China almost all the corn is set, or it is first sown in forcingbeds, and then transplanted. The English call the labour with the sowing-machine drilling, and the planting of wheat they name dibbling.

[Several sowing-machines have been invented, and patents taken out for them in late years. As it is very difficult to give a description of them, and still more so for the reader to comprehend them without figures, we refer to the Penny Cyclopædia, art. "Sowing-machine," for an account of the more important.]

MANGANESE1.

THAT the art of glass-making may have arisen from an accident, such as that mentioned by Pliny 2, 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 glassmaker? 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 frit3 of its colour. It produces this decoloration only

1 [The word manganese, strictly speaking, designates the metal itself, the peroxide of which is understood by the author whenever the word manganese occurs in the text.]

2 Lib. xxxvi. 26, § 25.-See Hambergeri Vitri Historia, in Comment. Societ. Götting. tom. iv. anni 1754, p. 487.

3 Under this appellation, writers on the art of glass-making understand a mixture of sand or siliceous earth and alkaline salts, which at the German glass-houses, where the above word is seldom heard, is called Einsatz. It appears to have been brought to us, along with the art, from Italy,

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'. 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, where it is written at present fritta, and to be derived from fritto, which signifies something broiled or roasted. It seems to be the same word as freton, which occurs in Thomas Norton's Poem, Crede mihi, sive Ordinale, where it however signifies a particular kind of solid glass, fused together from small fragments. This Englishman lived about the year 1477. His treatise was several times printed.

1 [The action of peroxide of manganese (the only compound of the metal used in the manufacture of glass) is simple and clearly understood. The sand (silica) used in the manufacture of glass frequently contains iron, which by the heat necessary for the fusion of the glass becomes reduced to the state of protoxide, giving the glass a greenish or yellowish colour; also, if any organic substance be present in the materials (and where sulphate of soda is used, charcoal is added), the glass is not colourless. When peroxide of manganese is added, it parts with some of its oxygen, becoming reduced to the protoxide, which remains colourless in the glass, the protoxide of iron absorbing the oxygen, becomes at the same time converted into the peroxide, which also imparts no colour to the glass, which is thus rendered colourless. If more of the peroxide of manganese be added than the carbon or protoxide of iron can reduce, it will tinge the glass of an amethyst colour, as stated in the text.]

communicate various colours to glass, has been already proved'. 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.

The period however when this great improvement in one of the most useful arts was fortunately introduced, cannot with certainty be determined; but it is very probable that it was practised in the time of Pliny. Were not this the case, what should have induced him, more than once, to remark that the magnet was employed in glass? Under this name the ancients certainly comprehended manganese; which, in general, had a resemblance to the magnet, and was considered as such by Agricola, Kircher, and others, at a more modern period. Pliny3, in one passage, speaks of a kind of magnet which was found in Cantabria, not in veins, but interspersed or in nuclei; and he adds that he did not know whether it was useful in glass-making, because no one had ever tried it. This use of manganese then must at that time have been very common, since it occurred so readily to a writer in speaking of a supposed magnet.

Another passage of Pliny has been supposed to allude to manganese, but in my opinion with much less probability. It is that where he says Alabandicus flows in the fire, and is

See the History of Ruby-glass in vol. i. p. 123.

2 Plin. xxxvi. 26, p. 759, and lib. xxxvii. cap. 6, p. 769; he says that artists could make glass vessels nearly similar to those of rock crystal, but he remarks that the latter had nevertheless risen in price.

3 Lib. xxxvii. 24, § 66.

fused at the glass-houses'. But by that term he seems to understand a kind of marble, according to the opinion of Isidorus, by whom the word is repeated. As a calcareous earth it was perhaps added to promote the fusion of the sand. Camillus Leonardus, however, considered the Alabandicus as manganese2.

It is not improbable that the ancients employed manganese, if not for glazing, at any rate for painting their pottery or earthenware, as soon as they became acquainted at the glass-houses with its susceptibility of being converted into a coloured vitreous mass.

But this is far from being proved, though count Caylus, Genssane and others positively assert that the so-called Etruscan vases and lamps were painted with the same manganese that we use for our earthen-ware.

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 1 Plin. xxxvi. 8, § 13, p. 735.

2 Speculum Lapidum, Parisiis, 1610, 8vo, p. 71. It may not be superfluous here to remark, that this Alabandicus of Pliny must not, as is often the case, be confounded with the precious stone to which he gives the same name, lib. xxxvii. cap. 8. The name properly denotes only a stone from Alabanda in Caria. It occurs, but much corrupted, as the name of a costly stone, in writings of the middle ages. See in Du Cange Alamandinæ, Alavandinæ, Almandinæ; and even in our period so fertile in names, a stone which is sometimes classed with the ruby and sometimes with the garnet, and which is sometimes said to have an affinity to the topaz and hyacinth, is called Alamandine and Alabandiken. See Brückman on Precious Stones, who in the second continuation, p. 64, deduces the word from Allemands, without recollecting the proper derivation, which he gives himself, i. p. 89 according to Pliny.

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.

This use of it then has been retained through every age to the present time, and it is mentioned by all those authors who