Lecture Xvi.

Of Alkalis.

The class of substances that seems particularly formed by nature to unite with acids, and thereby form neutral salts, are the alkalis. They have all a peculiar acrid taste, not easily defined. They change the blue juices of vegetables green, or purple, and in common with acids have an affinity with water, so as to be capable of being exhibited in a liquid form; though when this water is expelled by heat, some of them will assume a solid form.

Alkalis are of two kinds; the fixed, which have no smell, and the volatile which have a pungent one.

The fixed alkalis are of vegetable or mineral origin. When in a solid form, they both melt with a moderate heat, and uniting with earthy substances, make glass. With an intense heat they are volatilized.

Vegetable alkali is procured by burning plants, and lixiviating the ashes; a purer kind by the burning of tartar, hence called salt of tartar; but the purest of all is got by the deflagration of nitre; the charcoal uniting with the acid as it assumes the form of dephlogisticated air, and the alkali being left behind.

Mineral alkali is found in ashes of sea-weed. It is likewise the basis of sea-salt; from which it is separated by several processes, but especially by the calx of lead, which has a stronger affinity with the marine acid with which it is found combined.

Alkalis united with fixed air are said to be mild, and when deprived of it caustic, from their readiness to unite with, and thereby corrode, vegetable and animal substances. To render them caustic, they are deprived of their fixed air by quick-lime; and in this state they unite with oils, and make soap.

Alkalis have a stronger affinity with acids than metals have with them; so that they will precipitate them from their solutions in acid menstruums.

The vegetable fixed alkali has a strong attraction to water, with which it will become saturated in the common state of our atmosphere, when it is said to deliquesce; and having the appearance of oil, the salt of tartar is thus said to become oil of tartar per deliquium. On the other hand, the mineral, or fossil alkali, is apt to lose its water in a dry atmosphere, and then it is said to effloresce. In this state it is often found on old walls.

Volatile alkali is procured by burning animal substances; in Egypt (from whence, as contained in sal ammoniac, we till of late imported it) from camel's dung; but now from bones, by distillation. To the liquor thus procured they add vitriolic acid, or substances which contain it. This acid unites with the alkali, and common salt being put to it, a double affinity takes place. The vitriolic acid uniting with the mineral alkali of the salt, makes Glauber salt, and the marine acid uniting with the volatile alkali, makes sal ammoniac. Slaked lime added to this, unites with the marine acid of the ammoniac, and sets loose the volatile alkali in the form of alkaline air, which combining with water, makes the liquid caustic volatile alkali. If chalk (containing calcareous earth united with fixed air) be mixed with the sal ammoniac, heat will make the calcareous earth unite with the marine acid, while the fixed air of the chalk will unite with the volatile alkali, and assume a solid form, being the sal volatile of the apothecaries.