236 HEAT. 



either by directly heating the gas and measuring the radiation from it, 

 or by a very ingenious method suggested by an apparent anomaly 

 noticed in the course of his work. On admitting air into a tube pre- 

 viously containing vapour, he noticed that, apparently, the vapour became 

 much more transparent, as the thermopile indicated a larger amount 

 of heat falling on to it. But he found that this was really due to the 

 heating of the vapour by compression. This suggested that he might do 

 away with the external source of heat, and allow the gas in his tube to 

 be heated by compression. He was able to measure its radiation when so 

 heated. He thus found that radiating and absorbing powers were in 

 the same order. One result obtained by Tyndall is that water-vapour 

 has a very considerable absorption for dark radiations, a result which 

 was contradicted by other experimenters. The various methods, how- 

 ever, by which Tyndall obtained evidence of the absorption, and the 

 latter experiments of Paschen and others, leave little doubt that his 

 experiments warranted his conclusion. It is confirmed by the observa- 

 tions of meteorologists on the greater or slower rate of cooling of the 

 earth at night, when the air contains less or greater amounts of water- 

 vapour. If we accept this conclusion, we can also account for the great 

 variation in the absorption of the solar spectrum noticed by Langley * 

 and others as occurring with changes of weather. 



We may notice one very interesting result obtained by Tyndall, that 

 water-vapour was especially opaque to the radiation from a hydrogen 

 flame, while a thickness of about a quarter of an inch of liquid water 

 quite cut off this radiation. 



The foregoing account of the results obtained by measurement of 

 quantity without regard to exa,ct quality of radiation, amply confirms the 

 general conclusion that radiating and absorbing powers go together, both 

 being either great or small together. But they do not tell us anything 

 as to the relation to each other of the wave-lengths of radiations 

 emitted and absorbed by the same body ; and for the long wave-length 

 radiations the experimental difficulties in the way of such an investiga- 

 tion have only recently been overcome. But certain general conclusions 

 have been arrived at, chiefly by the aid of the Theory of Exchanges, of 

 which we shall now give some account. These conditions have been 

 directly tested and found to hold good with regard to light radiations 

 and, in some few cases of dark radiations, and we cannot doubt that 

 they are true with regard to all radiations. 



* Rfiearch.cs on Solar Heat, p. 184. 



