The custom of dividing wax, ivory, wood and metal MSS. into pages and in this way into book form is said by Suetonius to have been introduced by Julius Caesar, whose letters to the Senate were so made up, and after whose time the practice became usual for all documents either addressed to, or issuing from that body, or to or from the Emperors. As that form subsequently crept into general use, the books were known as "codices;" and hence the ordinary term as applied to manuscript volumes. All classes of "books," the reeds for writing in them, the inkstands, and the "capsae" or "scrinia," the boxes in which the "scapi" or rolls were kept, are minutely portrayed in ancient wall-paintings and ivory diptychs (double tablets), and which may belong to a period near the beginning of the Christian era. Pliny and Dioscorides have given the formulas for the writing inks used by the Greek and Roman scribes immediately before and during their time. Pliny declares that the ink of the bookmakers was made of soot, charcoal and gum, although he does not state what fluid was employed to commingle them. He does, however, mention to an occasional use of some acid (vinegar) to give the ink a binding property on the papyrus. Dioscorides, however, specifies the proportions of this "soot" ink. Another formula alluded to by the same author calls for a half ounce each of copperas (blue) and ox-glue, with half pound of smoke black made from burned resin. He adds, "is a good application in cases of gangrene and is useful in scalds, if a little thickened and employed as a salve." De Vinne speaks of this as a "crude" receipt which will enable one to form a correct opinion of the quality of scientific knowledge then applied to medicine and the mechanical arts; also that these mixtures which are more like shoe blacking than writing fluid were used with immaterial modifications by the scribes of the dark ages. The old Greeks and Romans had no substitute for the papyrus, which was so brittle that it could not be folded or creased. It could not be bound up in books, nor could it be rolled up unsupported. It was secure only when it had been wound around a wooden or metal roller.