It is chiefly in connection with such work that there arose a curious literature, if that term may be used for the barbarous treatises in question. Already in Roman times we hear of writings that describe the manufacture of artificial gems: Pliny says that he purposely abstains from mentioning the names of these works—he would not help to spread so objectionable an industry. But at that time and even later it was in Egypt that treatises of the kind chiefly originated. The mysteries of glass-making were there early associated with more dangerous arts. It is mainly to writers on magic—white or even black—and to those on alchemy that we must turn to find the earliest examples of those strange recipes for the manufacture, and especially the colouring, of glass, of which I shall have more to say later on. This connection between the arts of the glass-maker and of the alchemist arose from many causes, some of them obscure. For example, the vessels used in the experiments of the alchemists were from an early date made of glass. Again, the strange changes of colour observed when glass was stained by copper or by gold were regarded as steps to the great discovery itself. So that from the days of the Ptolemies in Egypt, if not from an earlier date, down to the time of the German alchemists and Rosicrucians of the seventeenth and eighteenth centuries, we find along with the grotesque and cryptic formulas for the preparation of gold an almost continuous chain of recipes, equally absurd for the most part, for the colouring of glass. In addition to this, many of these treatises, although professing to deal with the general problem of the transmutation of matter, are in reality concerned with the more practical questions of making plausible imitations of gold, silver, and precious stones—they are, in fact, handbooks for the fraudulent goldsmith. This is especially the case with the earliest example of the class that has come down to us, the famous papyrus of Leyden, which alone has survived the destruction that the Roman law again and again attempted to enforce in the case of all books of magic. M. Berthelot, whom I follow for these early writers,[83] calls this papyrus the working notes of an ‘_artisan faussaire et d’un magicien charlatan_.’ This little work, found long ago at Thebes, is a Greek manuscript of the third century; it contains, however, little or nothing about glass, and is of interest merely as an early specimen of this class of composition.