42 POPULAR SCIENCE MONTHLY 



determination of these values. The desire to test this hypothesis was 

 one of the chief motives for some of the most careful determinations 

 of atomic weights which have ever been made. These determinations 

 resulted in proving that the divergences of the atomic weights from 

 whole numbers were greater than could be accounted for on the basis 

 of experimental errors. This precluded the possibility that the atom 

 of hydrogen was the common ultimate unit, but did not dispose of the 

 possibility that a half, or quarter, or some other fraction, of the 

 hydrogen atom might play that role. 



In 1901 Strutt 1 applied the mathematical methods of the theory 

 of probabilities to the most accurately determined atomic weights, and 

 calculated that the chance that they should fall as close to whole num- 

 bers as they do was only one in one thousand. The inference from 

 this is that it is not a matter of chance, but that there is a regularity 

 in the atomic weights which we do not understand; a regularity which 

 points to the probability that our elements are complex substances, 

 constructed according to some system, from some simpler substance. 



All the facts comprised in that great generalization, the periodic 

 law, which states that the properties of the elements, both chemical 

 and physical, are functions of their atomic weights, and most of them 

 are periodic functions, point unmistakably to the same conclusion. 



The evidence from spectroscopic analysis is so abundant that it is 

 not easy to compress it into a few general statements. 



In the first place, the spectrum of each of our elements consists of 

 numerous lines, a fact not exactly compatible with the notion of ex- 

 treme simplicity of the particles emitting the light. 



In the second place, one and the same element, contrary to common 

 belief, frequently has two or three distinctly different spectra, the 

 particular spectrum which appears depending upon the pressure and 

 the temperature at which the element is while emitting the light. 

 In fact the extraordinary spectroscopic results obtained when highly 

 rarefied gases enclosed in tubes (variously called Plucker, Hittorf, 

 Geissler or Crookes tubes) were made luminous by the passage of high 

 potential electricity, induced Crookes to suggest in 1887 a theory that 

 the elements were all built up by gradual condensation with falling 

 temperature from a fundamental substance to which he gave the name 

 protyl. 2 



In the third place, the lines in the spectrum of one element may be 

 separated out into several series. Each line corresponds, as is well 

 known, to light of a definite wave length. The wave lengths of the 

 lines comprised in one series are related to each other in such a way 

 that a general formula may be derived for them. This means that, 

 given some of the lines, the wave lengths, and thus the positions, of 



1 ~R. J. Strutt, Philosophical Magazine, March, 1901, p. 311. 

 2 ' The Genesis of the Elements,' W. Crookes. 



