40 
DR. A. SCHUSTER ON THE SPECTRA OF METALLOIDS. 
a greater or smaller space, and do not gradually fade away on either side, but seem to 
have the same intensity throughout, with a sudden fall at both edges. Such are the 
bands which appear at the negative pole of oxygen tubes. With high dispersion 
I was able to resolve these bands into a series of lines. Their appearance, under these 
circumstances, is represented in Plate 1, figs. 1 and 2. 
It seems remarkable that most bodies should, under certain conditions, give spectra 
which in character resemble each other so nearly as these channelled space spectra ; 
and the idea seems natural, and almost necessary, that such a simikrity of spectra 
should be produced by a similarity in molecular structure. We know that certain 
bodies giving line spectra show, when they combine with others, spectra of fluted 
bands ; and this seems to me to lead to a necessary explanation of double spectra—the 
one which indeed has been adopted by most experimentalists. They hold that when 
a spectrum suddenly changes from a line spectrum to a band spectrum, we have to 
deal with a molecular combination either of the molecules of the same body or of 
different bodies. 
Line spectra .-—The appearance of these spectra is too well known to need any 
description. Until lately, line spectra have kept out of the way of the many difficul¬ 
ties which beset their brethren of different orders, but at last they could no more 
resist the invasion. It gradually appeared that lines which are prominent at one tem¬ 
perature may be comparatively weak at another. This fact is sometimes exaggerated to 
such a degree that all the lines which form the spectrum of lower temperature 
disappear without a remnant at higher temperatures, while the lines of higher tem¬ 
perature are not to be seen at lower temperatures. Oxygen, which is a typical gas 
for every difficulty in spectroscopy, is a case in point. Oxygen seems to have two 
perfectly distinct line spectra, each of which generally appears by itself and unmixed 
with a trace of the other sj^ectrum. 
Let us for a moment consider how we may explain these facts without having 
recourse to molecular combinations. We have to assume that, when the vibrations of 
a molecule are small, they take place in a series of periods, and consequently show 
lines in the spectroscope at places corresponding to these periods. Now imagine the 
temperature to be raised. We must assume that, as the intensity of vibration in¬ 
creases, other periods come into play which rapidly increase in intensity, while those 
which were prominent at first gradually disappear, so that, after a while, a different 
set of vibrations is formed. I do not know that there is any fact which could be 
adduced against such an explanation, nor do I think that the explanation is theo¬ 
retically impossible. Yet the passage of one line spectrum into the other resembles so 
very closely the passage of a band spectrum into that of a line spectrum, that one feels 
inclined to refer both phenomena to the same cause. In the case of oxygen, for 
instance, one line spectrum appears under precisely the same circumstances as the 
band spectrum of nitrogen, and it changes into the other line spectrum almost identi¬ 
cally in the same way as the band spectrum of nitrogen changes into the line spectrum. 
