CONTEMPORARY ADVANCES IN PHYSICS 107 



predictions meet for testing, and it is likely that in six months more a 

 great deal will be learned.^ 



The Denton 



The newly-discovered isotope of hydrogen of mass-number 2 — H', 

 "heavy hydrogen," or, to adopt Urey's name for it, "deuterium"-^— 

 has suddenly become the most popular and the most eagerly sought- 

 after of all chemical substances. This is because of the notable 

 chemical and physical differences between it and its compounds on 

 the one hand, H^ and the corresponding compounds of H^ on the other. 

 So great are these differences that by the usage of twenty years ago 

 H^ would probably have been called a new element, and indeed it 

 deserves all the prestige that would accrue to it from being so denoted ; 

 but to violate the present and most wisely-based of usages, whereby 

 an element is characterized by atomic number rather than by the 

 ensemble of its propeities, would be mistaken.'' 



Deuterium is so rare by comparison with H^ (Urey's "protium") 

 that it would still be very unfamiliar, but for the unexpected and 

 remarkable efficacy of the electrolytic method of separating water 

 molecules comprising H^ atoms from water molecules comprising none 

 but H^ atoms. It turns out that if an aqueous solution is electrolyzed 

 until only a very tiny fraction of the original liquid remains, the 

 proportion of the former kind of molecule in that tiny residue is 

 anonialously large. Washburn seems to have been the first to suspect 

 that this might happen; he procured samples of the residues from 

 electrolytic cells which had been operated continuously in commercial 

 plants for two and three years, and sent them to Urey, who performed 

 a spectrum-analysis and observed "a very definite increase in the 

 abundance of H^ relative to H^." Shortly afterwards the method was 

 put into operation on a grand scale by G. N. Lewis and his collabora- 

 tors, with spectacular results. In one experiment, for instance, they 

 started out with twenty liters of water, electrolyzed it until there 

 remained but half a cc. of liquid, and found that in this residue deu- 

 terium atoms made up two-thirds of all the hydrogen atoms which 

 were left. For months thereafter, nearly every paper on deuterium 

 and on the deuton which was published began with an acknowledgment 

 to Lewis for a small amount of water rich in heavy hydrogen which the 

 fortunate author had received from him. 



^ For a fuller account of the situation as it now stands, see an article of mine in the 

 Scientific Monthly, January 1934; also one by P. M. S. Blackett, Nature 132, pp. 917- 

 919 (Dec. 16, 1933), which incidentally contains some further data. 



" I should think that the case of deuterium by itself would make it necessary 

 henceforth to define the concept "element" altogether from the concept "atomic 

 number," forsaking all the earlier definitions. 



