so 
PROFESSOE TYNDALL ON THE ABSORPTION AND 
former length, and the following results were obtained : — 
O 
Sulphuric ether 11 
Formic ether 12 
Acetic ether 15 
Olefiant gas 39 
The verification of the above theoretic reasoning is here complete. It is proved that 
in a long tube the dynamic radiation of the vapour exceeds that of olefiant gas^ while in a 
short tube the dynamic radiation of the gas far exceeds that of the vapour. 
§ 8 . 
The apparatus with which these experiments were made is capable of very diverse 
uses. Attached to a compression pump, with it the relation between the mechanical 
force expended in compressing a gas and the heat developed might be accurately deter- 
mined. If oxygen, hydrogen, nitrogen, or air were the body compressed, the conversion 
of vis viva into heat might he declared by a modicum of vapour always kept in the tube, 
while a compound gas would tell its own tale. 
Another interesting point might be, and indeed has been settled by the apparatus. 
Some years ago a discussion was carried on between Professors Challis and Stokes on 
Laplace’s correction for the velocity of sound. Professor Challis contending that 
Laplace had no right to his correction, inasmuch as the heat developed by the local 
compression of a mass of air of indefinite extension would be instantly wasted by radi- 
ation. Experiments, he argued, conducted in confined vessels furnish no ground for 
drawing conclusions regarding what occurs in the atmosphere, where the heat developed 
has an indefinite space to lose itself. In our experimental tube, though it is mecha- 
nically closed, indefinite extension, as regards the radiation of heat, is secured in one 
direction, and the means also exist of measuring the flux of heat in this direction. What 
is true for one direction is of course true for all, so that the apparatus will inform us of 
what must occur in the open atmosphere. Now with the most powerful radiating gases 
which I have examined the radiation continues a very sensible time, while the heat 
acquired by air on entering the tube is often a source of inconvenience on account of the 
inability of the air to disperse its heat by radiation. The question is therefore experi- 
mentally decided in favour of Laplace and his supporter. 
I would here dwell for a moment on this comparative absence of radiating power on 
the part of air, and of the elementary gases generally. The air is the sole source of the 
heat which has warmed the vapours in our experiments on dynamic radiation ; it is related 
to them precisely as a hot polished metal plate is to the coat of varnish which makes it a 
radiator. The air and the metal, both elements or mixtures of elements, are incompetent 
to impart motion to the luminiferous ether without the intermediation of a second body. 
They possess the motion, but they are so related to the ether that they cannot commu- 
nicate this motion to it directly., or only in an extremely feeble degree. The atoms of 
