204 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
and permitting air to bubble through the liquid and enter the tunnel X Y, we divide 
the tunnel into spaces of air saturated with the vapour, and spaces of air in its ordinary 
condition. The action of such a medium upon the sound-waves issuing from the bell B 
is very energetic, instantly reducing the violently agitated flame to stillness and steadiness 
as long as it continues to be the vehicle of the sound. The removal of the hetero- 
geneous medium instantly restores the noisy flaring of the flame. 
That differences of density due to differences of temperature produce partial echoes 
and waste of sound is also capable of experimental demonstration. Across a tunnel 
resembling X Y sixty-six platinum wires were stretched, all of them being in metallic 
connexion. The bell B, in its padded box, was placed at one end of the tunnel, 
and the sensitive flame F, near its flaring-point, at the other. When the bell rang 
the flame flared. A current from 50 cells being sent through the platinum wires, 
layers of warm air rose from them through the tunnel, and immediately the agitation 
of the flame was stilled : on stopping the current the agitation recommenced. In this 
experiment the platinum wires were not heated to redness. Employing half the number 
and the same battery, they were raised to a red heat, the action in this case upon the 
sound-waves being also energetic. Employing one third of the number, with the 50 
cells, the wires were raised to a white heat ; and here also the flame was immediately 
rendered tranquil by the stoppage of the sound. A number of experiments executed 
by Mr. Cottrell at my request showed that 66 wires at a dark heat act a little more 
strongly upon the sound-waves than 38 at a red heat or than 22 at a white heat*. 
A few details of the experiments on the action of non-homogeneous atmospheres pro- 
duced by the saturation of layers of air with the vapours of volatile liquids may follow 
here. 
To secure a more uniform distribution of the layers of vapour in the tunnel X Y, the 
box r s (fig. 2, Plate XVIII.) was subsequently divided into three compartments, because, 
with the arrangement described in § 9, the air at the centre of the tunnel remained 
homogeneous after its two ends had become heterogeneous. In all cases the saturated 
air was forced in layers upwards, while common air was forced downwards ; with the 
vapours of the liquids here mentioned the following results were obtained : — 
Bisulphide of carbon. — Flame very sensitive; the action of the non-homogeneous 
atmosphere prompt and strong. 
Chloroform. — Flame still very sensitive ; action similar to the last. 
Iodide of ethyl. — Action decided, but weak. 
Iodide of methyl. — Action prompt and energetic. 
Benzol. — Action decided, but very weak. 
Amylene. — Very fine action, prompt and energetic. Bendered a short and violently 
agitated flame tall and quiescent. 
Sulphuric ether. — Action prompt and energetic. 
* Mr. Cottrell himself has shown, in a very striking manner, the reflection of sound from a single layer of 
heated air (Proc. Roy. Soc. vol. xxii. p. 190). The action of a hot poker is also quite distinct. 
