226 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1920. 



further complicated system with several nuclei and many outlying 

 orbits. According to Aston's work, as we shall see, such a system 

 also may lose a single negative ion, and the remainder take part in 

 a positive ray by virtue of its resulting excess of one positive unit 

 of electricity. 



The chemical and optical properties of atoms are thus supposed 

 to be governed by the number and arrangement of their outlying 

 ions, but their gravitational properties inhere almost wholly in their 

 nuclei. In atoms of a considerable number of outer ions, nuclei may 

 be built up in different ways of different numbers of positive par- 

 ticles, and yet present the necessary number of free positive charges 

 to neutralize the equal groups of outer ions. Thus there ma}' be 

 atoms of identical chemical and optical properties but different atomic 

 weights. Such atoms are called " isotopes." 



In this view it is evident that the isotopes are to be separated only 

 by methods which react differently depending on the masses of the 

 nuclei. One such is the method of diffusion. Naturally if some 

 molecules of a gas are heavier than others they will tend to diffuse 

 through a narrow orifice at different rates. By collecting the slower 

 moving fractions several thousand times, the heavier molecules can 

 be partially separated from the lighter ones. In this way a partial 

 separation of the gas neon into two fractions, differing somewhat in 

 their atomic weight, was accomplished in 1913 in Sir J. J. Thom- 

 son's laboratory. Doctor Aston has, however, perfected a method of 

 separating isotopes based on the deflections of the positive rays in 

 the electric and magnetic fields, which has led to the most beautiful 

 results. 



To understand his method, it should oe recalled that there are 

 three quantities involved besides the strength of the electric and 

 magnetic fields. These are the mass (m), the electric charge (e), 

 and the velocity (v) of each positive particle taking part in the 

 positive ray which is being experimented upon. In general the 

 values of the electric charge (e) are equal. That is, each particle 

 carries one electronic unit excess of positive charge equal but op- 

 posite in sign to the negative charge which is carried by the ion 

 which is knocked off by the electric field in the vacuum tube. I say 

 " in general " but two or three electronic units of excess positive 

 charge are possible. The electric charges being, then, generally 

 equal, Doctor Aston's problem was to eliminate the variable velocity 

 from the observation by combined influences of electric and magnetic 

 fields, so as to be free to observe differences of mass of the several 

 atoms composing a single gas. We will now let Doctor Aston's 

 own words tell his story. 



