33 
DR. A. SCHUSTER OH THE SPECTRA OF METALLOIDS. 
Continuous spectra .—During the first years of spectrum analysis it was generally 
supposed that a continuous spectrum was characteristic either of a solid or of a liquid 
body; but as our experimental knowledge gradually increased, it was found that the 
fact of a body being in its solid, liquid, or gaseous state only indirectly influenced the 
order of the spectrum. It was believed at first that a chemical compound always 
showed the spectrum of its individual molecules; in other words, that the vibrations 
of the molecules were made up of the vibration of the component atoms. If we accept 
this theory, then, of course, we have only one explanation of the different orders of 
spectra, viz., the influence of one molecule on another. According to this view, the 
state of aggregation of the body must be the direct cause of the change in the spec¬ 
trum. Since the work of Mitcherlich, however, it has gradually been recognised 
that a combination of atoms changes entirely the spectrum. This change is not slight, 
but fundamental. The whole vibrating system seems to have been altered ; and there 
is at present, in the vast majority of cases, no hope that we may consider the vibra¬ 
tions of a molecule to be only the slightly disturbed vibrations of the atoms. We 
have now a second cause for a change in the different orders of spectra. We need no 
more look on a continuous spectrum as a line spectrum, disturbed by the shocks of 
other molecules ; but we may explain it by the changes which have taken place in the 
individual molecule. 
According to this view, liquid and solid bodies give generally continuous spectra, 
not because they are liquid or solid, but because in these states the molecules have a 
more complicated structure than in the gaseous state. Experiment has to decide 
between the two theories : the theory of molecular disturbance, and the theory of mole¬ 
cular structure. I think that the facts are decidedly in favour of the latter theory. 
Mr. Lockyer’s investigations have shown that most bodies give us a continuous spec¬ 
trum, as a gas, before they condense, and many at a considerable temperature above 
the boiling point. Mr. Lockyer has rightly drawn the conclusion from these facts, 
that the atomic aggregation of the molecules is the cause of the different orders of 
spectra. If we observe the changes in a spectrum which gradually take place on 
heating or cooling a vapour, we find that the continuous spectrum is produced, not by 
a widening of the bands, but by a direct replacement, which is sometimes sudden and 
sometimes gradual, and which leaves no doubt in the observer’s mind that he has to 
deal with two vibrating systems, and not simply with a disturbed one. I do not, of 
course, mean to say that the impacts of other molecules have no observable influence. 
If the hydrogen lines widen through increased pressure, it is very likely that the 
alteration is produced by impacts ; but the change from a line spectrum to a con¬ 
tinuous spectrum, as a rule, is quite different from the change which takes place with 
hydrogen. According to the theory of molecular aggregation, it seems quite possible 
that a liquid should give the same spectrum as its vapour, and this, indeed, seems to 
be true in some cases. 
I shall show that we can obtain a continuous spectrum of oxygen, not by an 
