SECT. II.]
PROPERTIES OF STEAM.
81
The idea of employing very powerful pressures, acting through a short space,
seems more valuable at first sight than it proves on examination. It is considered
that an engine of high power can be got into a small place, and will be of less
weight. But the real inconveniences are, the large mass of fuel required to
supply the engine a given time, and the immense surface that must be exposed
to an intense heat to obtain a given quantity of heat in a given time. Besides,
when we attempt to use high degrees of pressure, an accuracy of workmanship,
and attention to the elasticity of materials, becomes necessary, which renders the
work expensive and of short duration.
The success of Mr. Faraday in reducing various gases into the liquid state is
not however the less important. His method consisted in generating the sub
stances in a bent tube of glass hermetically sealed at both ends. Then, by cooling
one end of the bent tube and heating the other, when heat was necessary, the gas
was condensed in a liquid state at the cold end of the tube.
117. Carbonic acid required the greatest precautions to effect the conden
sation with safety. The liquid obtained is a limpid, colourless body, extremely
fluid, and floated upon the contents of the tube without mixing. It distils readily
at the difference of temperature between 32° and 0°: its refractive power is much
less than that of water, and its vapour exerts a pressure of 36 atmospheres
at a temperature of 32°. In endeavouring to open the tubes which contained
it, at one end, Mr. Faraday states, that they uniformly burst with powerful
explosions. 1
The gases reduced to a liquid state by Mr. Faraday, with their densities as far
as they are known, are collected in the following table, with a column to show
the mechanical power compared with steam. 2
1 The ingenious Mr. Brunei is attempting to work an engine where the acting vapour is to be
from liquid carbonic acid. It is to he regretted that his great talent for mechanical combination
should be employed where there is so little chance of success.
2 The power is as the force and the space through which the gas passes in its reduction to the
state of liquid. (See Sect, iv.) The space is found by comparing the density of the body in the
liquid state with its density in the gaseous under the same pressure ; and as the weight of air is
to water as 1: 828, to find the mechanical power of equal volumes of the liquid, we have simply
to multiply 828 by the specific gravity of the liquid, and divide the product by the specific gravity
of the body in the state of gas. The force does not enter into the calculation, because the density
of the gas must obviously be greater in the same proportion. The quantity of heat is most pro
bably in the ratio of the power : if this be the case, all substances will afford equal powers with
equal quantities of heat.
L