AS A VEHICLE OE SOUND. 
203 
being connected with an india-rubber bag filled with carbonic-acid gas. The apparatus 
looked at “ end on ” is shown in fig. 3. Here also is shown the section of a second box, 
r \ in all respects similar to the first, furnished like the first with a tube behind, and 
connected by a trunk-tube with the gas-main of the Institution. The course pursued 
by the two gases when the cocks are turned on will be evident on inspecting fig. 3. 
The carbonic acid, entering from its box into the tunnel X Y, falls in layers across the 
tunnel, and escapes by a series of apertures placed vertically under its places of entrance ; 
the lower arrows in fig. 1 mark the course of the carbonic acid. The coal-gas, on the 
other hand, entering from its box below rises in layers between the carbonic-acid layers, 
and escapes through a series of apertures at the top of the tunnel. The upper arrows in 
fig. 1 mark the course of the coal-gas. In this way five-and-twenty layers of the heavier 
gas, separated from each other by layers of the lighter one, are obtained, the limiting 
surfaces at which reflection takes place being therefore fifty in number. 
With the ear properly defended and applied to the end of the tunnel, this apparatus 
proves effective ; but an excellent objective test is furnished by one of the sensitive flames 
discovered by Lecomte, observed some years subsequently by Barrett, and described, 
with variations and extensions, in my sixth Lecture on Sound. Such a flame is repre 
sented at fig. 1 issuing from the burner S. It is protected from air-currents by a glass 
bulb surrounding its lower portion, and through the bulb passes the shank of a funnel 
intended to concentrate the sound. 
The tube t leading to the sensitive flame being connected with a gas-holder, the bell 
B is set ringing, and the pressure on the gas is exalted until the flame F, acted upon 
by the sonorous waves from the bell, trembles violently and roars. A few preliminary 
trials instruct us as to the necessary pressure. The two gases are now turned on, and 
after a few seconds the light and heavy layers establish themselves within the tunnel ; the 
sound is stopped, the roaring trembling flame becomes immediately tranquil and burns 
steadily at a multiple of its height when agitated. The stoppage of the sound by aerial 
reflections is thus demonstrated. When the tunnel is explored with the beam of the 
electric lamp, it is found perfectly clear optically. The cause which so powerfully affects 
the sound does not sensibly affect the light ; and it will be shown subsequently that agents 
which powerfully affect light have no sensible influence upon sound. By cutting off the 
two gases their places are immediately taken by air, the homogeneity of the medium is 
restored, and the flame is again shortened to a fraction of its normal height and thrown 
into violent agitation. The experiment can be repeated at pleasure, with the same 
unfading result. 
Not only do gases of different densities act thus upon sound, but atmospheric air 
saturated in different degrees with the vapours of volatile liquids can be shown by expe- 
riment to produce the same effect. Introducing, for example, into the path pursued 
by the coal-gas in the last experiment one of the flasks which I have so frequently 
employed to charge air with vapour*, partially filling the flask with a volatile liquid, 
* Philosophical Transactions, 1870, vol. clx. p. 337. 
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