Novembers, 19 ig] 



NATURE 



223 



which Soddy has recently introduced the term 

 isotope, by which he defines very closely related 

 elements which are chemically inseparable but 

 have different atomic weights. The non-separa- 

 bility of isotopes by chemical methods has recently 

 been confirmed by Richards and his co-workers, 

 who found that the atomic weight of lead obtained 

 from Australian carnotite (containing uranium- 

 lead) was unaltered even after the nitrate into 

 which the lead was converted had been subjected 

 to more than a thousand fractional crystallisa- 

 tions. Furthermore, Richards has determined the 

 atomic weight of uranium-lead, and the number 

 found (20608) is less by as much as 025 per cent, 

 than that of ordinary lead, which differs from it 

 in other physical properties involving weight. It 

 is possible that lead descended from thorium (208) 

 and lead descended from uranium (206) have 

 enough in common to be each called lead, but 

 are varieties or isotopes of the same element, 

 common lead (207-2) being a mixture of the 

 two. 



We may conclude, therefore, that in radioactive 

 substances there is a continual transformation of 

 one element into another of lower atomic weight, 

 such transformation (apparently quite independent 

 of temperature and external electrical conditions) 

 being accompanied by the liberation of enormous 

 amounts of energy, compared with which the 

 magnitudes of energy of chemical reactions fade 



to insignificance. Has the earth passed through 

 its element-building epoch? Instead of spinning 

 "for ever down the ringing grooves of change," 

 are we mounting backwards up the spiral as our 

 larger empires of matter disintegrate into smaller 

 and perhaps more stable states? 



Just as the beginning of the last half-century 

 was marked by the epoch-making discovery of the 

 periodic system of the elements, so in effect is the 

 close of it marked with another- — namely, 

 Moseley's discovery of the atomic numbers of the 

 elements, the importance of which we have as yet 

 scarcely realised. 



The atomic number of an element as suggested 

 by van der Broek defines the place-number occu- 

 pied by the element in the periodic table, and at 

 the same time is the nurrber of electrons in the 

 atom or nuclear charge of it. Moseley showed 

 from a spectroscopic examination of the fre- 

 quencies of characteristic X-rays emitted when 

 X-rays bombard anticathodes of various metals, 

 that the square roots of the frequencies are pro- 

 portional to the atomic numbers. The latter are 

 known for all elements up to uranium — thus, 

 hydrogen one, helium two, lithium three, and so 

 on until finally uranium 92, and the anomalies 

 which appear in Mendeleeff's table disappear, as 

 In all cases the correct chemical order is niain- 

 tained. The atomic numbers appear to be even 

 more fundamental than the atomic weights. 



PHYSICAL CHEMISTRY— PAST AND PRESENT. 



Bv Prof. J. C. 



npHE cultivation of the border-lands between 

 -*- the various sciences, so actively prosecuted 

 in the last few decades, has nowhere led to more 

 notable results than on the frontiers of physics 

 and chemistry. This particular field of investiga- 

 tion, covering phenomena in some measure com- 

 mon to both these sciences, has gradually taken 

 shape, and has attracted crowds of workers, keen 

 to apply the exact methods of physics to the 

 wealth of problems and material presented by 

 chemistry. With the passing of the year's 

 physical chemistry has ultimately emerged as a 

 definite branch of natural knowledge, full of in- 

 trinsic interest, but comprising also much that is 

 of value for other sciences. 



Fifty years ago the foundations of physical 

 chemistry had to some extent been already laid. 

 Faraday's experiments on electrolysis and the 

 liquefaction of gases, Graham's observations on 

 gaseous and liquid diffusion, and Hittorf's in- 

 vestigations of electrolytic migration had been put 

 on record, although in some cases, notably the 

 last-mentioned, the full significance of the "work 

 was not to be realised for many years to come. 

 .\vogadro's hypothesis and the kinetic theorv 

 were also before the scientific world, and the 

 Brownian movement of minute particles 

 NO. 2610, VOL. 104] 



Philip, F.R.S. 



suspended in water, destined ultimately to figure 

 ! so prominently in the physical chemistry of recent 

 years, had been not only recorded but, for the 

 time, forgotten. 



During the period in which Nature first ap- 

 peared, new methods of investigating chemical 

 change, and new conceptions of chemistrv as a 

 quantitative science were being developed. The 

 work of Harcourt and Esson, of Guldberg and 

 Waage, on the action of mass as a factor in 

 equilibrium and velocity, as well as Horstmann's 

 application of thermodynamics to chemistry, in- 

 augurated a new epoch, with which, in both 

 directions, the name of van't Hoff was afterwards 

 so brilliantly associated. It was van't Hoff who 

 put the science of chemical dynamics on a secure 

 experimental basis, and thus prepared the way 

 for a rational study of catalysis, a particular 

 development of vital significance for the growth 

 of important chemical industries. It represents 

 part of the contribution which physical chemistrv 

 has made to the advance of chemical knowledge 

 from the purely descriptive to the rational and 

 quantitative stage. 



Appreciable progress towards the recognition 

 of physical chemistry as a distinct branch of 

 knowledge resulted, at a somewhat later date, 



