(>XV(iKN AND ITS SALINE CCLMI51NATX >NS 181 



combined with another element, and by a method of double decom- 

 position this element is replaced by oxygen, or a substance is taken 

 which easily evolves oxygen, and is brought into contact with the given 

 substance. The oxygen then acts at the moment of its evolution. If 

 the conditions are such that the substance to be oxidised is liberated 

 at the same moment, then oxidation proceeds with greater ease. 

 (The explanation of this phenomenon was given in the last chapter.) 

 It must be remarked that substances which do not directly combine 

 with oxygen, but form compounds with it by an indirect method, often 

 readily lose the oxygen which was absorbed by them by double decomposi- 

 tion or at the moment of its evolution. Such, for example, are the com- 

 pounds of oxygen with chlorine, nitrogen, and platinum, which evolve 

 oxygen on heating. They, like other substances which easily evolve 

 oxygen on heating, may serve as a means for obtaining oxygen, or for 

 oxidation. They, in the presence of substances which are capable of 

 combining with oxygen, are decomposed, give up their oxygen to them, 

 and may thus be themselves employed for indirect oxidation. In this 

 respect oxidising agents, or those compounds of oxygen which are em- 

 ployed in chemical and technical practice for transf erring oxygen to 

 other substances, are especially remarkable. The most important 

 among these is nitric acid or aquafortis a substance rich in oxygen, 

 and capable of evolving it when heated, and which easily oxidises a great 

 number of substances. Thus nearly all metals and organic substances 

 containing carbon and hydrogen are more or less oxidised when heated 

 with nitric acid. If strong nitric acid be taken, and a piece of burning 

 charcoal be immersed in the acid, it continues to burn, the combustion 

 proceeding in this case at the expense of the oxygen contained in 

 the liquid nitric acid. Chromic acid acts like nitric acid ; alcohol 

 burns when mixed with it. Although the action is not so marked, 

 even water may oxidise with its oxygen. Sodium is not oxidised in 

 perfectly dry oxygen at the ordinary temperature, but it burns very 

 easily in water and aqueous vapour. Charcoal can burn in carbonic 

 anhydride a product of combustion forming carbonic oxide. Mag- 

 nesium burns in the same gas, separating carbon from it. Generally, 

 combined oxygen can pass from one compound to another. 



The products of combustion or oxidation and in general the definite 

 compounds of oxygen are termed oxides. Some oxides are not capable 

 of combining with other oxides or combine with only a few, and then 

 form unstable compounds with the evolution of very little heat ; 

 others, on the contrary, enter into combination with very many other 

 oxides, and in general have remarkable chemical energy. The oxides 

 incapable of combining with others, or only showing this quality in a 



