90 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
have experimented with air moistened in various ways, sometimes by allowing small 
bubbles of it to ascend through water, sometimes dividing it by sending it through the 
pores of common cane immersed in water. Between the drying apparatus and the 
experimental tube I have introduced tubes containing fragments of glass moistened 
with water, and allowed the air to pass over them ; large effects were in all such cases 
obtained, the absorption being usually more than eighty times that of dried air. Frag- 
ments of unwetted glass, which had been merely exposed to the air of the laboratory, 
had dry air led over them into the experimental tube ; the absorption was fifteen times 
that of dried air. A roll of bibulous paper, taken from one of the drawers of the labo- 
ratory, and to all appearance perfectly dry, was enclosed in a glass tube, and dry air 
carried between its leaves. The experiment was made five times in succession rvith the 
same paper, and the following absorptions were observed : — 
Absorption. 
No. 1 72 
No. 2 62 
No. 3 62 
No. 4 47 
No. 5 47 
In fact, the action of aqueous vapour is exactly such as might be expected from the 
vapour of a liquid which Mellon: found to be the most powerful absorber of radiant 
heat of all he had examined. 
Every morning, on commencing my experiments, I had an interesting example of the 
power of glass to gather a film of aqueous vapour on its surface. Suppose the tube 
mounted, and the air of the laboratory removed as far as the air-pump was capable of 
removing it. On allowing dry air to enter for the first time, the needle would move 
from 0° to 50°. On pumping out it would return to 0°, and on letting in dry air a second 
time it would swing almost to 40°. Repeated exhaustions would cause this action to 
sink almost to nothing. These results were entirely due to the vapour collected during 
the night in an invisible film on the inner surface of the tube, and which was removed 
by the air on entering, and diffused through the tube. If the dry air entered at the 
end of the tube nearest to the source of heat, on the first and second admissions, and 
sometimes even on a third, the vapour carried from the warm end to the cold end of the 
tube was precipitated as a mist upon the latter, for a distance sometimes of nearly a foot. 
The mist always disappeared on pumping out. It is needless to remark that facts of this 
character, of which I could cite many, were not calculated to promote incautiousness or 
rashness on my part. I saw very clearly how easy it was to fall into the gravest errors, 
and I took due precautions to prevent myself from doing so. 
Knowing that a solution of salt was almost as opake to radiant heat as water itself, 
I was careful to examine whether the effects which I had observed with aqueous vapour 
might not be due to the precipitation of the vapour on the surfaces of the plates of 
