76 NON-METALS AND THEIR COMBINATIONS. 



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. 



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 

 called oxufysj/M^gents, as, for instance, nitric acid, potassium chlorate, 



pota^siujn_permanganate, etc. 



In all combustions we have at least two substances acting chemically upon 

 one another, which substances are generally spoken of as combustible bodies 

 and supporters of combustion. Illuminating gas is a combustible substance, 

 and oxygen a supporter of combustion ; but these terms are only relatively 

 correct, since oxygen may be caused to burn in illuminating gas, whereby it is 

 made to assume the position of a combustible substance, whilst illuminating 

 gas is the supporter of combustion. 



While some substances, such as iron and phosphorus, undergo slow combus- 

 tion at the ordinary temperature, there is a certain degree of temperature, 

 characteristic of each substance, at which it inflames. This point is known as 

 kindling temperature, and varies widely in different substances. Zinc ethyl 

 ignites at the ordinary temperature, phosphorus at 50 C. (122 F.), sulphur at 

 about 450 C. (842 F.), carbon at a red heat, and iron at a white heat. The 

 heat produced by the combustion is generally higher than the kindling tern- 



