490 BARON VON WREDE ON THE ABSORPTION OF LIGHT 
tion, or rather the form which we must suppose matter to possess in 
order to produce them, I cannot but remark en passant a circumstance 
which has excited my attention, and which perhaps deserves considera- 
tion. 
Most of the gases retain, when they are in any manner brought 
into another aggregate condition, very nearly the same colour. The 
retarding cause to which we ascribe the colour of the body must there- 
fore be almost independent of the aggregate condition. The other re- 
tarding cause, on the contrary, suffers a certain change when that con- 
dition is changed, because the spectrum of the light which has traversed 
a solid or liquid body does not possess the black stripes which it would 
have if the body had been gaseous. In this fact we have a certain rea- 
son for referring the first to the particles of the body, and the latter to 
their distance from one another, because we properly think these to be 
changeable. A reflection in the inderior of a particle, or a kind of pro- 
pagation of light through it, we are not able to imagine, inasmuch 
as we consider it as an elementary particle. Here then we have an in- 
creased probability of the truth of the supposition that bodies consist 
of such groups of elementary particles as Ampére* has supposed in 
order to explain the propagation of caloric; an hypothesis which Herschel 
also in other respects thinks probable+. Be this as it may, we must 
not be thought too bold when we suggest that by observations on the 
absorption of light we may find a new way opened to us of viewing 
the constitution of matter which may perhaps lead to results that 
could be attained in no other way. 
The other facts stated by Brewster, which, as he thinks, remain in- 
capable of explanation according to the undulatory theory, are, on the 
above supposition, all exceedingly easy to be explained. 
In a spectrum of light which has traversed oxalate of chromium and 
potash, all the colours are absorbed with the exception of the red, which 
contains black stripes. Brewster mentions as a consequence of this, 
that this body permits ether to undulate freely to a red ray whose index 
of refraction, in flint-glass, is 16272, and also to another red ray whose 
index is 1:6274; while it is absolutely opake, or its ether will not undu- 
lute at all, to a red ray of intermediate refrangibility whose index is 
16273. 
Set forth under this form, the fact must surely appear as a paradox. 
It is, however, easily explained if we suppose two interruptions, one of 
nearly the same magnitude as the length of the wave of the red light, 
and the other greater, for instance ten times as great. In consequence 
of the first retardation, the curve of the intensity obtains the form of 
AB (fig.4), and, through the second, the form C D (in the same figure): 
the resultant of both must possess the form of E F. If now we consi- 
* Pogeendorff's dnnalen, vol. xxvi. p. 161. 4 Tbid., vol. xxxi. p. 255. 
