ATOMIC WEIGHTS — ASTON. 185 



against a fluorescent screen can be visibly detected, the ionization 

 caused by its passage through a suitable gas can be measured on a 

 sensitive electrometer and, in the beautiful experiments of C. T. R. 

 Wilson, its path in air can be both seen and photographed by means 

 of the condensation of water drops upon the atomic wreckage it 

 leaves behind it. 



In the first complete Atomic Theory put forward by Dalton in 

 1803 one of the postulates states that : " Atoms of the same element 

 are similar to one another and equal in weight." Of course, if we 

 take this as a definition of the word " Element " it becomes a truism, 

 but, on the other hand, what Dalton probably meant by an element, 

 and what we understand by the word to-day, is a substance such as 

 hydrogen, oxygen, chlorine, or lead, which has unique chemical prop- 

 erties and can not be resolved into more elementary constituents by 

 any known chemical process. For many of the well-known elements 

 Dalton's postulate still appears to be strictly true, but for others, 

 probably the majority, it needs some modification. 



The idea that atoms of the same element are all identical in 

 weight could not be challenged by ordinary chemical methods, for 

 the atoms are by definition chemically identical and numerical ratios 

 were only to be obtained in such methods by the use of quantities 

 of the element containing countless myriads of atoms. At the same 

 time it is rather surprising, when we consider the complete absence 

 of positive evidence in its support, that no theoretical doubts were 

 publicly expressed until late in the nineteenth century, first by 

 Schutzenberger and then by Crookes, and that these doubts have 

 been regarded, even up to the last few years, as speculative in the 

 highest degree. In order to dismiss the idea that the atoms of such 

 a familiar element as chlorine might not all be of the same weight, 

 one had only to mention diffusion experiments and the constancy of 

 chemical equivalents. It is only within the last few years that the 

 lamentable weakness of such arguments has been exposed and it has 

 been realized that the experimental separation of atoms differing 

 from each other by as much as 10 per cent in weight, is really an 

 excessively difficult operation. 



There are two ways by which the identity of the weights of the 

 atoms forming an element can be tested. The one is by the direct 

 comparison of the weights of individual atoms, the other is by 

 obtaining samples of the element from different sources or by differ- 

 ent processes, which, although perfectly pure, do not give the same 

 chemical atomic weight. It was by the second and less direct of 

 these methods that it was first shown that substances could exist 

 which, though chemically identical, had different atomic weights. 



In 1906 Boltwood, 2 at Yale, discovered a new element in the radio- 



2 Boltwood : Amer. J. Sci., 22, 537, 1906 ; 24, 370, 1907. 



