Anhydrous

Anhydrous is the term applied to a chemical substance free from water. Thus, ordinary lime- shell as it comes from the kiln is simply lime, $\backslash text\{CaO\}$, without any water, and is called anhydrous lime; but when water is thrown upon the lime-shell, the liquid disappears by combination with the lime, which very much increases in volume and becomes hydrated lime, $\backslash text\{CaOH\}\_2\backslash text\{O\}$. Again, ordinary stucco, before being used by the modeller, contains only lime and sulphuric acid, $\backslash text\{CaSO\}\_4$, with no water, and is therefore anhydrous; but when water is added, and the stucco sets into its mould, it combines with two equivalents of water, and becomes hydrated stucco, $\backslash text\{CaSO\}\_4\; \backslash cdot\; 2\backslash text\{H\}\_2\backslash text\{O\}$. Examples of anhydrous substances are also found amongst liquids; thus, alcohol free from water is called anhydrous alcohol; and in like manner we speak of anhydrous acetic acid, anhydrous nitric acid, &c.