OXYGEN. 141 



For practical, including medical, purposes oxygen is sold stored in 

 strong steel cylinders, the gas being condensed by a pressure, gen- 

 erally, of 225 pounds to about -fa of its volume. 



The temperature above which a gas cannot be liquified by pressure is 

 known as its critical temperature. The failure of former attempts to liquefy 

 oxygen and a few other gases was due to the fact that, though an enormous 

 pressure was used, the gas was not brought to the critical temperature. 



Oxygen is but sparingly soluble in water (about 3 volumes in 100 

 at common temperature). A liter of oxygen under 760 mm. pressure, 

 and at the temperature of C. (32 F.), weighs 1.429 grammes. 



Chemical properties. The principal feature of oxygen is its great 

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

 with nearly all of which it combines in a direct manner. The more 

 important elements with which oxygen does not combine directly are : 

 Cl, Br, I, F, Au, Ag, and Pt; but even with these it combines in- 

 directly, excepting F. 



The act of combination between other substances and oxygen is 

 called oxidation, and the products formed, oxides. The large number 

 of oxides are divided usually into three groups, and distinguished as 

 basic oxides (sodium oxide, Na 2 O, calcium oxide, CaO), neutral oxides 

 (water, H 2 O, manganese dioxide, MnO 2 , lead dioxide, PbO 2 ), and 

 acid-forming or acidic oxides, also called anhydrides (carbon dioxide, 

 CO 2 , sulphur trioxide, SO 3 ). Whenever the heat generated by oxida- 

 tion (or by any other chemical action) is sufficient to cause the emis- 

 sion 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 dis- 

 tinguish oxygen from most other gases. Processes of oxidation evolv- 

 ing no light are called slow combustion. An instance of slow combus- 

 tion is the combustion of the different organic substances in the living 

 animal, the oxygen being supplied by respiration. 



In some cases the heat generated by the slow combustion of a sub- 

 stance may raise its temperature sufficiently high to cause ignition, 

 which is then called spontaneous combustion. Thus, greasy rags or wet 

 hay, when piled in heaps, may ignite spontaneously, because some oils 

 and damp hay undergo slow oxidation, which raises the temperature. 



For a process of oxidation it is not absolutely necessary that free 

 oxygen be present. 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 



