatever source they may be 

 derived. The corpuscles were first detected in the rays which 

 are projected from the cathode when an electric discharge passes 

 through a vacuum tube, and it was found that whatever the nature 

 of the residual gas in the tube, or whatever the metal used for the 

 electrodes, the corpuscles were always the same. Other sources of 

 corpuscles soon came to light ; they were found to be projected from 

 incandescent metals, from metals illuminated by ultra-violet light, 

 and from radio-active substances ; but whatever their source the cor- 

 puscles were always the same. This fact, in conjunction with their 

 small mass, suggests that these corpuscles form a part of the atom, 

 and my object this evening is to discuss the properties of an atom 

 built up of corpuscles. As these corpuscles are all negatively elec- 

 trified, they will repel each other, and so if an atom is a collection of 

 corpuscles, there must in addition to the corpuscles be something to 

 hold them together ; if the corpuscles form the bricks of the structure, 

 we require mortar to keep them together. I shall suppose that 

 positive electricity acts as the mortar, and that the corpuscles are 

 kept together by the attraction of the positive electricity. We do 

 not know nearly so much about positive as we do about negative elec- 

 tricity ; we have never obtained positive electricity associated with 

 masses less than the mass of an atom ; in fact, appearances all point 

 to the conclusion that positive electrification is produced by the 

 withdrawal of corpuscles from a previously neutral body. These con- 

 ditions are satisfied, if we suppose with Lord Kelvin that in the atom 

 we have a sphere uniformly filled with positive electricity, and that 

 the corpuscles are immersed in this sphere. The attraction of the 

 Vol. XVIII. (No. 99) e 



