282 CONSIDERATION OF CARBON COMPOUNDS. 



filled. Upon heating the combustion-tube in a suitable furnace, the 

 organic matter is burned by the oxygen of the cupric oxide, the. 

 hydrogen is converted into water (steam), which is absorbed by the 

 calcium chloride, and the carbon is converted into carbon dioxide, 

 which is absorbed by the potassium hydroxide. The apparatus repre- 

 sented in Fig. 38 shows the gas-furnace in which rests the combustion- 

 tube with calcium chloride tube and potash bulb attached. Upon 

 re-weighing the two absorbing vessels at the end of the operation, the 

 increase in weight will indicate the quantity of water and carbon 

 dioxide formed during the combustion, and from these figures the 

 amount of carbon and hydrogen present in the organic matter may 

 easily be calculated. 



For instance : 0.81 gramme of a substance having been analyzed, 

 furnishes, of carbon dioxide 1.32 gramme, and of water 0.45 gramme. 

 As every 44 parts by weight of carbon dioxide contain 12 parts by 

 weight of carbon, the above 1.32 gramme contains of carbon 0.36 

 gramme, or 44.444 per cent. As every 18 parts of water contain 2 

 parts of hydrogen, the above 0.45 gramme consequently contains 0.05 

 gramme, or 6.172 per cent. 



Oxygen is scarcely ever determined directly, but generally indi- 

 rectly, by determining the quantity of all other elements and deduct- 

 ing their weight, calculated to percentages from 100. The difference 

 is oxygen. 



If, in the above instance, 44.444 per cent, of carbon and 6.172 per 

 cent, of hydrogen were found to be present, and all other elements, 

 except oxygen, to be absent, the quantity of oxygen is, then, equal 

 to 49.384 per cent, and the composition of the substance is as follows : 



Carbon . . . . . . . 44.444 per cent. 



Hydrogen 6.172 " 



Oxygen 49.384 " 



100 000 



Determination of nitrogen. Nitrogen is generally determined 

 by heating the substance with soda-lime and passing the generated 

 ammonia gas through hydrochloric acid contained in a suitable glass 

 vessel. Upon evaporation of the acid solution in a weighed platinum 

 dish over a water-bath, ammonium chloride is left, from the weight 

 of which compound the quantity of nitrogen may be calculated. Or 

 the ammonia gas may be passed through a measured volume of 

 normal hydrochloric acid and the unsaturated portion of the acid 

 determined volumetrically. 



