The Egyptians thought fit to call saltpetre “Chinese snow,” but this does not justify the conclusion that the discovery was made by the Chinese. Consider our own phrases “Jerusalem” artichoke, “Welsh” onion, and “Turkey” cock. Jerusalem is a gardener’s corruption of _girasole_, the Turkey came from America, and the home of the Welsh onion is Siberia. The Persians called their native alkaline salt _jamadi Chini_, and no one will suggest that this substance came from China. It is evident from the way in which it is mentioned by the alchemists of the thirteenth century, and from their primitive methods of refining it, that saltpetre was then in its infancy. Roger Bacon speaks of it as one would speak of a substance recently discovered and still little known—“that salt which is called saltpetre” (_illius salis qui sal petræ vocatur_).[36] Marcus Græcus thought it necessary to explain what the word means, in his 14th recipe which probably belongs to the latter years of the thirteenth century.[37] The methods of refining the salt given by Marcus and Hassan leave no possible doubt that in their time it had but just come into use. It is true that Bacon’s method was much superior, if the solution of his steganogram given in Chap. viii. be accepted. But it would have been past all explanation had the method of the greatest natural philosopher of the age been found to be no better than that of an Arabic druggist or a European fireworker. As the matter is one of the greatest importance, the methods of all three are given in full, together with that of Whitehorne, 1560. The Waltham Abbey method is added, as a standard by which to judge them. To admit of easy comparison, the corresponding operations are marked with the same letter. The five methods are summed up in Table I. WALTHAM ABBEY, 1860. A. _Preparation of grough from natural saltpetre._[38] Natural saltpetre is dissolved in boiling water, the insoluble impurities removed, and the solution evaporated by the sun or artificial heat. The solid residue is grough saltpetre, and contains 1 to 10 per cent. of impurities, consisting of the chlorides of potassium and sodium, sulphates of potash, soda, and calcium, vegetable matter, sand, and moisture. B. _Boiling the solution of grough saltpetre._ The grough saltpetre is placed in an open copper with a false bottom; water is added, and heat applied until the mixture boils at 110° C. C. _Removal of the insoluble impurities._ The scum which rises to the surface during this operation is removed by ladles; the sand and heavy impurities fall upon the false bottom, which is removed just before the mixture boils. The boiling is continued until the scum ceases to rise. D. _Second boiling of the solution._ Cold water is added; the solution is boiled for a few minutes, and then allowed to cool somewhat. E. _Filtration._ At 104.5° C. the mother liquid is transferred to a tank with holes in its bottom, closed by filters.