12 CHEMISTRY. 
the substance of plants and animals almost always consists of such 
combinations. Hence we see that to ascertain the properties of an 
elementary body, we must carefully separate it from its combinations. In 
the following articles we shall present the principal methods of doing this 
for a number of these elements. 
1. Apparatus for Obtaining the Gaseous Elements. 
Oxygen gas, while in a state of mixture, and not of combination, is a 
principal constituent of atmospheric air and the great instrument of organic 
vitality. Inhaled into the lungs, or brought into contact with the 
respiratory apparatus of animals in general, whether skin, gills, lungs, or 
branchie, it furnishes the chief means for purifying the blood by 
eliminating the effete carbon. This gas, however, does not exist alone. 
The stimulus to these various organs would, in many cases, be too great in 
the case of pure oxygen alone; we therefore find this gas diluted with a 
large proportion of nitrogen. ‘ 
So intimate, however, is the mixture of oxygen and nitrogen in the air, 
that it is impossible to separate them by mechanical means; it therefore 
becomes necessary to do this by chemical agencies, which may also be 
applied in obtaining’ oxygen from other mixtures or compounds, the 
latter, in fact, being by far the best sources from which to procure the gas 
in question. Oxygen combines with almost all the other elementary bodies ; 
at least, the only one in respect to which this fact has not been ascertained, 
is fluorine. To obtain oxygen pure, therefore, we may use one of several 
different combinations. Thus, for instance, we may select the protoxyde 
of mercury, a reddish substance, known in the arts as red precipitate. A 
sufficient quantity of this is introduced into the retort, b (pl. 31, fig. 18), 
the neck of this being firmly held by the clamp of the stand C. The neck 
of the retort terminates in the balloon, c, in which it is secured by a cork. 
From the second opening of this balloon a bent tube, d, passes into the 
water of the trough A. On one of the shelves of the trough is placed the 
receiver B, first filled with water, and then inverted so as to keep it entirely 
full; the end of the tube, d, must be just under the bottom of the receiver 
B. The inversion of the receiver, when filled with water, will be more 
practicable if the open end be closed by a plate of ground glass, which is to 
be removed under the surface of the water in the trough. After seeing that 
all the joints are rendered perfectly air-tight by means of the proper luting, 
a red heat is applied to the bottom of the retort by means of the spirit lamp 
a. At the temperature of a low red heat, the red oxyde of mercury is 
decomposed into its elements, and the gaseous oxygen first drives out the 
contained air through the conducting tube, d; for this reason the first 
bubbles, when formed, must be allowed to escape and not pass into the 
receiver. When all the air has been expelled, the succeeding oxygen passes 
from the end of the tube d, and rising through the water in a succession of 
bubbles, occupies the top of the receiver, displacing as much water as the 
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