CHEMISTRY. 33 
products of combustion of a substance composed of carbon, hydrogen, 
nitrogen, and oxygen, can only consist of water, carbonic acid, and nitrogen, 
there remains in the graduated tube g, after all the carbonic acid has been 
abstracted by the potassa, only the nitrogen, which may then be measured. 
In this experiment, it is true that the air has passed over from the 
combustion tube into the graduated tube g, but since the volume has been 
determined no error will arise from this mixture with the nitrogen. The tube 
a reaches with its bent leg nearly to the cover of the graduated cylinder g, 
and as soon as the combustion tube, ab, is cooled, as much of its gaseous 
contents passes from the cylinder g into the combustion tube as is necessary 
to fill it. ‘The tube g being divided into cubic inches, or cubic centimetres, 
we can readily calculate the weight of the contained nitrogen, knowing the 
weight of a cubic inch or centimetre. 
The volume of nitrogen may be more accurately determined than in the 
preceding method by means of the apparatus (p/. 31, jig. 37°). The 
combustion tube, a, b, c, d, is filled from its posterior extremity, @ to b, with 
carbonate of lead, from b to c with the mixture of oxyde of copper and the 
substance to be burned, and from ¢ to d with copper turnings. The first 
half of the carbonate of lead is heated, by which means its carbonic acid is 
liberated, and the air thereby expelled from the whole series of tubes. To 
assist in the expulsion of air from the tube, the air-pump A, with the tube 
B, is applied to the intermediate joint of brass, provided with the cock f, 
this joint carrying at one side the vertical glass tube h, a little over twenty- 
eight inches in length. The lower end of this tube dips into the mercurial 
trough C. The cock. f, is to be opened and the air pumped out from the 
whole apparatus, by which means the mercury ascends to a height of nearly 
twenty-eight inches in the tube h. The cock, f, is now to be closed, and the 
carbonate of lead heated red hot, until the carbonic acid liberated depresses 
the mercury in / again into the trough, andescapes from the mercury in 
bubbles. The cock, f, is then again opened, and the previous operations 
repeated, until all the air has been expelled by means of the carbonic acid 
gas and the air-pump. The cock is now to be closed, and the bent lower 
extremity of the tube, 4, brought under the cylinder D, filled with mercury, 
this being held erect by the holder KE; the substance also is to be burned 
by means of coals surrounding the combustion tube, a, b, c,d. As soon as 
a considerable quantity of gas, consisting of watery vapor, carbonic acid, 
and nitrogen, has accumulated, a sufficient amount of solution of potassa is 
introduced into the cylinder D by means of a pipette; this solution will 
take up both the water and the carbonic acid, leaving the nitrogen, which, 
however, will still contain a little vapor of water. Afterwards, to measure 
the amount of nitrogen, we must introduce a shallow dish under the 
opening of the cylinder, and remove it with the cylinder from the trough, 
the mouth of the cylinder still remaining closed by the mercury filling the 
dish. In this way the cylinder is brought into a high vessel ( fig. 38), A, 
filled with water, and the dish removed. The mercury will fall to the 
bottom, and the gas remain included by the water. The graduated cylinder 
is depressed in the water until the surface of the water inside stands just at 
463 
