BLACK. 105 became mild calcareous earth by re-uniting with this air. a This theory was demonstrated by plain and incotro- vertible experiments; and it is not to be wondered at, that it should immediately supplant the then received hy po- thesis, that the causticity of lime depended on the union de enail belon of igneous particles. Lime being discovered to take this air from alkalies, and thereby render them caustic, the same beautiful theory of causticity was extended to these substances, and thus the true reason of alkalies being rendered caustic by lime was given. Lime was also observed to attract this air from magnesia. This air was shewn to be different from the common atmospheric air; and he concluded that it was either a peculiar species of elastic fluid dispersed through the atmosphere, or an exceedingly subtle powder. This newly discovered substance he named fixed air; improperly, in- deed, as he was himself aware, but the name was naturally enough applied to a substance which he looked upon as having been fixed in the substance of the bodies with (! ever 2011 which it was combined. Such is a brief sketch of the luminous experiments of Dr. Black, by which were demonstrated the peculiar nature of magnesia; the existence of a new species of air, in mild alkalies, magnesia, and calcareous earth; the cause of the effervescence of these substances with acids; the cause of the loss of weight in these substances by acids or fire; that the causticity of alkalies and lime depended on the separation of this new air; and the relative affinities of this air to alkalies and earths. Important as these facts were, considered merely as belonging to the substance discovered and investigated by Dr. Black, they were infinitely more so, on account of the new field they opened to the view of chemical philosophers, of substances of different species, in a gaseous form, of which they had no idea before; the opinion of Hales and others being, that aeriform matter was of the same species as
