ALKALINE EARTHS. By the early chemists, the term earth was used to denote those non-metallic substances which were insoluble in water and were unaffected by strong heating; and as some of these substances (e.g. lime) were found to be very similar in properties to those of the alkalis, they were called alkaline earths. The alkaline earths, lime, strontia and baryta, were assumed to be elements until 1807, when Sir H. Davy showed that they were oxides respectively of the metals, calcium, strontium and barium. The metals are never found in the uncom bined condition, but occur most often in the form of carbonates and sulphates; they form oxides of the type RO, and peroxides of the type The oxides of type RO are soluble in water, with generation of heat (e.g., the slaking of lime) ; the resulting solution possesses a strongly alkaline reaction, due to the presence of the hydroxide R(OH which rapidly absorbs carbon dioxide on exposure. These oxides and hydroxides are basic in character and dissolve readily in acids with the formation of the correspond ing salts. As the atomic weight of the elements increases, it is found that the solubility of the hydroxides in water increases although that of the sulphates decreases.
The metals calcium, strontium and barium oxidize rapidly on exposure. Their salts usually crystallize well, the chlorides and nitrates dissolve readily in water, whilst the carbonates, phos phates and sulphates are either very sparingly soluble or insoluble in water.