METALS. 97 



and copper are oxidized slowly by moist air. Iron, 

 zinc, copper, tin, &c., require to be heated to red- 

 ness. Although the perfect metals cannot be 

 oxidized by any degree of ordinary heat, they may 

 by the effect of electricity and galvanism. All the 

 metals, that are converted into oxides by atmo- 

 spheric air, undergo this change still more rapidly 

 in oxygen gas, as was shown in the burning of iron 

 wire in oxygen. 



Metals are also converted into oxides by the ac- 

 tion of acids, but in different manners. Some acids 

 which contain oxygen loosely combined part with 

 it to the metal ; while others, as the sulphuric and 

 muriatic acids, do not act upon iron or zinc, except 

 they are diluted with water, and then it is the wa- 

 ter, and not the acid, which supplies the oxygen. 



Metals cannot be made to combine with all pro- 

 portions of oxygen, but are susceptible only of cer- 

 tain degrees or stages of oxidation. Thus iron has 

 only two oxides ; the black oxide composed of 29.5 

 parts of oxygen, and 100 parts metal ; and the 

 red oxide of 43.5 parts of oxygen, and 100 parts of 

 metal : and there are no intermediate degrees of 

 oxidation, nor will iron combine with a larger or 

 smaller proportion of oxygen. Metals differ in the 

 number of oxides which they form : thus some 

 have two, some three, and others four oxides: 

 and, according to the law of the atomic theory, 

 the different oxides of the same metal contain oxy- 

 gen in proportions that are simple multiples of each 

 other. 



The different oxides of the same metals have dif- 

 ferent colours, which render them very valuable as 

 pigments. They have also distinct chemical pro- 

 perties, and combine, in different proportions, with 

 the acids, forming distinct salts. 



VOL. ir. H 



