260 



SCIENCE 



[N. S. Vol. LIV. No. 1395 



be originally produced in the stars through 

 hydrogen and helium. 



These results are of extraordinary interest 

 as bearing on the question of the essential 

 unity of matter and the mode of genesis of 

 the elements. Members of the British As- 

 sociation may recall the suggestive address 

 on this subject of the late Sir William 

 Crookes, delivered to the Chemical Section 

 at the Birmingham meeting of 1886, in which 

 he questioned whether there is absolute uni- 

 formity in the mass of the atoms of a chemic- 

 al element, as postulated by Dalton. He 

 thought, with Marignac and Schutzenberger, 

 who had previously raised the same doubt, 

 that it was not improbable that what we term 

 an atomic weight merely represents a mean 

 value around which the actual weights of the 

 atoms vary within narrow limits, or, in other 

 words, that the mean mass is " a statistical 

 constant of great stability." ITo valid experi- 

 mental evidence in support of this surmise 

 was or could be offered at the time it was 

 uttered. Maxwell pointed out that the pheno- 

 mena of gaseous diffusion, as then ascertained, 

 would seem to negative the supposition. If 

 hydrogen, for example, were composed of 

 atoms of varying mass it should be possible 

 to separate the lighter from the heavier atoms 

 by diffusion through a jwrous septum. " As 

 no chemist," said Maxwell, " has yet obtained 

 specimens of hydrogen differing in this way 

 from other specimens, we conclude that all 

 the molecules of hydrogen are of sensibly the 

 same mass, and not merely that their mean 

 mass is a statistical constant of great stabil- 

 ity." '^ But against this it may be doubted 

 whether any chemist had ever made experi- 

 ments sufficiently precise to solve this point. 



The work of Sir ISTorman Lockyer on the 

 spectroscopic evidence for the dissociation of 

 " elementary " matter at transcendental tem- 

 peratures, and the jwssible synthetic intro- 

 stellar production of elements, through the 

 helium of which he originally detected the ex- 

 istence, will also find its due place in the 

 history of this new philosophy. 



1 Clexk-Maswell, Art. ' ' Atom, ' ' Eney. Brit., 

 Qth Ed. 



Sir J. J. Thomson was the first to afford 

 direct evidence that the atoms of an element, 

 if not exactly of the same mass, were at least 

 approximately so, by his method of analysis 

 of positive rays. By an extension of this 

 method Mr. F. W. Aston has succeeded in 

 showing that a number of elements are in 

 reality mixtures of isotoi)es. It has been 

 proved, for example, that neon, which has a 

 mean atomic weight of about 20.2, consists 

 of two isotopes having the atomic weights 

 respectively of 20 and 22, mixed in the pro- 

 portion of 90 per cent, of the former with 10 

 per cent, of the latter. By fractional dif- 

 fusion through a porous septum an apparent 

 difference of density of 0.7 per cent, between 

 the lightest and heaviest fractions was ob- 

 tained. The kind of experiment which Max- 

 well imagined proved the invariability of the 

 hydrogen atom has sufficed to show the con- 

 verse in the case of neon. 



The element chlorine has had its atomic 

 weight repeatedly determined, and, for spe- 

 cial reasons, with the highest attainable ac- 

 curacy. On the oxygen standard it is 35.46, 

 and this value is accurate to the second deci- 

 mal place. All attempts to prove that it is a 

 whole number — 35 or 36 — have failed. When, 

 however, the gas is analyzed by the same 

 method as that used in the case of neon it 

 is found to consist of at least two isotopes of 

 relative mass 35 and 3Y. There is no evidence 

 whatever of an individual substance having 

 the atomic weight 35.46. Hence chlorine is 

 to be regarded as a complex element consist- 

 ing of two principal isotopes of atomic weights 

 35 and 37 present in such proportion as to 

 afford the mean mass 35.46. The atomic 

 weight of chlorine has been so frequently 

 determined by various observers and by vari- 

 ous methods with practically identical results 

 that it seems difficult to believe that it consists 

 of isotopes present in definite and invariable 

 proportion. Mr. Aston meets this objection by 

 pointing out that all the accurate determina- 

 tions have been made with chlorine derived 

 originally from the same source, the sea, which 

 has been perfectly mixed for aeons. If samples 

 of the element could be obtained from some 



