Anhydrides, compounds which become acids upon the addition of water. In technical language, they are the oxides of acid radicals, and stand in the same relation to acids as the oxide of potassium, K2O, does to the hydrated potash, HKO. The most familiar anhydrides are sulphuric, nitric, hypochlorous, and acetic; these have long been called anhydrous sulphuacid, anhydrous nitric acid, anhydrous hy-pochlorous acid, and anhydrous acetic acid; but as they do not possess acid properties until combined with water, it is now proposed to call them sulphuric anhydride, nitric anhydride, etc. The following formulas will illustrate how an anhydride becomes an acid on the addition of water: SO3 (sulphuric anhydride) +H2O = H2 SO4 (sulphuric acid); N265 (nitric anhydride) + H20 = 2HNO3 (nitric acid); C12O (hypochlorous anhydride)+ H2O=2HC10 (hy-pochlorous acid). Anhydride has therefore a signification of its own, and must not be confounded with the term anhydrous, applied to substances which have no water either mixed or combined with them.