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XLY. Intelligence and Miscellaneous Articles. 
ON PEOFESSOE LANGLEY's " SELECTIVE ABSORPTION." 
BY C. H. KOYL*. 
"TilTBIXG- a series of years, beginning with 1859, Prof. Tyndall 
■*-J carried on experiments which demonstrated the great absorptive 
power of water, carbonic dioxide, and the vapour of water in tbe 
infra-red region of the spectrum, and which also demonstrated the 
non-absorptive character of these substances in the visible parts. 
Other investigators have, almost without exception, arrived at the 
same conclusions ; and it now appears beyond dispute that pure 
water with its vapour and pure carbonic dioxide deeply absorb the 
long-wave rays, but exercise Kttle influence upon the shorter. The 
same series of experiments proves also that dry oxygen and dry 
nitrogen, either singly or mixed, are almost without effect upon any 
part of the spectrum. 
Our atmosphere is composed principally of oxygen, nitrogen, 
watery vapour, and carbonic dioxide. It follows, then, that when 
there are not clouds or suspended haze the sky may be, as far as 
pure absorption is concerned, almost perfectly transparent to the 
visible rays of solar energy, but to the longer waves opaque, to a 
degree dependent upon the amount of dissolved aqueous vapour. 
Until within a short time actual experiments upon the absorption 
of the earth's atmosphere as a whole have been of little value because 
of deficient apparatus ; but since the invention and application to 
this work of the bolometer by Prof. Langley, we are able to arrive 
at approximate results. In his late paper he shows that the ratios 
of energy in different wave-lengths stopped by our atmosphere are 
not by any means such as we should expect from the laboratory 
experiments above mentioned, but that if e', e", e" represent the 
amounts of energy transmitted, (e') to our atmosphere, (e") through 
it on a clear day with a noon sun, and (e'") with a low sun, the 
ratios will be about as follows for three representative wave-lengths, 
in the ultra-violet, in the green, and in the infra-red : — 
X ... 
V. 
G. 
E, 
•375 
•500 
1-000 
e'" ... 
353 
112 
27 
1203 
570 
225 
309 
235 
167 
demonstrating immediately the fact that though some 54 per cent, 
of long-wave energy is transmitted at low sun, only about 8 per 
* Abstract of remarks at a meeting of the University Scientific Asso- 
ciation, Mav 2, 1883. 
