Decant the liquor from these crystals; mix it with twice its weight of hot water; evaporate and crystallize as before. Repeat the same operation till the liquor will yield no more crystals: it will then be very thick, and goes by the name of Mother of Nitre.
OBSERVATIONS.
Earths and stones that have been impregnated with animal or vegetable juices susceptible of putrefaction, and have been long exposed to the air, but sheltered from the sun and rain, are those which yield the greatest quantity of Nitre. But all sorts of earths and stones are not equally fit to produce it. None is ever found in flints or sands of a crystalline nature.
Some earths and stones abound so with Nitre, that it effloresces spontaneously on their surface, in the form of a crystalline down. This Nitre may be collected with brooms, and accordingly has the name of Salt-petre Sweepings. Some of this sort is brought from India.
Hitherto we are much in the dark as to the origin and generation of Nitre. Some Chymists pretend that the Nitrous Acid is diffused through the air, and gradually deposited in such earths and stones as are qualified to receive it.
Others, considering that none of it is ever obtained but from earths that have been impregnated with vegetable or animal juices, have from thence concluded those two kingdoms to be the general repositories of the Nitrous Acid; that if we do not perceive it to exist in such matters at all, or at least in any great quantity, till they have undergone putrefaction, and are in some measure incorporated with suitable earths and stones, it is because the Acid is so entangled with heterogeneous particles that it requires the assistance of putrefaction, and much more of filtration through an earth, to disengage it, and enable it to appear in its proper nature.
Lastly, others are of opinion that this Acid is no other than the universal or Vitriolic Acid; disguised indeed by a portion of the Phlogiston, which is combined with it in a peculiar manner by the means of putrefaction. They ground this opinion chiefly on the analogy or resemblance which they find between the Nitrous Acid and the Volatile Sulphureous Spirit. Its volatility, its pungent smell, its properties of taking fire, and of destroying the blue and violet colours of vegetables, serve them as so many proofs.
Their opinion is the more probable on this account, that even though the Nitrous Acid should actually be produced by vegetable and animal substances, yet as these substances themselves draw all their component principles from the earth, and as the Vitriolic Acid is diffused through all the soils which afford them nourishment, there is great reason to think that the Nitrous Acid is no other than the Vitriolic Acid altered by the changes and combinations it hath undergone in its passage into and through those substances. In 1750 the Royal Academy of Sciences at Berlin proposed an account of the generation of Nitre as the subject for their prize, which was conferred on a Memoir wherein this last opinion was supported by some new and very judicious experiments.
The process by which our Salt-petre makers extract Nitre in quantities, out of rubbish and nitrous earths, is very nearly the same with that here set down; so that I shall not enter into a particular account of it. I shall only take notice of one thing, which it is of some consequence to know; namely, that there is no nitrous earth which does not contain sea-salt also. The greatest quantities of this salt are to be found in those earths which have been drenched with urine, or other animal excrements. Now as the rubbish of old houses in great cities is in this class, it comes to pass that when the Salt-petre workers evaporate a nitrous lixivium drawn from that rubbish, as soon as the evaporation is brought to a certain pitch, a great many little crystals of sea-salt form in the liquor, and fall to the bottom of the vessel.