In this manner of operating it is impossible to determine the quantity
of carbonic acid gas disengaged, because a part of it is absorbed by the
water while passing through it; but, when the carbonic acid is absorbed,
the azotic gas remains; and, if it be agitated for a few minutes in
caustic alkaline solution, we obtain it pure, and can easily determine
its volume and weight. We may even, in this way, acquire a tollerably
exact knowledge of the quantity of carbonic acid by repeating the
experiment a great many times, and varying the proportions of charcoal,
till we find the exact quantity requisite to deflagrate the whole nitre
employed. Hence, by means of the weight of charcoal employed, we
determine the weight of oxygen necessary for saturation, and deduce the
quantity of oxygen contained in a given weight of nitre.
I have used another process, by which the results of this experiment are
considerably more accurate, which consists in receiving the disengaged
gasses in bell-glasses filled with mercury. The mercurial apparatus I
employ is large enough to contain jars of from twelve to fifteen pints
in capacity, which are not very readily managed when full of mercury,
and even require to be filled by a particular method. When the jar is
placed in the cistern of mercury, a glass syphon is introduced,
connected with a small air-pump, by means of which the air is exhausted,
and the mercury rises so as to fill the jar. After this, the gas of the
deflagration is made to pass into the jar in the same manner as directed
when water is employed.
I must again repeat, that this species of experiment requires to be
performed with the greatest possible precautions. I have sometimes seen,
when the disengagement of gas proceeded with too great rapidity, jars
filled with more than an hundred and fifty pounds of mercury driven off
by the force of the explosion, and broken to pieces, while the mercury
was scattered about in great quantities.
When the experiment has succeeded, and the gas is collected under the
jar, its quantity in general, and the nature and quantities of the
several species of gasses of which the mixture is composed, are
accurately ascertained by the methods already pointed out in the second
chapter of this part of my work. I have been prevented from putting the
last hand to the experiments I had begun upon deflagration, from their
connection with the objects I am at present engaged in; and I am in
hopes they will throw considerable light upon the operations belonging
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