the nitre of the ancients, in natural history, is a genuine, pure, and native salt, extremely different from our nitre, and indeed from all the other native salts, it being a fixed alkali, plainly of the nature of those made by fire from vegetables, yet capable of a regular crystallization, which those salts are not. It is found on the surface of the earth, or at very small depths within it; and is naturally formed into thin and flat cakes or crusts, which are of a spongy or cavernous substance, very light and friable, and, when pure, of a pale brownish white; but as its spongy texture renders it very subject to be fouled by earth received into its pores, it is often met with of a deep dirty brown, and not unfrequently reddish.

Natrum, whether native or purified, dissolves in a very small quantity of water; and this solution is, in many parts of Asia, used for washing; where it is also made into soap, by mixing it with oil. Natrum reduced to powder, and mixed with sand or flints, or with any other stone of which crystal is the basis, make them readily run into glaigs. Gold heated red-hot, and sprinkled with a small quantity of this salt, melts in the same manner; as does also iron, copper, and the regulus of antimony; which melt much more easily than they otherwise would do. Mercury cannot be mixed with it by any art, and indeed will not amalgamate with metals if only a little of this salt be added. It is found in great abundance in many parts of Asia, where the natives sweep it up from the surface of the ground, and call it soap-earth. The earliest account we have of it is in the scriptures, where we find that the salt called nitre in those times would ferment with vinegar, and had an abrasive quality, so that it was used in baths and in washing things. Solomon compares the fingering of songs with a heavy heart, to the contrariety of vinegar and nitre; and Jeremiah says, that if the sinner wash himself with nitre, his sin is not cleansed off. These are properties that perfectly agree with this salt, but not at all with our saltpetre.