<>XY<;KN AND ITS SALINE COMBINATIONS 157 



( >.\ygen is evolved with particular ease by a whole series of un- 

 stable oxygen compounds, of which we will proceed to take a general 

 survey, remarking that many of these reactions, although not all, belong 

 to the number of reversible reactions ; 8 so that in order to ob- 

 tain many of these substances (for instance, potassium chlorate) rich 

 in oxygen, recourse must be had to indirect methods (see Intro- 

 duction), with which we shall become acquainted in the course of this 

 book. 



1. The- compounds of oxygen with certain metals, and especially 

 with the so-called noble metals that is, mercury, silver, gold, and 

 platinum having been once obtained, retain their oxygen at the ordi- 

 nary temperature, but part with it at a red heat. The compounds are 

 solids, generally amorphous and infusible, and are easily decomposed by 

 heat into the metal and oxygen. We have seen an example of this in 

 speaking of the decomposition of mercury oxide. Priestley, in 1774, 

 obtained pure oxygen for the first time by heating mercury oxide by 

 means of a burning-glass, and clearly showed its difference from air. 

 He showed its characteristic property of supporting combustion ' with 

 remarkable vigour,' and named it dephlogisticated air. 



into the other end, and the heat of the stove is increased to a bright-red heat (800). At 

 this temperature the barium peroxide gives up all that oxygen which it acquired at a dark- 

 red heat i.e., about one part by weight of oxygen is evolved from twelve parts of barium 

 peroxide. After the evolution of the oxygen there remains the barium oxide which was 

 originally taken, so that air may be again passed over it, and thus the preparation of oxygen 

 from one and the same quantity of barium oxide may be repeated many times. Oxygen 

 has been procured one hundred times from one mass of oxide by this method ; all the neces- 

 sary precautions being taken, as regards the temperature of the mass and the removal of 

 moisture and carbonic acid from the air. Unless these precautions be taken, the mass 

 of oxide soon spoils. 



As oxygen may become of considerable technical use, from its capacity for giving 

 high temperatures and intense light in the combustion of substances, its preparation 

 directly from air by practical methods forms a problem whose solution many investi- 

 gators continue to work at up to the present day. The most practical method is that of 

 Tessie du Motoy. It is based on the fact that a mass of equal weights of manganese 

 peroxide and caustic soda at an incipient red heat (about 850) absorbs oxygen from air, 

 with the separation of water, according to the equation MnO.j + 2NaHO + O = Na.MnO4 

 + H._,O. If superheated steam, at a temperature of about 450, be then passed through 

 the mixture, the manganese peroxide and caustic soda orginally taken are regenerated, and 

 the oxygen held by them is evolved, according to the reverse equation Na.^MnO 4 

 + H.jO = MnOo + '2NaHO + O. This mode of preparing oxygen may be repeated for an 

 infinite number of times. The oxygen in combining separates out water, and steam, 

 acting on the resultant substance, evolves oxygen. Hence all that is required for the 

 preparation of oxygen by this method is fuel and the alternate cutting off the supply of 

 air and steam. 



8 Even the decomposition of manganese peroxide is reversible, and it may be re- 

 ol.tained from that suboxide (or its salts), which is formed in the evolution of oxj'gen 

 (Chap. XI. note 6). The compounds of chromic acid containing the trioxide CrO 5 in 

 evolving oxygen give chromium oxide, Cr.,O 3 , but they re-form the salt of chromic acid 

 when heated at a red heat in air with an alkali. 



