CHEMISTRY. 25 
than is possessed by carbonic acid, the latter will be displaced by the 
former, and will be liberated in the form of carbonic acid gas, the sulphuric 
acid combining with the lime to form a sulphate of lime. The evolution 
of the carbonic acid gas will depress the liquid in B, causing it to rise in 
A, and finally this depression will be so great as to leave the lime uncovered 
by the dilute acid. Further evolution of gas will then stop until the cock 
a is opened for the escape of carbonic acid gas, this escape being facilitated 
by the pressure of the liquid in A. As the gas escapes, the liquid is again 
enabled to attack the carbonate of lime, and to generate fresh gas, as before. 
This apparatus thus serves an excellent purpose in enabling us to keep 
constantly on hand a small quantity of this and some other gases, as 
hydrogen, for immediate use. : 
An apparatus by means of which nitric acid may be manufactured on a 
large scale is represented in pl. 31, fig. 55. It consists principally of a 
furnace of peculiar construction, called a galley furnace, the general 
relation of whose parts is shown by the figure. AA is the masonry 
inclosing the inner furnace space, D the chimney, F and E the fuel and 
ash-doors. BB indicates a series of cups in which glass retorts, as CB, are 
set. The retorts are filled half full of nitrate of potassa or saltpetre, and 
sulphuric acid poured in through a tube reaching nearly to the bottom of 
the retort. All the cups being provided with retorts properly prepared, 
and these provided with properly cooled receivers, heat is applied to the 
whole row by means of a single fire. As soon as the mixture in the retorts 
is raised to a certain heat, the sulphuric acid takes the potassa from the 
nitric acid, forming sulphate of potassa, and leaving the nitric acid free to 
escape into the receivers, there to be condensed. 
An illustration of an entirely different method of separating one substance 
from another is seen in the ordinary assaying of silver and gold. If, for 
example, it be desired to ascertain how great an amount of silver is 
contained in any object, as a coin, some grains of the object are placed 
with a certain quantity of lead in a cupel (fig. 43) composed of bone earth 
and ashes. In mints and silver mines, where such operations occur daily, 
several such cupels are placed in a clay muffle (fig. 44). This muffle is 
then set in the opening, A, of the muffle furnace (fig. 42), and surrounded 
by red hot coals. In the glowing current of air within the muffle. the lead 
and copper (if copper compose the alloy) are oxydized, one portion of the 
oxyde of lead escaping in vapor, the rest, however, melting with the 
copper, and being absorbed by the cupel. The silver remains pure in the 
form of a round granule, which is weighed, and the amount of weight 
compared with that ascertained before the operation. 
Having thus mentioned the principal generalities, with respect to the 
combinations of simple and compound bodies, we shall pass to the 
consideration of some more complex chemical operations. Thus, supposing 
it be required to obtain the metal aluminum from clay, which is its oxyde, 
we shall not be able to procure this directly from the clay itself. 
Nevertheless, we may replace the oxygen of the alumina by chlorine, and 
then we may abstract the chlorine by some metal, as potassium, which has 
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