706 PRELIMINARY OBSERVATIONS. 



combustion tube is chosen 24 inches long, 6 inches of which at 

 the closed end are filled with carbonate of copper, then follow 

 2 inches of pure oxide of copper, next the mixture of the subs- 

 tance with oxide of copper, then another layer of pure oxide, 

 and lastly a layer of copper turnings. The air is exhausted 

 from the tube by a syringe, and the tube filled with carbonic 

 acid by heating one half of the carbonate of copper ; the ex- 

 haustion and evolution of carbonic acid are several times re- 

 peated, till the whole air is certainly withdrawn from the tube, 

 and the latter is filled with carbonic acid. The combustion of the 

 mixture is then conducted as in the previous case, the gases 

 however are received in a large graduated jar, over mercury, half 

 full of a strong solution of caustic potash. After the com- 

 bustion is completed, heat is again applied to the end of the 

 tube containing the remaining half of the carbonate of copper, 

 and carbonic acid evolved which sweeps out the last portions 

 of nitrogen into the receiver, where the volume of that gas is 

 observed. 



MM. Will and Varrentrapp have lately proposed the follow- 

 ing excellent method of determining the nitrogen in organic 

 substances, which is likely to supersede every other. The sub- 

 stance is mingled with a mixture of caustic lime and hydrate of 

 soda, and heated to redness in a combustion tube. All the 

 nitrogen of the substance escapes as pure ammonia, which may 

 be condensed in a small apparatus containing dilute hydrochloric 

 acid. This liquid is afterwards mixed with chloride of platinum, 

 and brought to dryness in a water-bath; the double chloride 

 of platinum and ammonium remaining is washed with a mixture 

 of alcohol and ether, in which it is perfectly insoluble. The 

 quantity of nitrogen is calculated from the weight of the chloride 

 of platinum and ammonium, or from the metallic platinum 

 which it leaves behind when heated to redness. 



Oxide of copper is not applicable for the combustion of subs- 

 tances containing chlorine, owing to the volatility of the 

 chloride of copper, a portion of which passes into the chloride 

 of calcium tube, and vitiates the determination of the hydrogen. 

 Chromate of lead is then employed in the combustion tube, 

 with the same manipulations as with oxide of copper. This 

 salt must first be strongly ignited till it begins to melt and 

 then be reduced to a very fine powder ; the chloride of lead is 



