Professor Forbes on the Colours of the Atmosphere. S3 



with more care, has detected a specific action of the earth's 

 atmosphere affecting every part of the spectrum by absorbing 

 or annihilating certain luminous rays of every colour. The 

 analogy which he has observed to exist between tiie deficient 

 lines of the atmospheric spectrum, and those of the common 

 solar spectrum, (which SU" David supposes to have been pro- 

 duced 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 fur- 

 ther to conclude, "that the same absorptive elements exist" 

 in all those media*. Now, since it is the strata of air nearest 

 to the earth whose effect 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 ana- 

 lysis. The air, containing as it does the constituents of nitrous 

 acid gas, is naturally first looked to for their origin. But this 

 supposition, even if it be true, for the atmospheric lines of the 

 spectrum, cannot explain the extraordinary variety of absorp- 

 tive 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 at- 

 tended to the subject of light-house illumination, and in con- 

 sequence of which crimson signal-lights were proposed a few 

 years ago for adoption in hazy weather by Sir John Robisonf, 

 on account of the persistence of such rays in a foggy atmo- 

 sphere. The absorptive elements 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 pure white mass of particles generally called ve- 

 sicular J. 1 have shown that it passes through a third or in- 

 termediate 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 black- 

 ness. 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 par- 

 ticular 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 



* Ed. Trans, xii. .530. f Phil. Mag. 1833. 



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



Phil. Mag. S, 3. Vol. 15. No. 93. July 1839. D 



