ALKALIS are a class of chemical bodies distinguish ed by the following properties: They impress the tongue with a peculiar acrid taste, which has been termed caus tic or urinous, a sensation commonly considered as the contrary of sour; they have a strong affinity for water, with which they combine with rapidity, and in great quantity ; they change the blue vegetable colours to green, the brown to yellow ; they corrode and dissolve animal substances ; they unite with the oils and fats, and thus form the well-known compound, soap ; they com bine readily with several chemical agents, and particu larly with the acids, with which they form the neutral salts ; they arc capable of being fused and volatilized by heat.
Some of these properties are discovered in two or three of the earths; and two of these, barytes and stroll tiles, have been considered as alkalis by Vauquelin, Fourcroy, and others of the French chemists. But this arrangement has not been very generally received ; be cause, as has been observed, if we admit these amongst the alkalis, there is hardly any good reason for exclud ing lime, magnesia, and perhaps some other of the earthy substances ; and because the greater solubility and fusibility of the alkalis sufficiently distinguish them from all these substances, which have also properties common to themselves. If, however, as there is now some reason to believe, the chemical composition of any of these earths should turn out to be similar to that of the alkalis, it is more than probable that they will he universally acknowledged to belong to this class of che mical bodies.
The alkalis, hitherto acknowledged as such, are only three in number; the two called fixed, potass and soda. and the volatile alkali, or ammonia. Potass being obtain ed by lixiviation from the incinerated ashes of most Ve getables, has also been distinguished as the vegetable alkali ; while soda abounding in the mineral kingdom. in common or sea salt, in the ashes of marine plants, and found native in the soils of Egypt, Syria, and India, has received the appellation of mineral alkali.* Both these alkalis are solid, and eomparatively fixed bodies; on the contrary, when pure, exists only in the gaseous form; for liquid ammonia is nothing more than a solution of this gas in water. Hence the name of volatile alkali. The great source of ammonia is the decomposition of animal substances; but it is most rea dily obtained in its pure state by decomposing sal am moniac (mu•iate of ammonia) by lime.
Au early experiment of 1)r Priestley, by which he demonstrated that the volume of pure ammoniacal gas was greatly enlarged, and its properties totally changed by the electric spark ; and the observations of Scheele on the detonation of fulminating gold, sufficiently war ranted the conclusion, that this alkali was not a simple but a compound body. Its real composition was at last, in the year 1785, fully explained by Berthollet, since which time it has been generally acknowledged by che mists to be a compound of hydrogen and nitrogen. By subjecting ammoniacal gas to the influence of the elec tric spark, or by passing it through heated porcelain tubes, it is easily resolved into these two elements. Dr Austin afterwards sheaved, that ammonia might be di rectly formed by presenting nascent hydrogen to nitro gen gas, and so confirmed synthetically the analytical proof of the composition of this alkali.
According to the best experiments, it has been Con luded, that 100 parts or ammonia are composed of 80 of nitrogen, and 20 of hydrogen.
The discovery of the composition of ammonia natu rally suggested a belief, that the other alkalis were also compound bodies; and as oxygen appeared to be the universal principle of acidity, it was inferred by analogy, that nitrogen or hydrogen might be the principle on which depend the common properties of alkaline bodies. Certain observations and experiments seemed even to give more than probability to these conjectures. The large and repeated productions of nitre (nitrate of pot ass,) from the artificial compost of animal matters, and carbonate of lime, even after successive lixiviations, dis posed Chaptal to conclude that potass must be a com pound of lime and nitrogen. And from a converse ex periment, in which phosphate of lime was precipitated from a solution of oxymuriate of potass and phosphoric acid on the addition of ammonia, and in which there fore there was a seeming production of lime, Desormes and Guyton inferred this to be a component part of potass, and derived from its decomposition, and that this alkali was a compound of lime and hydrogen. From somewhat similar experiments, and from an observation of Vauquelin on the existence of magnesia in the salsola, from the ashes of which soda is more abundantly pro cured than from any other plant, it was in like manner supposed that magnesia might he the basis of soda.