TRANSACTIONS OF SECTION B. 485 



adduces) a remarkable experiment in confirmation of this supposition. We know, 

 too, the power which our atmosphere, and especially the water vapour in it, has of 

 absorbing the infra-red rays, and that amongst the Fraunhofer lines some of the 

 strongest groups are due to aqueous vapour ; and the capital observation made 

 by the spectroscopic observers at the last total eclipse, that the group of lines known 

 as ' B,' which is one of those produced by aqueous vapour, is greatly strengthened 

 when the sun's light passes by the edge of the moon and so through the lunar 

 atmosphere, may be taken as a confirmation of the theory that gases like our atmo- 

 sphere are difi'used through space and concentrated about the planets. But if it 

 be true that the compounds are decomposed by absorbing the sun's rays, we ought 

 to find in our atmosphere the products of decomposition, we ought to find in it Iree 

 hydrogen, carbonic oxide, and acetylene or some other hydrocarbons. The hydro- 

 gen, from its small specific gravity, would not be concentrated in the lower regions 

 of our atmosphere in the same proportions as the denser gases, but carbonic oxide 

 and hydrocarbons could not fail to be detected in the air if they formed any sen- 

 sible proportion of the gases in the planetary space. That a large portion of solar 

 radiation is intercepted before it reaches the earth is no doubt true, for there are 

 not only tlie dark bands which are increased by our atmosphere and may reasonably 

 be attributed to the action of like gases pervading planetary space, but there is a 

 continuous absorption of the ultra-violet spectrum beyond tlie line U, and Cornu 

 has found that this absorption is not sensibly aff'ected by our atmosphere, so that 

 the substance, whatever it be that produces it, may be an agent in the process 

 imagined by Dr. Siemens, but cannot be the means of restoring to the sun any por- 

 tion of his radiant energy which reaches our distance from him. 



Dr. Siemens explains the self-luminous character of comets by the theory that 

 the streams of meteoric stones of which tliey are supposed to consist bring from 

 stellar space hydrocarbon and other gases occluded within them, and that in con- 

 sequence of the rise of temperature due to the frictional resistance of such a divided 

 mass moving with enormous velocity, aided by attractive condensation, the occluded 

 gases will be driven out and burnt, the flame giving rise to the original light emitted 

 by the nucleus. Now the spectrum of most comets shows only the principal bands 

 of a Bunsen burner, and is therefore adequately explained bj' the flame of gas con- 

 taining hydrocarbons such as have been found in meteorites; but Dr. Huggins has 

 observed in the spectrum of more than one comet, not only hydrocarbon, but cyano- 

 gen bands ; and, although carbon and nitrogen combine readily in the electric arc, 

 a coal-gas flame in air shows no trace of the spectrum of cyanogen, and it would 

 certainly put some strain on our credulity if it were asserted that cyanogen was one 

 of the gases brought ready-formed by meteorites from stellar space. Dewar and I 

 have, however, recently shown that if nitrogen already in combination, as, for 

 instance, ammonia, be brought into a hydrocarbon flame, cyanogen is produced in 

 sufficient amount to give in a photograph (though not so as to be directly visible) the 

 characteristic spectrum of cyanogen as it appears in the comet. It is therefore no 

 longer necessary to make any other supposition to account for the cyanogen bands in 

 the spectra of comets, than that ammonia, or some such compound of nitrogen, is 

 present as well as hydrocarbons in a state of ignition. 



Quite recently Dr. Huggins has observed that the principal comet of this year 

 has a spectrum of an entirely difi'erent character, but he is not yet able to say to 

 what elements or compounds it is probably due. The notion that comets may bring 

 us news of distant parts of stellar space, towards which our system is driving, where 

 the atmosphere is not like ours — oxygen and nitrogen — but hydrogen and hydro- 

 carbons, may fascinate the fancy, but the laws of occlusion oblige us to think that 

 the meteorites have not merely wandered through an attenuated atmosphere of 

 hydrogen and hydrocarbons, but have cooled in a much denser atmosphere of those 

 substances, which we can only conceive as concentrated by the presence of a star 

 or some large aggregation of matter. They may, perchance, have come from some 

 nebulous mass, for Draper and Huggins tell us that in the great nebula in Orion 

 hydrogen is dense enough and hot enough to show some of its characteristic lines, 

 besides the F line which is seen in other nebulse, and is the last to disappear by 

 reduction of density. No comet on visiting our system a second time can repeat 



