ABSOEPTION OF GASEOUS MEDIA. 
873 
action of hydrobromic acid much exceeds that of the hydrochloric, and that of hydriodic 
acid is greater than of either. 
The abrupt termination of the spectrum in coal-gas is remarkable. The absorption 
appears to he due not to the permanent gases, but to the vapours of benzol and other 
heavy hydrocarbons which it contains. The four compounds of sulphur, viz., sulphurous 
acid*, sulphuretted hydrogen, bisulphide of carbon, and dichloride ’of sulphur, are espe- 
cially active in absorbing the chemical rays; and the vapours of the terchloride and oxy- 
chloride of phosphorus exhibit a similar though less intense absorptive power. 
19. Coloured gases, whether elementary or compound, such as chlorine, bromine, and 
nitrous gas, have long been known to exert an absorptive action upon the luminous raysf ; 
and their effect is not less marked upon the invisible prolongation of the electric spectrum. 
The effects of the three halogens, chlorine, bromine, and iodine, in the form of vapour, 
are particularly remarkable. As a general rule, when a body exerts an absorptive influ- 
ence the absorption is greatest in the most refrangible portions ; but the reverse of this 
occurs in the case of chloiine and of bromine. A column of chlorine two feet in depth, 
cuts off the whole of the less refrangible portion as far as 143’5 ; beyond that a distinct 
impression is obtained as far as about 159. With Iromine diffused in the form of diluted 
vapour, the impression commences at 106, and is continued distinct, though rather feeble, 
to the extreme end of the spectrum. The apparatus required a slight modification to 
adapt it for the experiment with iodine. I had a glass tube 6 inches long, the open ends 
of which were ground fiat so as to admit of being closed by thin plates of quartz ; this 
was enclo.sed in a brass tube ; a few grains of iodine were introduced, and the quartz plates 
fixed by metallic caps perforated to admit the passage of the rays ; this tube could then 
be supported as usual betAveen the spark and the prism, and could be raised to and kept 
at a temperature beyond that necessary for the volatilization of the iodine. The electric 
light, after traversing such a column of vapour of an intensely deep violet colour, gave 
a strong spectrum, extending from 96'5 as far as 112, then it gradually faded till it 
disappeared at about 118 ; the impression became again rather faintly but distinctly 
visible at 142, and gradually disappeared at about 156. It is interesting to notice a 
somewhat similar interrupted absorption of the rays, though at a different part of the 
spectrum, in the case of the metallic iodides. 
Both peroxide of nitrogen and peroxide of chlorine, in a stratum of 2 feet in depth, 
wholly absorb the chemical rays ; but when more dilute or in shorter columns, they each 
give characteristic absorption-bands. 
20. There appears to be little or no connexion between the absorptive power of 
any particular gas for the chemical rays, and its power of absorbing radiant heat as 
determined by the experiments of Dr. Tyndall J. Aqueous vapour is highly diactinic, 
though not diathermic ; olefiant gas exhibits a similar difference ; and various other 
instances might be pointed out. 
* An aqueous solution of sulphurous acid cuts olf the spectrum at the same point as the gas itself does. 
t For an historical sketch of the progress of discovery in relation to the production of bands in the 
spectrum, the reader is referred to a paper by the author in the ‘ Pharmaceutical Journal,’ February 1S62, 
p. 17 et seq. ^ Phil. Trans. 1861. 
