260 Professor Tyndall [March 16, 



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 90 to about 25 per cent., 

 75 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, subtle, 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 term " theoretical radiation " introduced into the title of this 

 discourse is, I hope, thus justified. 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 realise mentally the mechanism 

 on which radiation and absorption depend. We will turn in a moment 

 to what I have called practical " radiation." It is pretty well known 

 that for a long series of years I conducted an amicable controversy with 

 one of the most eminent experimenters 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 sj)ace, 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 vapour, the 

 amount of which does not exceed 1 per cent, of the whole atmosphere. 

 Minute, however, as its quantity is, the life of our planet depends upon 

 that vapour. Without it, in the first place, the clouds could drop no 

 fatness. In this sense the necessity 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 reduced 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 

 vapour upon radiant heat has been established by irrefragable experi- 

 ments in the laboratory ; and these experiments, though not unopposed, 

 have been substantiated by some of the most accomplished meteoro- 

 logists 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 mo to bring my 

 views to a practical test. Thanks to an individual who devotes her 

 life to taking care of mine, a little iron hut, embracing a single room, 

 has been placed for my benefit, upon the wild moorland of 

 Hind Head. From the plateau on which the hut stands, there is a 

 free outlook in all directions. Here, amid the heather, I had two 



