METALLIC OXIDES.

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.relative proportions, and form different oxides. Thus,for example, when lead is melted over a fire and keptfor a long time exposed to the air, it gradually combineswith oxygen, and becomes converted into a yellow-coloured substance, called litharge ; this is an oxide oflead, and if it be kept heated, it absorbs an additionalquantity of oxygen, and passes into a brilliant red sub-stance, called minium, or red lead ; this is a secondoxide, and contains more oxygen than litharge does.Besides these two oxides, there is yet a third, whichcannot be formed by merely heating red lead and ex-posing it to the air, but which is easily made by boilingred lead in nitric acid ; it then acquires a third dose ofoxygen, and becomes dark brown.

217. These oxides are not all bases ; they are not allable to combine with acids and form salts, but usuallyonly one, or sometimes two, of the oxides of a metal arebases: thus, only one of the three oxides of lead, thatcontaining least oxygen, is a base, and when combinedwith carbonic acid, forms white lead, a substance ofvery great importance, as being better suited than anyother we are acquainted with for the manufacture ofpaint. The other two oxides of lead are unable tocombine with acids, and therefore, when acted on bystrong acids, they part with some of the oxygen theycontain, and are then able to combine with the acid,being converted into the protoxide, which is a base.

218. Modern discoveries have shown that both theearths and alkalies are, in fact, the oxides of peculiar