300 Mr. F. W. Aston [Feb. 11, 



atomic weight of an element is a mean ; that when we say the atomic 

 weight of oxygen is 16 we merely state that the average atomic 

 weight is 16 ; and that it is not inconceivable that a certain number 

 of molecules have a weight somewhat higher than 32, while a certain 

 number have a lower weight." 



That such conjectures were then regarded as wildly speculative 

 shows how strong was the faith in Dalton's postulate, which is all 

 the more remarkable when we consider that at that time not one 

 single direct experimental proof of it had been offered. Such proof, 

 obviously, can only be obtained by some method which measures the 

 masses of atoms individually, and at that time none had been 

 developed. ' 



The first direct evidence that the atoms of an element were at 

 least approximately equal in mass appears to be that obtained by 

 Sir J. J. Thomson in 1910 by his well-known method of analysis of 

 positive rays. The fact that sharply defined parabolic streaks were 

 obtained at all proves that the ratio of the masses of the separate 

 particles causing them to the charges of electricity they carry is 

 constant. The latter was known to be a definite unit, or a simple 

 multiple of it, so that if the masses of the individual atoms varied 

 amongst each other in an arbitrary manner an indistinct blur would 

 result instead of a clear-cut parabola. 



Before going on to consider the evidence of positive rays in 

 greater detail it will be as well to re-state briefly the evidence upon 

 which the theory of isotopes was founded. The first indication that 

 it might be possible to obtain substances having identical chemical 

 properties but different atomic weights was afforded by the brilliant 

 researches on the radioactive elements made by Sir E. Rutherford 

 and his colleagues. Investigations on the transformations of the 

 different radioactive families showed that certain products, such as 

 lead, could be formed in several ways. Each of the leads so formed 

 was found to have chemical properties identical in every respect with 

 those of ordinary lead, but their method of production precluded any 

 possibility of them all having the same atomic weight. Such bodies, 

 although having different atomic weights, must occupy the same 

 position in the Periodic Table of the elements, and on this account 

 have been called '"isotopes" by Prof. Soddy. 



Moseley's epoch-making discovery has shown us that chemical 

 properties depend not upon atomic weight but upon something much 

 more fundamental, namely, atomic number. The atomic number of 

 an element is the number of units of positive electric charge on the 

 nucleus of its atoms ; the nuclear charge of hydrogen is 1, of helium 2, 

 of lithium 3, and so on. AYe see, therefore, that isotopes are elements 

 having the same atomic number but different atomic weights. 



The theory of isotopes was triumphantly vindicated during the 

 war by the researches of Soddy, Richards, Honigschmidt, and others, 

 on the atomic weights of lead found in various radioactive minerals. 



