68 NON-METALS AND THEIR COMBINATIONS. 



nent or stable gas, as all attempts to liquefy or solidify it had 

 failed. Lately, however, these efforts have been successful, and 

 oxygen has been converted (though in very small quantities) 

 into a liquid by the application of intense cold and an enormous 

 pressure. 



Oxygen is but sparingly soluble in water (about 3 volumes in 

 100 at common temperature). A litre of oxygen at the standard 

 temperature and pressure weighs 1.43 gram. 



Chemical properties. The principal feature of oxygen is its 

 great affinity for almost all other elements, both metals and non- 

 metals; with nearly all these elements it combines in a direct 

 manner. The more important elements with which oxygen 

 does not combine directly are : Cl, Br, I, Fl, Au, Ag, and Pt, 

 but even with these it combines indirectly, excepting Fl. 



The act of combination between other substances and oxygen 

 is called oxidation, and the products formed, oxides. Whenever 

 the heat generated by oxidation (or by any other chemical 

 action) is sufficiently high to cause the emission of light, the 

 process is called combustion. Oxygen is the chief supporter of 

 all the ordinary phenomena of combustion. Substances which 

 burn in atmospheric air burn with greater facility in pure 

 oxygen. This property is taken advantage of to recognize and 

 distinguish oxygen from most other gases. Processes of oxida- 

 tion evolving no light are called slow combustion. An instance 

 of slow combustion is the combustion of the different organic 

 substances in the living animal; the oxygen being supplied 

 during the process of respiration. 



It is not absolutely necessary for a process of oxidation that 

 free oxygen be present, as many substances contain oxygen in 

 such a form of combination that they part with it easily when 

 brought in contact with substances having a greater affinity for 

 it. Such substances are called oxidizing agents, as, for instance, 

 nitric' acid, potassium chlorate, potassium permanganate, etc. 



Ozone is an allotropic modification of oxygen, which is formed 

 when non-luminous electric discharges pass through atmospheric 

 air or through oxygen ; when phosphorus, partially covered 

 with water, is exposed to air, and also during a number of 

 chemical decompositions. Ozone differs from ordinary oxygen 



