according to M. Berthollet, is a chemical combination of azot of hydrogen and carbon. It appears Annals, vol. much less akin to acids than to ammonia; it has, however, too many properties in common with other acids not to place it in the same class, the rather because our classifications are always in a degree arbitrary, and ought to be considered rather as useful methods, than as divisions formed by nature. When the Prussian acid is combined with alkali and oxyd of iron, it cannot be separated by any other acid, unless heat be employed, or it be exposed to light; and nevertheless, when it is disengaged by one of these means, it cannot separate iron, even from the weakest acid, unless it be by a double affinity. It appears that this property is connected with the elastic state, which is unfavourable to these combinations: it must have lost this state, in other words its specific heat must be diminished, in order that it may possess its affinities with metallic oxyds and alkalis. Nitrous gas, oxygenated muriatic acid, and sulphureous acid, present analogous phenomena.