10 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
A slow current of air sent through the tube gradually removed the gas, and the needle 
returned accurately to zero. 
The gas within the holder being under a pressure of about 12 inches of water, the 
cock attached to the cube was turned quickly on and off ; the quantity of gas which 
entered the tube in this brief interval was sufficient to cause the needle to be driven to 
the stops, and steadily held between 60° and 70°. 
The gas being again removed, the cock was turned once half round as quickly as pos- 
sible. The needle was driven in the first instance through an arc of 60°, and was held 
permanently at 50°. 
The quantity of gas which produced this last effect, on being admitted into a graduated 
tube, was found not to exceed one-sixth of a cubic inch in volume. 
The tube was now taken away, and both sources of heat allowed to act from some 
distance on the thermo-electric pile. When the needle was at zero, olefiant gas was 
allowed to issue from a common argand burner into the air between one of the sources 
of heat and the pile. The gas was invisible, nothing was seen in the air, but the needle 
immediately declared its presence, being driven through an arc of 41°. In the four 
experiments last described, the source of heat was a cube of oil heated to 250° Centi- 
grade, the compensation cube being filled with boiling water'*. 
Those who like myself have been taught to regard transparent gases as almost per- 
fectly diathermanous, will probably share the astonishment with which I witnessed the 
foregoing effects. I . was indeed slow to believe it possible that a body so constituted, 
and so transparent to light as olefiant gas, could be so densely opake to any kind of 
calorific rays ; and to secure myself against error, I made several hundred experiments 
with this single substance. By citing them at greater length, however, I do not think 
I could add to the conclusiveness of the proofs just furnished, that the case is one of 
true calorific absorption f. 
§ 6 . 
Having thus established in a general way the absorptive power of olefiant gas, the 
question arises, “ What is the relation which subsists between the density of the gas 
and the quantity of heat extinguished! ” 
I sought at first to answer this question in the following way : — An ordinary mer- 
curial gauge was attached to the air-pump ; the experimental tube being exhausted, and 
the needle of the galvanometer at zero, olefiant gas was admitted until it depressed the 
mercurial column 1 inch, the consequent deflection being noted ; the gas was then 
admitted until a depression of 2 inches was observed, and thus the absorption effected 
by gas of 1,2, 3, and more inches tension was determined. In the follo'wing Table 
the first column contains the tensions in inches, the second the deflections, and the third 
the absorption equivalent to each deflection. 
* "With a cube containing boiling water I have since made this experiment visible to a large audience. 
t It is evident that the old mode of experiment might be applied to this gas. Indeed, several of the solids 
examined by Melloni are inferior to it in absorptive power. Had time permitted, I should have checked 
my results by experiments made in the usxial way; this I intend to do on a future occasion. 
