[555] Of Segni an account may be found in Notizie degli Scrittori Bolognesi raccolte da Giovanni Fantuzzi. In Bologna 1784–1794, 9 vols. 4to, vii. p. 377. Segni, who died in 1610, wrote a great many ascetic books, the names of which are there given.
[556] Dell’ agricultura, dell’ arti e del commercio. Lettere di Antonio Zanon. In Venezia 1764, 8vo, vol. iii. p. 325.
[557] Prodromo, overo saggio di alcune inventioni nuove, premesso all’ arte maestra. In Brescia 1670, fol. p. 96, fig. 26.
[558] See the excellent account of the agriculture in Suffolk in my Journal, the Beytragen zur Oekonomie, &c., i. p. 1. It was written by M. F. Wild, of Durlach, who in the year 1767 was one of my pupils, and afterwards became teacher in the Institute of Education at Colmar. But alas! I do not know whither he has now been swept by the vortex of the revolution.
[559] Leske Reise durch Sachsen. Leipzig, 1785, 4to, p. 319.
[560] Sylva Sylvarum, cent. 5, § 442.
MANGANESE[561].
That the art of glass-making may have arisen from an accident, such as that mentioned by Pliny[562], I am ready to admit; but by what accident were artists made acquainted with the use of manganese, a mineral the outward appearance of which seems to announce nothing that could be useful to the glass-maker? It is not found in such abundance as to allow us to suppose that it naturally presented itself; nor do we know that any older application of it may have induced the ancients to employ and examine it in such a manner that the present use of it might be accidentally discovered. In general, it resembles some kinds of iron-stone, which it was considered to be till a very late period. That iron, however, colours glass must have been very early remarked; and therefore it could occur to no one to employ manganese for depriving frit[563] of its colour. It produces this decoloration only when it is added sparingly, and according to a determinate proportion; otherwise it gives to the glass a violet colour, something similar to that of the amethyst.
The application of manganese was certainly taught by accident, and not by theory. But in regard to the question, why it frees glass from its dirty colour, it must be admitted, if we readily acknowledge the truth, that we can offer only hypotheses; as the old chemists called in the aid of phlogiston, and the new that of oxygen[564]. Did a false hypothesis, then, conduct to this discovery? That this was the case, has been asserted by old as well as more modern writers, and is no doubt possible. Thus Kepler, from an erroneous hypothesis in regard to the revolution of the planets, discovered the ratio of their motion, according to their distance from the sun; and such instances may be adduced in favour of hypotheses which have done more harm than good. But, in my opinion, in examining the origin of the ancient arts, we ought not to give credit to any cause assigned for an invention until no other can be found. In regard to the art in question, I think I can mention one which, at any rate, has probability in its favour, and which I shall here submit to the reader’s decision.