ORGANIC ANALYSIS. 



705 



FIG. 94. 



and oxide are at a red heat, the tube is heated from the closed 

 end, a second screen n being gradually advanced from that 

 extremity. When the proportion of nitrogen is not inconsi- 

 derable, it is generally sufficient to determine its relation by 

 volume to the carbonic acid evolved at the same time. This is 

 done by attaching a bent quill tube (Fig. 94) to the mouth of 

 the combustion tube, by which the gas evolved is conveyed to 

 the mercurial trough, and collected at different stages of the 

 combustion in a small graduated jar. By passing up a few 

 drops of caustic potash into the gas in the jar, the carbonic 

 acid is absorbed and the nitrogen remains. If the proportion 

 between the volumes of the two gases thus observed is the 

 same in several successive trials, and no red nitrous fumes be 

 perceived on mixing the gas with air, this result is sufficient 

 for the nitrogen. The whole quantity of carbonic acid pro- 

 duced in the combustion being known from a previous analysis 

 conducted in the usual way, we can obtain the volume of ni- 

 trogen by calculation, and thence its weight. Or, as two volumes 

 of carbonic acid and of nitrogen represent both one atom of 

 carbon and nitrogen, the atoms of these two gases are in the 

 same number as the volumes observed ; consequently when 

 the weight of the carbonic acid is known, that of the nitrogen 

 may be calculated from the atomic weights of carbonic acid 

 and nitrogen. 



For the analysis of uric acid, in which the volumes of the ni- 

 trogen and carbonic collected are as 4 to 10, and of bitartrate of 

 ammonia in which they are as 1 to 8, this method answers very 

 well, but when the proportion of nitrogen is smaller than the last, 

 or when the nitrogen appears in a variable proportion at different 

 stages of the analysis, then it is necessary to collect and measure 

 the whole nitrogen evolved, which is not easily done with accuracy. 

 To get rid of the nitrogen of the air contained in the tube, a 



