ANHYDRIDE

Most, if not all modern chemists, adopting Gerhardt’s practice of limiting the title of acid to a particular cla of substances which contain hydrogen, now regard all true acids as salts of hydrogen. Formerly many bodies, such as silica or white arsenic, were looked upon as acids, though if we adopt the foregoing definition they are not really so until they have combined with water. Such bodies, because they contain no hydrogen, are now distinguished as anhydrides; the substances, for example, familiarly known as carbonic, sulphurous, and phosphoric acids, must, upon the above principle, be designated carbonic, sulphurous, and phosphoric anhydrides. We may also define an anhydride to be an oxide which forms an acid on treatment with water.