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BOOK XII.

vats. If when tested it tastes very salty, and at the same time acrid, it isgood ; but, if not, then it is condemned, and it must be made to percolateagain through the same material or through a fresh lot. Even two or threewaters may be made to percolate through the same earth and become fullof saltpetre, but the solutions thus obtained must not be mixed togetherunless all have the same taste, which rarely or never happens. The first ofthese solutions is poured into the first vat, the next into the second, the thirdinto the third vat; the second and third solutions are used instead of plainwater to percolate through fresh material ; the first solution is made inthis manner from both the second and third. As soon as there is an abun-dance of this solution it is poured into the rectangular copper caldron andevaporated to one half by boiling; then it is transferred into a vat coveredwith a lid, in which the earthy matter settles to the bottom. When thesolution is clear it is poured back into the same pan, or into another, andre-boiled. When it bubbles and forms a scum, in order that it shouldnot run over and that it may be greatly purified, there is poured into it threeor four pounds of lye, made from three parts of oak or similar ash and one ofunslaked lime. But in the water, prior to its being poured in, is dissolved rock-alum, in the proportion of one hundred and twenty librae of the former to five

to understand. In this case the saltpetre is melted and the sulphur added and set alight.Such an addition to saltpetre would no doubt burn brilliantly. The potassium sulphateformed would possibly settle to the bottom, and if the " greasy matter ” were simplyorganic impurities, they might be burned off. This method of refining appears to havebeen copied from Biringuccio (x., 1), who states it in almost identical terms.

Historical Note. —As mentioned in Note 6 above, it is quite possible thatthe Ancients did include efflorescence of walls under nitrum ; but, so far as we are aware,no specific mention of such an occurrence of nitrum is given, and, as stated before, thereis every reason to believe that all the substances under that term were soda and potash.Especially the frequent mention of the preparation of nitrum by way of burning, arguesstrongly against saltpetre being included, as they would hardly have failed to notice thedecrepitation. Argument has been put forward that Greek fire contained saltpetre, but itamounts to nothing more than argument, for in those receipts preserved, no salt of any kindis mentioned. It is most likely that the leprosy of house-walls of the Mosaic code (Leviticusxiv., 34 to 53) was saltpetre efflorescence. The drastic treatment by way of destruction ofsuch “ unclean ” walls and houses, however, is sufficient evidence that this salt was not used.The first certain mention of saltpetre (sal petrae) is in Geber. As stated before, the dateof this work is uncertain ; in any event it was probably as early as the 13th Century. Hedescribes the making of “ solvative water ” with alum and saltpetre, so there can be nodoubt as to the substance (see Note on p, 460, on nitric acid). There is also a work by anebulous Marcus Graecus, where the word sal petrosum is used. And it appears that RogerBacon (died 1294) and Albertus Magnus (died 1280) both had access to that work. Baconuses the term sal petrae frequently enough, and was the first to describe gunpowder (DeMirabili Potestate Artis et Naturae 1242). He gives no mention of the method of making hissal petrae. Agricola uses throughout the Latin text the term halinitrum, a word he appearsto have coined himself. However, he gives its German equivalent in the Interpretatio assalpeter. The only previous description of the method of making saltpetre, of which we areaware, is that of Biringuccio (1540), who mentions the boiling of the excrescences from walls,and also says a good deal about boiling solutions from “ nitrous ” earth, which may or may notbe of “ plantation ” origin. He also gives this same method of refining with sulphur. Inany event, this statement by Agricola is the first clear and complete description of the salt-petre “ plantations.” Saltpetre was in great demand in the Middle Ages for the manufactureof gunpowder, and the first record of that substance and of explosive weapons necessarilyinvolves the knowledge of saltpetre. However, authentic mention of such weapons onlybegins early in the 14th Century. Among the earliest is an authority to the Council of Twelveat Florence to appoint persons to make cannon, etc., (1326), references to cannon in thestores of the Tower of London, 1388, &c.