388 PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. ' 



fic action of the earth's atmosphere affecting every part of the spectrum by ab- 

 sorbing, or annihilating certain luminous rays of every colour. The analogy 

 which he has observed to exist between the deficient lines of the atmospheric 

 spectrum, and those of the common solar spectrum, (which Sir David supposes 

 to have been produced in the transit of light through the sun's atmosphere), and 

 those developed in artificial light by the absorptive action of nitrous acid gas, is 

 truly remarkable, and has led him farther to conclude, " that the same absorp- 

 tive elements exist" in all those media.* Now, since it is the strata of air 

 nearest to the earth whose efiect is chiefly conspicuous in producing the tints of 

 evening, it is to be presumed that the elements which produce this action, are 

 within reach of chemical analysis. The air, containing as it does the constituents 

 of nitrous acid gas, is naturally first looked to for their origin. But this supposi- 

 tion, even if it be true, for the atmospheric lines of the spectrum, cannot explain 

 the extraordinary variety of absorptive action observed in hazy weather, when, as 

 we have said, the atmosphere at a thickness of but a few miles suffers only the 

 red rays to pass ; a fact familiar to those who have attended to the subject of light- 

 house illumination, and in consequence of which crimson signal-lights were pro- 

 posed a few years ago for adoption in hazy weather by Sir John RoBisoN,f on ac- 

 count of the persistence of such rays in a foggy atmosphere. The absorptive ele- 

 ments are clearly within our reach ; can they be nitrous gas, or what are they ? 

 The experiment detailed in my last paper comes in to answer the question. Vapour 

 has hitherto been known (to philosophers at least) under but two characters, — a 

 colourless gaseous body, and a translucent pm-e white mass of particles generally 

 called vesicular. :t: I have sheA\Ti that it passes through a third or intermediate 

 state, in which it is very transparent, but having a more or less intense colour 

 graduating through the very shades which nitrous acid gas assumes, — that is, 

 tawny yellow, orange, deep orange-red, intense smoke-red, verging on blackness. 

 I say that this discovery, to a great extent, supplies the gap which was wanting 

 to make the absorption theory intelligible. It is the " mixture of air and vapour in 

 a particular state," which Count Maistre supposed (see the passage quoted above), 

 but could not prove to exist. The threefold condition of vapour in the sky we 

 can now exhibit in a room ; — the pure elastic fluid devoid of colour, which gives 

 even to pure air its greatest transparency, — next, the transition state, when, still 

 invisible in form, and almost certainly not vesicular, it transmits a steady orange 

 glare, not the play of colour which is often seen in clouds and fogs forming a 

 glory round a radiant body ; — and lastly, the vesicular steam, such as we every 

 day see issuing from the spout of a tea-kettle reflecting iridescent colours, just as 

 the semi-opake clouds do which seem to float across the disk of the sun or moon. 



" Ed. Trans, xii. 530. f P^iil- Mag. 1833. 



\ See Robison's Works, ii. 2, &c. 



