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 interior 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 Ampere * has supposed in 

 order to explain the propagation of caloric; an hypothesis which Herschel 

 also in other respects thinks jjrobablef. 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 ichose index 

 of refraction, in flint-fflass, is 1"6272, and also to another red ray whose 

 index is I'GS?^ ; ivhile it is absolutely opahe, or its (ether will not undu- 

 late at all, to a red ray of intermediate refrangihility whose index is 

 1-6273. 



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 

 A B (fig.4), and, through the second, the form C D (in the same figure): 

 the resultant of both must possess tiie form of E F. If now we consi- 



• Vogg^nAoiS's Annalen, vol.xxvi. p. 161. f Ibid., vol. xxxi. p. 255. 



