164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



mensions or structure of the atom. It was only necessary to assume 

 that the atoms acted as individual units, and to know the relative 

 masses of the atoms of the different elements. In the next stage, 

 for example, in the kinetic theory of gases, it was possible to explain 

 the main properties of gases by supposing that the atoms of the 

 gas acted as minute perfectly elastic spheres. During this period, 

 by the application of a variety of methods, many of which were due 

 to Lord Kelvin, rough estimates had been obtained of the absolute 

 dimensions and mass of the atoms. These brought out the minute 

 size and mass of the atom and the enormous number of atoms neces- 

 sary to produce a detectable effect in any kind of measurement. 

 From this arose the general idea that the atomic theory must of 

 necessity forever remain unverifiable by direct experiment, and for 

 this reason it was suggested by one school of thought that the 

 atomic theory . should be banished from the teaching of chemistry, 

 and that the law of multiple proportions should be accepted as the 

 ultimate fact of chemistry. 



While the vaguest ideas were held as to the possible structure of 

 atoms, there was a general belief among the more philosophically 

 minded that the atoms of the elements could not be regarded as 

 simple unconnected units. The periodic variations of the properties 

 of the elements brought out by Mendeleeff were only explicable if 

 atoms were similar structures in some way constructed of similar 

 material. We shall see that the problem of the constitution of 

 atoms is intimately connected with our conception of the nature of 

 electricity. The wonderful success of the electromagnetic theory 

 had concentrated attention on the medium or ether surrounding the 

 conductor of electricity, and little attention had been paid to the ac- 

 tual carriers of the electric current itself. At the same time the 

 idea was generally gaining ground that an explanation of the re- 

 sults of Faraday's experiments on electrolysis was only possible on 

 the assumption that electricity, like matter, was atomic in nature. 



The name "electron" had even been given to this fundamental 

 unit by Johnstone Stoney," and its magnitude roughly estimated, 

 but the full recognition of the significance and importance of this 

 conception belongs to the new epoch. 



For the clarifying of these somewhat vague ideas, the proof in 

 1897 of the independent existence of the electron as a mobile elec- 

 trified unit, of mass minute compared with that of the lightest atom, 

 was of extraordinary importance. It was soon seen that the elec- 

 tron must be a constituent of all the atoms of matter, and that 

 optical spectra had their origin in their vibrations. The discovery 

 of the electron and the proof of its liberation by a variety of 

 methods from all the atoms of matter was of the utmost signifi- 

 cance, for it strengthened the view that the electron was probably 



