Familiar Letters on Chemistry | Page 8

effervescence as a gas, but if the
decomposition is effected in a strong, close, and suitable vessel of iron,
we obtain the carbonic acid in the state of liquid. In this manner it may
be obtained in considerable quantities, even many pounds weight.
Carbonic acid is separated from other bodies with which it is combined
as a fluid under a pressure of thirty-six atmospheres.
The curious properties of fluid carbonic acid are now generally known.
When a small quantity is permitted to escape into the atmosphere, it
assumes its gaseous state with extraordinary rapidity, and deprives the

remaining fluid of caloric so rapidly that it congeals into a white
crystalline mass like snow: at first, indeed, it was thought to be really
snow, but upon examination it proved to be pure frozen carbonic acid.
This solid, contrary to expectation, exercises only a feeble pressure
upon the surrounding medium. The fluid acid inclosed in a glass tube
rushes at once, when opened, into a gaseous state, with an explosion
which shatters the tube into fragments; but solid carbonic acid can be
handled without producing any other effect than a feeling of intense
cold. The particles of the carbonic acid being so closely approximated
in the solid, the whole force of cohesive attraction (which in the fluid is
weak) becomes exerted, and opposes its tendency to assume its gaseous
state; but as it receives heat from surrounding bodies, it passes into gas
gradually and without violence. The transition of solid carbonic acid
into gas deprives all around it of caloric so rapidly and to so great an
extent, that a degree of cold is produced immeasurably great, the
greatest indeed known. Ten, twenty, or more pounds weight of mercury,
brought into contact with a mixture of ether and solid carbonic acid,
becomes in a few moments firm and malleable. This, however, cannot
be accomplished without considerable danger. A melancholy accident
occurred at Paris, which will probably prevent for the future the
formation of solid carbonic acid in these large quantities, and deprive
the next generation of the gratification of witnessing these curious
experiments. Just before the commencement of the lecture in the
Laboratory of the Polytechnic School, an iron cylinder, two feet and a
half long and one foot in diameter, in which carbonic acid had been
developed for experiment before the class, burst, and its fragments
were scattered about with the most tremendous force; it cut off both the
legs of the assistant and killed him on the spot. This vessel, formed of
the strongest cast-iron, and shaped like a cannon, had often been
employed to exhibit experiments in the presence of the students. We
can scarcely think, without shuddering, of the dreadful calamity such
an explosion would have occasioned in a hall filled with spectators.
When we had ascertained the fact of gases becoming fluid under the
influence of cold or pressure, a curious property possessed by charcoal,
that of absorbing gas to the extent of many times its volume,--ten,
twenty, or even as in the case of ammoniacal gas or muriatic acid gas,

eighty or ninety fold,--which had been long known, no longer remained
a mystery. Some gases are absorbed and condensed within the pores of
the charcoal, into a space several hundred times smaller than they
before occupied; and there is now no doubt they there become fluid, or
assume a solid state. As in a thousand other instances, chemical action
here supplants mechanical forces. Adhesion or heterogeneous attraction,
as it is termed, acquired by this discovery a more extended meaning; it
had never before been thought of as a cause of change of state in matter;
but it is now evident that a gas adheres to the surface of a solid body by
the same force which condenses it into a liquid.
The smallest amount of a gas,--atmospheric air for instance,--can be
compressed into a space a thousand times smaller by mere mechanical
pressure, and then its bulk must be to the least measurable surface of a
solid body, as a grain of sand to a mountain. By the mere effect of
mass,--the force of gravity,--gaseous molecules are attracted by solids
and adhere to their surfaces; and when to this physical force is added
the feeblest chemical affinity, the liquifiable gases cannot retain their
gaseous state. The amount of air condensed by these forces upon a
square inch of surface is certainly not measurable; but when a solid
body, presenting several hundred square feet of surface within the
space of a cubic inch, is brought into a limited volume of gas, we may
understand why that volume is diminished, why all gases without
exception are absorbed. A cubic inch of charcoal must have, at the
lowest computation, a surface of one hundred square feet. This property
of absorbing gases