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BOOK XII.
561
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Saltpetre9 is made from a dry, slightly fatty earth, which, if it be retained for a while in the mouth, has an acrid and salty taste. This earth,
together with a powder, are alternately put into a vat in layers a palm deep. The powder consists of two parts of unslaked lime and three parts of ashes of oak, or holmoak, or Italian oak, or Turkey oak, or of some similar kind. Each vat is filled with alternate layers of these to within three-quarters of a foot of the top, and then water is poured in until it is full. As the water percolates through the material it dissolves the saltpetre ; then, the plug being pulled out from the vat, the solution is drained into a tub and ladled out into small eSaltpetre was secured in the Middle Ages in two ways, but mostly from the treatment
of calcium nitrate efflorescence on cellar and similar walls, and from so-called saltpetre plantations. In this description of the latter, one of the most essential factors is omitted until the last sentence, i.e., that the nitrous earth was the result of the decay of organic or animal matter over a long period. Such decomposition, in the presence of potassium and calcium carbonates—the lye and lime—form potassium and calcium nitrates, together with some magnesium and sodium nitrates. After lixiviation, the addition of lye converts the calcium and magnesium nitrates into saltpetre, i.e., Ca (NO3)2 + K2C03 = Ca CO3 + 2ΚΝΟ* The carbonates precipitate out, leaving the saltpetre in solution, from which it was evaporated and crystallised out. The addition of alum as mentioned would scarcely improve the situation. The purification by repeated re-solution and addition of lye, and filtration, would
eliminate the remaining other salts. Thepurißcation with sulphur, however, is more difficult |
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