After a description of some unsuccessful attempts to render mild alkalies caustic by heat alone (_i.e._ to expel carbon dioxide from potassium carbonate), Black examines the action of the “sedative salt” or boracic acid on mild alkalies, by rubbing them together in presence of some water. At first there is no effervescence, but on adding successive quantities of boracic acid, brisk effervescence finally takes place, borax being formed. “This phenomenon may be explained by considering the fixed alkalis as not perfectly saturated with air ... if they expel a small quantity of air from some of the salt, this air is at the same time absorbed by such of the contiguous particles as are destitute of it.” And on “exposing a small quantity of a pure vegetable fixed alkali (carbonate of soda) to the air, in a broad and shallow vessel, for the space of two months,” crystals were obtained, which possessed a milder taste than that of ordinary salt of tartar, which effervesced with acids more violently than usual, and which could not be mixed with the smallest portion of boracic acid without emitting a sensible quantity of air (hydrogen sodium carbonate). It therefore follows that such alkaline substances have an attraction for fixed air; and this was proved by mixing _magnesia alba_ in fine powder with caustic alkali, and shaking for some time. The magnesia was converted into the variety which did not effervesce with acids, and the alkali was rendered mild, like a solution of salt of tartar. These are the principal results of Black’s researches, and he concludes with a table of affinity of acids for fixed alkali, calcareous earth, volatile alkali, and magnesia, contrasting it with the affinity possessed by fixed air for the same bases.