OXYGEN AND NITROGEN 249 



means of a blast of air, and then blowing in hydrocarbon oils as 

 long as the temperature is high enough. The blast of air is then 

 again applied. The gas requires to be scrubbed with oil of 

 vitriol and caustic soda. 



CHAPTER XV 



OXYGEN AND NITROGEN 



EVEEY schoolboy is acquainted with the process, common a few 

 years ago and still used on a small scale for producing oxygen 

 gas, by heating potassium chlorate, either alone or mixed with 

 a small quantity of manganese dioxide. Priestley's original 

 method of heating mercuric oxide (red precipitate) is described 

 in most chemical textbooks, as well as the decomposition by 

 heat of a considerable number of peroxides and other metallic 

 oxides, and highly oxidised substances such as potassium per- 

 manganate, bleaching powder, sulphuric acid. But for indus- 

 trial purposes and manufacture on a fairly large scale, a process 

 was introduced, by patent in 1880, by MM. Brin freres which 

 soon took the place of all the others and became established as a 

 successful commercial undertaking. This was based on the fact 

 that at a low red heat barium oxide in a stream of air, deprived 

 of carbon dioxide, is converted into barium dioxide, BaO+0 = 

 Ba0 2 . The latter compound heated to a higher temperature 

 gives off again the absorbed oxygen, while barium monoxide, 

 baryta, BaO, -is reproduced. 



The inconvenience of alternately raising and lowering the 

 temperature of the retorts in which the baryta was heated, and 

 the wear and tear involved in these operations, led to the sub* 

 stitution of change of pressure for change of temperature with 

 great advantage. 



An apparatus was devised in which, by means of automatic 

 reversing gear, the air could be alternately introduced into the 

 retorts under slight pressure, and after absorption of the oxygen 

 and escape of the nitrogen, the oxygen could be pumped off by 

 reducing the pressure below atmospheric, but without altering 

 the temperature of the retorts and their contents. Oxygen 

 made by this process had a purity of 93 to 96 per cent. Some of 

 the plants erected a few years ago may be still working, but 



