THE LINES OE THE SOLAE SPECTEHM. 
157 
Lines produced hy Absorbent Media. 
In the paper already referred to Sir David Beewstee described the remarkable 
series of dark lines and bands which make their appearance in the spectrum when 
nitrous acid gas is interposed between the prism and the source of light, whether that 
be the sun or a burning lamp. He mentions also the circumstance that heating the gas 
produces the same increase in the number and breadth of these lines as an increased 
thickness of the gaseous stratum itself does. From his drawings made about that time, 
fig. 10 is compiled. It is on half the scale of fig. 1, and the principal lines of the solar 
spectrum are inserted, with a -view of identifying the position of the nitrous bands. 
These bands are numbered in the figure, beginning not with 1 but with 10, so that any 
future observer publishing a map Avith those lines in the orange and red spaces, which 
require a gi’eater thickness of gas, or a higher temperature to develope them, may 
continue the numbers also in the same backward direction. 
Almost immediately after the publication of this effect of nitrous acid fumes on light, 
Professor "VY. H. Millee, of Cambridge, announced the discovery of different series of 
lines caused in the spectrum hy the interposition of bromine and iodine vapour, and 
euchlorine gasf. These lines differ wholly from the preceding. Subsequently Professor 
\Y. A. Millee, of King’s College, investigated the subject, and published J not merely a 
description, but coloured drawings also of the lines and bands of absorption produced by 
iodine, bromine, chlorous acid, nitrous acid, and perchloride of manganese. His delinea- 
tion of the nitrous bands does not profess to be very accurate, and differs considerably 
in detail from the much fuller drawings that are united together in fig. 10, partly per- 
haps because he employed a sufficiently dense stratum of gas to bring out lines in the 
red, and to intercept nearly the whole of the violet rays. 
It was observed by Sir David Beewstee in the same paper that a solution of oxalate 
of chromium and potash has the remarkable property of giving rise to a sharp and narrow 
black band in the prismatic image. This band coincides with the bright space between 
the solar bands a 6 and a 7. Analogous absorption bands have been remarked when light 
has been transmitted through solutions of other salts; for instance, blue compounds of 
cobalt, salts of the protoxide and of the peroxide of uranium, permanganate of potash, 
and salts of didjunium, beside such substances as chlorophyll, alizarine, and purpurine||. 
Origin of Lines. 
The origin of these fixed lines and bands in the solar spectrum is a question still 
unresolved. It may be conceived — 
* Phil. Trans. Edinb. vol. xii. p. 522. f Phil. Mag. 1834. J Ibid. August, 1845, p. 81. 
§ P. 525. See also Philosophical Transactions, 1835, p. 92. 
II See the papers of Sir John Heeschel, Phil. Trans. Edinb. ix. ; of Sir David Beewstee, Phil. Trans. 
Edinb. xii. p 538 ; of Professor Stokes, Philosophical Transactions, 1852, pp. 487, 517, 522, 558, and Quart. 
Journ. Chem. Soc. 1859, p. 219; and of Dr. Gladstone, Quart. Journ. Chem. Soc. 1857, pp. 79 and 219, 
and Phil. Mag. Dec. 1857. 
MDCCCLX. Y 
