It is universally agreed, that the best gum is that produced from the Egyptian thorn;[860] it is of variegated appearance, of azure colour, clean, free from all admixture of bark, and adheres to the teeth; the price at which it sells is three denarii per pound. That produced from the bitter almond-tree and the cherry[861] is of an inferior kind, and that which is gathered from the plum-tree is the worst of all. The vine, too, produces a gum,[862] which is of the greatest utility in healing the sores of children; while that which is sometimes found on the olive-tree[863] is used for the tooth-ache. Gum is also found on the elm[864] upon Mount Corycus in Cilicia, and upon the juniper,[865] but it is good for nothing; indeed, the gum of the elm found there is apt to breed gnats. From the sarcocolla[866] also—such is the name of a certain tree—a gum exudes that is remarkably useful to painters[867] and medical men; it is similar to incense dust in appearance, and for those purposes the white kind is preferable to the red. The price of it is the same as that mentioned above.[868] CHAP. 21.—THE PAPYRUS: THE USE OF PAPER; WHEN IT WAS FIRST INVENTED. We have not as yet taken any notice of the marsh plants, nor yet of the shrubs that grow upon the banks of rivers: before quitting Egypt, however, we must make some mention of the nature of the papyrus, seeing that all the usages of civilized life depend in such a remarkable degree upon the employment of paper—at all events, the remembrance of past events. M. Varro informs us that paper owes its discovery to the victorious[869] career of Alexander the Great, at the time when Alexandria in Egypt was founded by him; before which period paper had not been used, the leaves of the palm having been employed for writing at an early period, and after that the bark of certain trees. In succeeding ages, public documents were inscribed on sheets of lead, while private memoranda were impressed upon linen cloths, or else engraved on tablets of wax; indeed, we find it stated in Homer,[870] that tablets were employed for this purpose even before the time of the Trojan war. It is generally supposed, too, that the country which that poet speaks of as Egypt, was not the same that is at present understood by that name, for the Sebennytic and the Saitic[871] Nomes, in which all the papyrus is produced, have been added since his time by the alluvion of the Nile; indeed, he himself has stated[872] that the main-land was a day and a night’s sail from the island of Pharos[873], which island at the present day is united by a bridge to the city of Alexandria. In later times, a rivalry having sprung up between King Ptolemy and King Eumenes,[874] in reference to their respective libraries, Ptolemy prohibited the export of papyrus; upon which, as Varro relates, parchment was invented for a similar purpose at Pergamus. After this, the use of that commodity, by which immortality is ensured to man, became universally known.