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. That it was observed at an early period that metallic oxides, and particularly that of iron, which most frequently occurs, communicate various colours to glass, has been already proved[565]. It needs therefore excite no wonder that men should be induced to make experiments on colouring glass with various minerals, and especially such as contained iron. Now, since manganese, as already said, has a great resemblance to iron-stone, it was also occasionally employed; and it was soon found that this supposed species of iron-stone, according as it is used in greater or less quantity, gives to glass many beautiful shades of a violet, red, and dark brown colour. As it was necessary that the artist should weigh the manganese, in order to proportion it to the vitreous mass, according to the required colour, it is possible that the glass, when a very small quantity had been added, was found to be colourless. This observation must have been made with the greater satisfaction, and more readily turned to advantage, the higher colourless glass, which approached nearest to rock crystal, was at that time esteemed[566].