10 CHEMISTRY. 
withdrawn, the vapor again becomes a solid or liquid, we are fairly entitled 
to suppose that those substances which are gaseous at ordinary temperatures, 
owe this property to their latent heat; and we may legitimately presume 
that by removing this latent heat the conversion desired may be effected. 
The means at our command for removing latent heat consist in the 
application of an extraordinary pressure, since, in proportion to the amount 
of compression exerted, latent heat is rendered sensible, and thus by 
pressure alone may many gases be condensed into liquids. A second method is 
afforded by the use of the so-called freezing mixtures. By mixing together 
certain substances, an extraordinary amount of cold may be produced, and 
many gases rendered liquid by such artificial congelation. In cases which 
are very difficult to reduce, both methods may be applied simultaneously. 
Thus the gas may be placed in a cylinder of metal, in which fits a piston, 
air-tight, and depressed with great force at the same time that the cylinder 
is immersed in a freezing mixture. The condensation of carbonic acid gas 
may serve as an illustration of this method, as also of quite a peculiar mode 
of producing a great degree of cold. Carbonic acid gas is well known, by 
its effects at least, to everybody; it is one of the substances formed by the 
combustion of coal, and one which is developed in the fermentation of 
liquids containing sugar, as beer and wine, its presence being indicated in 
the frothing and foaming of these and similar liquids, when the cork 
inclosing them is withdrawn. Carbonic acid, to become liquid at the 
temperature of freezing water, or 32° F., requires a pressure of sixty 
atmospheres, and at a temperature of 86° one of seventy-three atmospheres. 
The apparatus employed is represented by pl. 31, fig. 6. <A is a cylinder 
of cast iron into which bi-carbonate of soda must be introduced before the 
piece C is inserted. At the same time a copper vessel is to be introduced 
into the cylinder contaiming as much sulphuric acid as will suffice to expel 
all the carbonic acid gas from its combination with the soda, by combining 
itself with the latter. The iron piece C, having a longitudinal canal, is now 
screwed into the cylinder. The copper tube, mm, is so inserted into the 
side of C as to communicate with its canal. In a similar manner this tube 
opens by its other extremity into the piece D of the smaller iron cylinder 
B, similar in construction to C, and having its inner canal in connexion 
with the inside of the cylinder. The iron pieces, c, d, are inserted into the 
similar pieces C and D, and through them pass the screws, a, b, by means 
of which an attached valve fig. '7 (upside down in the plate), may be so 
adjusted, that the openings of the tube, mm, may be shut or opened at 
pleasure. The clamps 7, 2, and E, E, hold the two vessels tightly together. 
The apparatus being thus arranged, the opening of the tube, mm, in C is 
closed by depressing the valve by means of the screw a. The apparatus is 
to be inclined until the sulphuric acid placed in the copper vessel may flow 
over the bi-carbonate of soda, and thus produce a chemical decomposition. 
Both valves are now opened, or that connected with the screws a and 8, 
and the carbonic acid generated passes into the cylinder, B, cooled by ice. 
So vast an amount of gaseous carbonic acid is generated from the 
bi-carbonate of soda, that this becomes fluid from the pressure produced by 
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