136 Professor Frederick Soddy [May 18, 



In the original electrical theory of matter, the whole mass of the 

 atom was attributed to electrons, of which there would have been 

 required nearly 2000 times the atomic weight in terms of hydrogen 

 as unity. With the more definite determination of this number, and 

 the realisation that there were only about half as many as the number 

 representing the atomic weight, it was clear that all but an insignifi- 

 cant fraction of the mass of the atom was accounted for. In the 

 nuclear hypothesis this mass is concentrated in the exceedingly 

 minute nucleus. The electro-magnetic theory of inertia accounts for 

 the greater mass if the positive charges that make up the nucleus 

 are very much more concentrated than the negative charges which 

 constitute the separate electrons. The experiments on scattering 

 clearly indicated the existence of such a concentrated central positive 

 charge or nucleus. 



The mathematical consideration of the results of a-ray scattering, 

 obtained for a large number of different elements, and for different 

 velocities of a-ray, gave further evidence that the number of electrons, 

 and therefore the + charge on the nucleus, is about half the number 

 representing the atomic weight. But van der Broek, reviving an 

 isolated suggestion from a former paper full of suggestions on the 

 Periodic Law, which were, I think, in every other respect at fault, 

 suggested that closer agreement with the theory would be obtained 

 if the number of electrons in the atom, or the nuclear charge, was 

 the number of the place the element occupied in the Periodic Table. 

 This is now called the atomic number, that of hydrogen being taken 

 as 1, helium 2, lithium :^, and so on to the end of the table, uranium 

 92, as we now know. For the light elements, it is practically half 

 the atomic weight ; for the heavy elements, rather less than half. 



I pointed out this accorded well with the law of radioactive 

 change that had been established to hold over the last thirteen places 

 in the Periodic Table. This law might be expressed as follows : 

 The expulsion of the a-particle carrying two positive charges lowers 

 the atomic number Ijy two, while the expulsion of the yS-particle, 

 carrying a single negative charge, increases it by one. In ignorance 

 of van der Brock's original suggestion, I had, in representing the 

 generalisation, shown the last thirteen places as differing by unit by 

 unit in the number of electrons in the atom. 



Then followed Moseley's all-embracing advance, showing how 

 from the wave-lengths of the X-rays, characteristic of the elements, 

 this conception explained the whole Periodic Taljle. The square 

 roots of the frequency of the cliaracteristic X-rays are proportional to 

 the atomic numbers. The total number of elements existing betweeen 

 uranium and hydrogen could thus be determined, and it was found to 

 be ninety-two, only five of the places being vacant. The " exceptions " 

 to the Periodic Law, such as argon and potassium, nickel and cobalt, 

 tellurium and iodine, in which an element with higher atomic weight 

 precedes instead of succeeds one with lower, was confirmed by the 



