Of the sixty-five or seventy chemical elements, only a very small number occur at all commonly in lavas. Eight elements, indeed, make up the great mass of all lavas--these are oxygen, silicon, aluminium, magnesium, calcium, iron, sodium, and potassium. But even these eight elements are present in very unequal proportions. Oxygen makes up nearly one-half the weight of all lavas. Almost all the other elements found in lavas exist in combination with oxygen, so that lavas consist entirely of what chemists call 'oxides.' This is a most remarkable circumstance, which, as we shall presently see, is of great significance. The metalloid silicon makes up about one-fourth of the weight of most lavas, and the metal aluminium about one-tenth. The other five elements vary greatly in their relative proportions in different lavas. In all lavas the substance which forms the greatest part of the mass is the compound of oxygen and silicon, known as silica or silicic acid. In its pure form, this substance is familiar to us as quartz, or rock-crystal and flint. Silica is present in all lavas in proportions which vary from one-half to four-fifths of the whole mass. Now, this substance, silica, has the property of forming more complex compounds by uniting with the other oxides present in lavas--namely, the oxides of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica is called by chemists an _acid_, the other oxides in lavas are termed _bases_, and the compounds of silica with the bases are known as _silicates_. Hence we see that lavas are composed of a number of different silicates--the silicates of aluminium, magnesium, calcium, iron, potassium, and sodium. The above statements will perhaps be made clearer by the accompanying table from which it will be seen that lavas are compounds in varying proportions of six kinds of salts--namely, the silicates of alumina, magnesia, lime, iron, potash, and soda. Composition of Lavas. Elements Binary Compounds Salts Acid Bases { Silicon Silica--+ { | O { Aluminum +--Alumina Silicate of Alumina x { | y { Magnesium +--Magnesia " " Magnesia g { | e { Calcium +--Lime " " Lime n { | { Iron +--Iron " " Iron { | { Potassium +--Potash " " Potash { | { Sodium +--Soda " " Soda