AQUAFORTIS , NITRE, SODA , &C. 151
with several other lesser establishments for lying-in women,&c. &c.
AQUAFORTIS , NITRE, SODA , ALKALI, POT-ASH, and GUNPOWDER.
Our saltpetre, commonly called nitrum, or sometimes salnitri, is a neutral salt, from its peculiar acid,—an acidcommonly called the acid of saltpetre, and that vegetablealkali to which our pot-asli usually belongs; the chief markswhich distinguish it from other salts are, its cooling taste, itsfusibility when exposed to even a moderate degree of heat,and its peculiar decrepitation, that is, the property, on beingplaced in the fire, or any ignited body, of sudden combus-tion in a very bright flame, by which it becomes alkalised,or loses its acid; so that nothing remains but the vegetablealkali. Its chief use in the works of art is in the manufac-ture of gunpowder, and the preparation of the well-knownacid termed aquafortis.
Aquafortis is a caustic useful in various arts, butparticularly as a metallic test, and in the engraving of variousmetals; but, however, more peculiarly used in the elegantart of multiplying pictures, called copper-plate engravings.The workers on metals, distinguish it into two kinds, calledprima and secunda; the former of these is the pure nitrousacid, and the latter is the same acid dilated with an equalportion of pure water.
The basis of this acid (properly prepared) is from a mine-ral alkali, a neutral salt, of which there are three chiefspecies found in the calx of the minerals following, andcalled nitre of silver, called also lunar crystals, of lead,ana of mercury.
It appears to be a point by no means settled among natu-ralists, whether the substance known as nitrum to theancients, was the same we denominate saltpetre. But upon -the most obvious comparison of the specific nature of bothsubstances, the prevailing opinion appears to be, that whatTheophrastus and Pliny knew by the name of nitrum, is thearticle we now denominate soda; under which general nameare included a great variety of salts of many specific na-tures.
Almost all the saltpetre obtained in Europe , 1 as its pro-