AMIDES (Acid-Amides), chemical compounds which may be considered as derived from ammonia by replacement of its hydro gen with acyl residues, which are the characteristic groups of organic acids, derivable from these by removal of the acidic hy droxyl group or groups; the substances produced are known as primary, secondary or tertiary amides, according to the num ber of hydrogen atoms replaced. Of these compounds, the primary amides of the type R•CO•NH2 are the most important, the group •CO•NH2 being called the amido group. They may be pre pared by the dry distillation of the ammonium salts of the acids (A. W. Hofmann), by the partial hydrolysis of the nitriles, by the action of ammonia or ammonium carbonate on acid chlo rides or anhydrides, or by heating the esters (q.v.) with ammonia. They are solid crystalline compounds (formamide excepted) ; the lower members of the series are soluble in water, the solubility, however, decreasing as the carbon content of the molecules in creases. They are easily hydrolysed, breaking up into organic acids and ammonia or when boiled with acids or alkalies. They form compounds with hydrogen chloride when this gas is passed into their ethereal solution ; these compounds, however, are very unstable, being readily decomposed by water. On the other hand, they show faintly acid properties, since the hydrogen of the amido group can be replaced by metals to give such compounds as mercury acetamide Nitrous acid de composes them, with elimination of nitrogen and the forma tion of the corresponding acid : When distilled with phosphoric anhydride they yield nitriles. By the action of bromine and alcoholic potash on the amides, they are converted into amines containing one carbon atom less than the original amide, a reaction which possesses great theoretical im portance (A. W. Hofmann), and is also of industrial interest in the Badische synthesis of indigo (q.v.), for in accordance with Hofmann's reaction phthalimide is converted by alkali hypo chlorite into anthranilic acid: R•CONHBr Formamide, is a liquid readily soluble in water, boiling at about 195°C with partial decomposition. Acetamide, a white deliquescent crystalline solid, which melts at 82-83°C and boils at is usually prepared by distilling ammonium acetate. Benzamide, crystallizing in leaflets which melt at 13o° C, is prepared by the action of am monium carbonate on benzoyl chloride. It yields a silver salt which with ethyl iodide forms benzimido-ethyl ether, a behaviour which points to the silver salt as being derived from the tautomeric imidobenzoic acid: NH) .OH