EARTHS

( Terres , Fr.; Erden , Germ.) Modern science has demonstrated that the substances called primitive earths, and which prior to the great electro-chemical career of Sir H. Davy, were deemed to be elementary matter, are all compounds of certain metallic bases and oxygen, with the exception of silica , whose base, silicon , being analogous to boron, has led that compound to be regarded as an acid; a title characteristic of the part it extensively performs in neutralizing alkaline bodies, in mineral nature, and in the proce es of art. Four of the earths, when pure, po e decided alkaline properties, being more or le soluble in water, having (at least 3 of them) an acrid alkaline taste, changing the purple infusion of red cabbage to green, most readily saturating the acids, and affording thereby neutro-saline crystals. These four are baryta , strontia , lime ( calcia ), and magnesia . The earths proper are five in number; alumina , glucina , yttria , zirconia , and thorina . These do not change the colour of infusion of cabbage or tincture of litmus, do not readily neutralize acidity, and are quite insoluble in water. The alkalies are soluble in water, even when carbonated; a property which distinguishes them from the alkaline earths. Lithia must for this reason be considered to be an alkali. See the above substances in their alphabetical places.