66 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
heating- of the gas on entering the tube, and kept dissipated by the flux of heat passing 
tlirongh it. At all events the closest scrutiny could detect no trace of mist or turbidity 
-s^ithin the tube ; it was perfectly transparent throughout. The chlorine, on the contrary, 
was intensely coloured. 
I made many experiments with chlorine which had been collected over water, but 
something (what 1 know not yet) appeared to be in all cases carried along with the gas 
from the water into the tube, which materially augmented its absorption. 
The above experiments were made in the early part of this inquhy, and before I had 
become aware of all the peculiarities of my apparatus. Subsequent experiments reduced 
in some degree the absorption both of chlorine and hydrochloric acid. Very careful 
experiments made on the 29th of October gave the following absorption for these two 
gases, at a tension of 30 inches : — 
Chlorine 39 
Hydrochloric acid . . . . 53 
The chlorine and hydrochloric acid were removed from the experimental tube in the 
following manner: — A cock and connecting piece were attached to one end of the 
experimental tube, and from them a length of india-rubber tubing led to the flue of the 
laboratory stove. A gas-holder of air was put in connexion with the other end of the 
experimental tirbe, a system of dryirrg-tubes intervening between the latter and the 
holder. By a slight water-pressure a stream of dry air was carried gently through the 
tube to the flue, arrd in this way the gases, which if pumped out would have injured the 
pistons, were speedily removed. As the dry air replaced the gases, the needle gradually 
descended to zero, its arrival there being indicative of the complete displacement of the 
gas. The perfect dryness of the air thus made use of was beautifully proved. Had the 
air contained moisture, it would instantly on its mixture with hydrochloric acid have 
rendered the medium -within the tube turbid. However slight this turbidity might be, 
and however in\asible to the eye, the galvanometer would have revealed it. But there 
was no movement in an upward direction ; the needle gradually sunk from the moment 
the air entered. 
As regards the influence of chemical union in the absorption of radiant heat, a still 
more severe test than that furnished by the substances last examined is presented by 
bromine and hydrobromic acid ; for the opacity of the former to light is far greater than 
that of chlorine, while the two compounds are equally transparent. The density of 
bromine, moreover, is 5-54, whereas that of hydrobromic acid is only 2*75. The difficulty 
of operating with the acid compound of bromine is at least equal to that attendant on 
hydrochloric acid, and several successive days were spent in endeavouring to arrive at 
safe conclusions in connexion with this subject. Bromine dried with phosphoric acid 
was introduced into a flask furnished with a screw cap, which enabled it to be attached 
to the experimental tube. By turning a stopcock, the pure vapour was allowed slowly 
to enter until the mercury column was depressed two inches. From more than twenty 
experiments made with this substance, I should infer that the absorption of the quantity 
