58 CHEMISTRY. 



can take its place in that respect. Cut off the supply of 

 oxygen to our lungs for only a minute or two, and life is 

 extinct. When death occurs by drowning, it is because 

 oxygen is shut out from the lungs. 



61. Oxides. Among the most common chemical com- 

 binations are the oxides of metals. Most metals have such 

 an affinity for oxygen that they readily unite with it and 

 form oxides, some much more readily than others. Ex- 

 posed to the air, they tarnish, that is, unite with oxygen. 

 Gold, silver, mercury, and platinum do not oxidize in this 

 way, and therefore are called the noble metals. Gold and 

 platinum are so reluctant, as we may express it, to be 

 united with oxygen, that when the chemist by certain 

 processes forces them to a union with it, they very easily 

 part with the oxygen and return to their metallic state. 

 Such oxides are said to be unstable compounds. On the 

 other hand, there are some metals which have so strong 

 an affinity for oxygen that they are never found native, 

 and can only be obtained by separating them from the ox- 

 ygen with which they are combined. Such are the metals 

 of which lime, potash, and soda are oxides. 



62. Different Degrees of Oxidation. While some of the 

 metals have but a single oxide, most of them have two or 

 more, made by having different amounts of oxygen united 

 with the metal. Thus while there is but one oxide of 

 zinc, lead has three, mercury two, copper three, etc. If 

 there be two or more oxides of a metal, they are named 

 thus: Monoxide, dioxide, trioxide, etc., the prefixes being 

 derived from Greek words meaning one, two, three, etc. 

 Thus the dioxide has twice as much oxygen as the mon- 

 oxide, the trioxide three times as much, and so on, the rel- 

 ative amount of the metal being the same in all cases. 



In some cases another system of nomenclature is em- 

 ployed, as mentioned in 18. Thus we have nitrous oxide, 



