34 CHEMISTRY. 
the same height as that in the vessel A; the number of cubic inches or 
centimetres contained is then to be read off. 
All these methods for nitrogen depend upon measurements of gases, 
which can never be so accurate as weighing, as they are exposed to many 
more errors of observation, since, besides the direct volume occupied, we 
should have reference to the height of the barometer and thermometer. The 
following method furnishes the nitrogen in a form in which it may be 
weighed: let fig. 37° represent the combustion furnace with the contained 
combustion tube, A. Into this tube is introduced the substance to be 
burned, mixed, however, with carbonate of soda instead of oxyde of copper. 
On heating the mixture the organic matter is burned, at the expense of the 
water combined chemically with the soda, this water giving off its oxygen 
to the carbon of the organic body, and its hydrogen to its nitrogen. 
Ammonia results from this latter combination, which is received in the 
apparatus B, inserted in the combustion tube, A, by a cork, and filled with 
hydrochloric acid. A salt (chloride of ammonium or sal-ammoniac) is 
produced by the combination of the ammonia and hydrochloric acid, which 
is not volatile at the boiling point of hydrochloric acid, so that after pouring 
out the liquid from B into a capsule, the sal-ammoniac may be obtained 
separate by evaporation. This is then dissolved in dilute alcohol, and 
chloride of platinum added, which, combining with the chloride of ammo- 
nium, forms a double salt, the double chloride of platinum and ammonium, 
which is insoluble in alcohol. This double salt, after drying, is weighed, and 
from this weight the weight of the contained nitrogen may readily be 
determined. 
Nitrogenous substances are generally of animal origin, and often contain 
sulphur and phosphorus in their composition. To‘obtain these substances 
in a form in which they may be weighed, they must be converted into 
acids ; for which purpose we may employ the apparatus, p/. 31, fig. 65. The 
substance whose proportion of phosphorus is to be ascertained, must be 
mixed with soda and saltpetre, and placed in a combustion tube, there to be 
burned. The carbonic acid formed during combustion escapes through the 
anterior open end of the tube, and in the reliquiz the sulphur will occur as 
sulphuric, and the phosphorus as phosphoric acid, which may then be 
separated. 
VI. Mecuanicau Separation, anp THE Apparatus NeEcEssARY. 
Use is often made in chemistry of methods which do not strictly belong to 
the operations of chemical analysis. Thus, suppose the oxyde of iron to 
have been precipitated from a solution of iron, and suppose it be necessary to 
weigh the former: the first condition necessary will be to separate the 
oxyde from the liquid from which it had been precipitated, and from the 
substances used in causing the precipitation. The liquid is first poured on 
a paper filter laid on the funnel, A ( pl. 31, fig. 50), and allowed to run off 
into a tumbler, C, placed beneath, the precipitate remaining on the filter. 
464 
