Properties of Pure Substances : Nitrogen. 7 



quite, free from objection. I refer to the copper-ammonia 

 and chromous-chloride method of Berthelot and Recoura 

 (' Watts' Dictionary/ new edition, and Ann. de Phys. et de 

 Chim. s.'6, vol. x. p. 5). When air is exposed to a large sur- 

 face of copper wet with strong ammonia, the oxygen is for the 

 most part absorbed. In my experience with the arrange- 

 ments about to be described, there is only about three per 

 cent, of oxygen left after the reagents have acted for half an 

 hour, and only one per cent, after three hours. Since in 

 practice with my apparatus the air often stands over the 

 copper for a day or even a week, the absorption is probably 

 nearly complete. The last traces of oxygen are absorbed by 

 a strong solution of chromous chloride, which perhaps exer- 

 cises a stronger absorptive power on oxygen than any other 

 liquid, and is for this purpose quite easily prepared. As the 

 method is not so well known as it deserves to be, I give the 

 following description from Recoura's paper : — 



A large flask is taken, and in it are placed 250 to 300 

 grammes of granulated redistilled zinc with 50 grammes 

 powdered crystals of potassium bichromate. The bichromate 

 must be finely powdered ; it should be pounded till it looks 

 bright yellow. Three hundred cubic centimetres of pure 

 hydrochloric acid are mixed in a beaker with two hundred 

 cubic centim. of water, and I have found it advantageous to 

 heat the mixture almost to boiling. The contents of the 

 beaker are then poured as quickly as possible into the flask, 

 and a violent evolution of hydrogen and steam at once com- 

 mences. If the flask is too small, some of the liquid will of 

 course be projected from it. Under the powerfully reducing 

 influence of the zinc and hydrochloric acid, the chromium is 

 rapidly reduced to the state of green oxide, which then 

 redissolves to a beautiful blue solution. The whole reaction 

 only takes about ten minutes or a quarter of an hour. If 

 the bichromate is not sufficiently finely powdered, a further 

 quantity of hydrochloric acid may have to be added to com- 

 plete the reaction, or the mixture may be left (excluding air) 

 for a few hours. The blue solution contains chromous chlo- 

 ride, chlorides of zinc and potassium, and possibly various 

 impurities. The slightest trace of oxygen destroys the perfect 

 colour of the chromous chloride by converting it into the 

 green or grey substances investigated by Recoura. On 

 standing over zinc for some time, however, if not too much 

 oxidized and slightly acid, the chromium compounds become 

 again reduced and the fine blue colour reappears. The colour 

 is as clear and bright and almost exactly of the same shade 

 as the colour of an ammoniacal solution of cupric hydrate. 

 Recoura states that a stream of gas containing free oxygen 



