xx.] CONCURRENCE OF TESTIMONY. 285 



ture as one would expect, and we have already referred to the 

 series of spectra given by these bodies at comparatively low 

 temperatures, from which we should have predicted these 

 changes of vapour density. 



To sum up, then, this concurrent testimony, we do find 

 evidence, other than spectroscopic, of the existence of mole- 

 cular groupings of the elementary bodies in different degrees 

 of fineness; and if we endeavour to explain the "anoma- 

 lous " vapour density of some of the substances to which atten- 

 tion has been drawn by continuing the same line of argument* 

 it is easy to see that it can be easily and simply explained in 

 this way. If the molecule present in the vapour at the time 

 it is weighed is that same molecule to which the mass of 

 the element is most broken down by chemical affinity, the 

 density will be normal. If the molecule is finer, we shall 

 have the condition of things represented by cadmium and 

 mercury ; if coarser, the state of things represented by phos- 

 phorus and arsenic. It is further interesting to note that, 

 of the permanent gases, the one most easily driven into 

 the line stage, hydrogen, is a monad ; while sodium, lithium, 

 potassium, chlorine, bromine, and iodine, bodies most easily 

 driven into vapour, are also monads. 



I wish here to give in confirmation of my own work a 

 reference to the work of others on the possible complex origin 

 of line spectra. One of the most exhaustive inquiries of this 

 nature was published by Dr. Schuster in the Philosophical 

 Transactions in 1879. The subject was Oxygen. He distinguishes 

 no less than four spectra of this substance, two of them being 

 true line spectra. In order to make the reference complete I 

 give his description of all four. 



I. The Elementary Line-Spectrum. This is the spectrum which 

 appears at the highest temperature to which we can subject oxygen. 

 That is, whenever the jar and air-break are introduced into the 



