Those who attempt to trace out the history of the arts must be very cautious not to admit, without sufficient proof, that what the ancients accomplished was effected by the same means as those employed by us for the same purpose. This, in some cases, may be true; but in many others false. Thus, they made a beautiful kind of blue and red glass, without being acquainted with our cobalt and mineral purple; and they performed very long sea voyages without our compass. It is the duty of the historian either to point out the means which the ancients employed, whether they were the same or not as those used at present, or to acknowledge that their processes are unknown to us. Those who invariably follow this rule will sometimes discover that, in ancient times, men were able to accomplish the same objects and to produce the same effects, by means totally different from those used at present; and then the question will sometimes arise, Which of the means, the old or the new, are the cheapest, the most convenient, and the surest? This leads to technological problems, the solution of which, notwithstanding the great superiority we possess in those auxiliaries of the arts, natural history, chemistry, &c., is impossible. I have indulged in these observations, in mentioning the celebrated Caylus, because I well know that he has often erred in not attending to them. I acknowledge and respect the service of this eminent man; but I am convinced that by the boldness of his assertions he acquired greater confidence and more celebrity than he deserved. The colours on the Etruscan vases have a resemblance indeed to those on our stone-ware, but it is also true that they might be produced by oxide of iron. The substances used by the ancient potters can be determined only by the testimony of the ancients or by experiments; but the former is not to be found; and the latter have never been made, though they would not be difficult to any chemist who might choose to sacrifice a few vessels of that kind. The question how the use of manganese was first found out, occurred even to Pliny; and his opinion on that subject deserves to be quoted, especially as it was long considered as true by Albertus Magnus, Caneparius, and many later writers. To understand it one must know that it was at first believed that the magnet, as it attracts iron, could attract other bodies also; and it was conjectured that other minerals might possess a similar property. Some imagined that they had found magnets for gold and silver. In the oldest times men had so erroneous an opinion of the art of glass-making, that they conceived that glass was obtained from sand, as metal from its ore; and Pliny thinks that they then conjectured that a magnet could attract glass as well as it does iron. Now as manganese, on account of its similarity, was considered to be a magnet, it was consequently subjected to experiments, which gave rise to the beneficial discovery that it renders glass colourless.