202 
PROFESSOR TYNDALL ON THE ABSORPTION AND 
The third paper* of the series to which I have referred was devoted to the exami- 
nation of one particular vapour, which on account of its universal diffusion possesses 
an interest of its own — I mean of course the vapour of water. In this paper I con- 
sidered all the objections which had been urged against my results up to the time when 
the paper was written ; I replied to each of them by definite experiments, removing 
them one by one, and finally placing, as I believe, beyond the pale of reasonable doubt 
the action of the aqueous vapour of our atmosphere. In this third paper, moreover, 
the facts established by experiment are applied to the explanation of various atmo- 
spheric phenomena. 
I have now the honour to lay before the Royal Society a fourth memoir, containing 
an account of further researches. Hitherto I have confined myself to experiments on 
radiation through gases and vapours which were introduced in succession into the same 
experimental tube, the heat being thus permitted to pass through the same thickness of 
different gases. A portion of the present inquiry is devoted to the examination of the 
transmission of radiant heat through different thicknesses of the same gaseous body. 
The brass tube with which my former experiments were conducted is composed of 
several pieces, which are screwed together when the tube is to be used as a whole ; 
but the pieces may be dismounted and used separately, a series of lengths being thus 
attainable, varying from 2-8 inches to 49-4 inches. I wished, however, to operate upon 
gaseous strata much thinner than the thinnest of these, and for this purpose a special 
apparatus was devised, and with much time and trouble rendered at length practically 
effective. 
Eig. 1. 
The apparatus is sketched in fig. 1. C is the source of heat, which consists of a plate 
of copper against the back of which a steady sheet of flame is caused to play. The 
plate of copper forms one end of the chamber F (the “ front chamber ” of my former 
memoirs). This chamber, as in my previous investigations, passes through the vessel 
* Philosophical Transactions, December 1862 ; and Philosophical Magazine, July 1863. 
