ON RADIATION. 463 



than any hitherto pointed out. For all ordinary sources of radiant 

 heat the bisulphide, both in the liquid and vaporous form, is the most 

 transparent, or diathermanous, of bodies. It transmits, for example, 

 ninety per cent of the radiation from our hydrogen-flame, ten per cent 

 only being absorbed. But when we make the carbonic-oxide flame our 

 source of rays, the bisulphide shows itself to be a body of extreme 

 opacity. The transmissive power falls from ninety to about twenty- 

 five per cent, seventy-five per cent of the radiation being absorbed. 

 To the radiation from the carbonic-oxide flame the bisulphide behaves 

 like the carbonic acid. In other words, the group of atoms constituting 

 the molecule of the bisulphide vibrate in the same periods as those of 

 the atoms which constitute the molecule of the carbonic acid. And 

 thus we have established a new, subtile, but most certain resemblance 

 between these two substances. The time may come when chemists 

 will make more use than they have hitherto done of radiant heat as an 

 explorer of molecular condition. 



The conception of these quivering atoms is a theoretic conception, 

 but it is one which gives us a powerful grasp of the facts, and enables 

 us to realize mentally the mechanism on which radiation and absorp- 

 tion depend. We will now turn to a more practical view of the sub- 

 ject. It is pretty well known that for a long series of years I con- 

 ducted an amicable controversy with one of the most eminent experi- 

 menters of our time, as regards the action of the earth's atmosphere 

 on solar and terrestrial radiation. My contention was that the great 

 body of our atmosphere its oxygen and nitrogen had but little 

 effect upon either the rays of the sun coming to us, or the rays of the 

 earth darting away from us into space ; but that mixed with the body 

 of our air there was an attenuated and apparently trivial constituent 

 which exercised a most momentous influence. That body, as many of 

 you know, is aqueous vapor, the amount of which does not exceed one 

 per cent of the whole atmosphere. Minute, however, as its quantity 

 is, the life of our planet depeuds upon this vapor. Without it, in the 

 first place, the clouds could drop no fatness. In this sense the neces- 

 sity for its presence is obvious to all. But it acts in another sense as 

 a preserver. Without it as a covering, the earth would soon be re- 

 duced to the frigidity of death. Observers were, and are, slow to 

 take in this fact, which nevertheless is a fact, however improbable it 

 may at first sight appear. The action of aqueous vapor upon radiant 

 heat has been established by irrefragable experiments in the labora- 

 tory ; and these experiments, though not unopposed, have been sub- 

 stantiated by some of the most accomplished meteorologists of our 

 day. 



I wished much to instruct myself a little by actual observation on 

 this subject, under the open sky, and my first object was, to catch, if 

 possible, states of the weather which would enable me to bring my 

 views to a practical test. About a year ago, a little iron hut, embrac- 



