INTRODUCTION. 23 



excess of perfectly dry copper oxide or lead chromate, 

 to a red heat, finally employing a current of pure oxy- 

 gen. The water formed during the process is taken 

 up by a tube filled with calcium chloride, the carbonic 

 acid by a small apparatus which is filled with a solu- 

 tion of potassium hydroxide, and, to secure absolute 

 safety, is joined to a small tube containing pieces of 

 solid potassium hydroxide. The gain in weight of 

 these three pieces shows the quantity of water and car- 

 bonic acid. 



The conversion of nitrogen into ammonia is accom- 

 plished by heating the body strongly with a large 

 excess of a dry mixture of sodium hydroxide and cal- 

 cium hydroxide. The ammonia formed is either taken 

 up by hydrochloric acid and weighed as ammonium 

 chloroplatinate, or by dilute sulphuric acid of a known 

 strength, and the quantity of acid which remains free 

 afterwards estimated by means of a standard test-solu- 

 tion of sodium hydroxide. This subtracted from the 

 quantity of acid employed shows how much of the 

 acid has been neutralized by ammonia, from which 

 the quantity formed and the nitrogen contained there- 

 in may be easily calculated. 



Nitrogen is not, however, given off in all cases by 

 heating with soda-lime. This applies especially to such 

 cases in which the nitrogen is in close combination 

 with the oxygen, as for instance in nitro-compounds. 

 In analyzing such substances, the nitrogen is set free 

 by heating the substance with an excess of finely pow- 

 dered copper oxide, and passing the escaping gases 

 over metallic copper for the purpose of destroying the 

 oxides of nitrogen. This operation is carried out in a 

 long tube, from which the air has been previously com- 

 pletely removed by means of carbonic acid. The mix- 

 ture of carbonic acid and nitrogen is collected in a 

 graduated tube over mercury, the carbonic acid ab- 

 sorbed by caustic potassa, the volume of nitrogen 

 which has remained unabsorbed, measured and its 

 weight calculated according to the formula 



- w-- 001256 ' 



