or MAGNESIA NIGRA, a dark-coloured mineral employed in glaze-works for purifying the glass, by taking away the colour it has already, or by superadding a new colour to it. It is also used in the glazing of earthen ware, where it communicates a black colour. From its property of rendering glass colourless, it has sometimes been called the soap of glass.

This substance, commonly called block or glazemaker's manganese, is scarcely any other thing than the calx of a new semimetal, whose properties were for the first time investigated by Mr Scheele in the Stockholm Memoirs for 1774; afterwards it was more fully investigated by Dr Gahn, and lately by several other chemists. Its colour is of a dusky white; and the surface is uneven and irregular, owing to its imperfect fusion. It is bright and shining when first broken, but tarnishes by exposure to air much sooner than any other metallic substance. Its specific gravity is 6.850; it equals, if it does not exceed, iron in hardness, as well as difficulty of fusion. When reduced to powder, it becomes magnetic, though large pieces of it are not so. When exposed to the air, it soon crumbles into a blackish brown powder, somewhat heavier than the regulus itself; and this effect is sooner produced in a moist than a dry air.

The regulus is obtained by making the calx or ore of manganese into a ball with pitch, and putting it into a crucible with powdered charcoal one-tenth of an inch thick on the sides and a quarter of an inch at the bottom. The empty space is then to be filled with powdered charcoal, covering the crucible with another. Having fitted the joints, the whole is to be exposed to the strongest heat of a forge for an hour or more. This regulus is soluble in all the acids, but most readily in the nitrous, the solution in which is generally of a brownish colour, though that in the others is mostly colourless. The brown colour in the nitrous solution arises from the mixture of a small portion of iron, and there is always a black residuum resembling plumbago left undissolved. Aerated alkalis throw down a white precipitate from these solutions, which by heat grows black, and is converted into the original calx of the metal.

Regulus of manganese melts readily with other metals, mercury alone excepted. Copper, united with a certain quantity of it, is extremely malleable; but scarce any traces of the red colour are to be seen on the surface when polished, but the mixture sometimes has a green efflorescence by age. Its decomposition by air is very remarkable. A piece of it newly made, when put into a dry bottle well corked, remained perfect for six months; but afterwards, when exposed only for two days to the open air of a chamber, contracted a brown colour on the surface, and became so friable as to crumble into powder between the fingers, the internal parts only retaining an obscure metallic splendour, which also disappeared in a few hours.

This surprising facility of decomposition might naturally lead us to suppose that no such thing as native manganese could exist in the earth. In the Journal de Physique for January 1786, however, M. de Peyrouse gives an account of a native regulus of manganese, the properties of which are as follow: 1. In appearance it very much resembles the artificial regulus already mentioned. 2. It dries the fingers by handling. 3. None of its particles are in the least affected by the magnet. 4. It is composed of laminae having a kind of divergence among themselves. 5. Its metallic brilliancy is the same with that of the artificial regulus, and it has a partial malleability. 6. When repeatedly hammered, it exhibits a kind of exfoliation, forming itself into very thin leaves. 7. Its opacity and density are so completely similar to the artificial regulus, that were it not for the matrix in which the latter is imbedded, it would be in a manner impossible to distinguish them.

This regulus is not found in large masses, or in any solid continuous body, but only in clots or lumps inclosed and intermixed with the powdery or calciform ore. These lumps are somewhat flattened or compressed in their form like the artificial ones, though for the most part they are of a larger size. This powdery magnesian ore, in which the reguline lumps are imbedded, has an argentine hue, as if the materials had been subjected to some violent heat upon the spot.—This regulus was found among the iron mines of Sem, in the valley of Viedoros, in the county of Foix, near the Pyrenean mountains.

Manganese is found in a calciform state, of various colours. M. de Magellan observes, that the aerial acid is the only mineralizer of this semimetal in its dry state; and that in proportion to the different degrees of phlogistication, a variety of colours is produced. When it contains as much phlogiston as possible, without being reduced to a regulus, it always appears of a white colour, and contains about 40 per cent. of fixed air. In proportion to its dephtlogistication and union with other substances, its colour is either blue, green, yellow, red, or black.

The black manganese seems to be the decayed particles of that which is indurated. The latter is met with either pure, or in form of balls seemingly composed of concentric fibres; sometimes, but very rarely, it is met with of a white colour. Cronstedt informs us, that he had a specimen of this from an unknown place in He found that it differed from the common kind by giving a deep red colour to borax in the fire. By calcination it assumes a reddish brown colour.

Blue manganese, according to Mr Scheele, acquires its colour from the phlogiston which it is enabled to retain by its union with fixed alkalies. Green arises from a mixture of the blue with the yellow calx of iron, and the yellow colour, from a prevalence of this calx; red from a flight deplogistication of the calx; and black from a thorough deplogistication of it.

The white kind, above mentioned, contains but a very small proportion of iron. Rinnan found it both in small white crystals and in round masses in the cavities of quartz, and adhering to glanz-bende. The hardness rather less than limestone, the texture sparry, and the substance scarcely magnetic even after roasting: it affords a colourless solution with nitrous acid, from which mild alkalies throw down a white precipitate turning black with heat, as already mentioned, of the regulus itself. The white ore has also been found vegetating on the surface of some iron ores, particularly hematites. Mr Rinnan also met with it in the form of calcareous spar of the colour of rosin, somewhat shining, and covered over in some places with a foamy powder. It is found also in thin pieces, transparent at the edges, but not hard enough to strike fire. This consists of manganese bedded in zeolite. It melts per se with the heat of a blow-pipe into a whitish grey porous flake; and with the addition of calcined borax gives a garnet colour to glass. According to Kirwan, many of the white sparry iron ores may be classed among those of manganese, as they contain more of this ferrimetal than of iron.

Red manganese is said to be found in Piedmont, but Cronstedt says he never saw it. He was told by an ingenious workman, that this variety is free from iron, and gives rather a red than violet colour to glass. Mr Kirwan says, that this kind has less fixed air and more iron than the white kind. It is also joined with ponderous earth, calcareous earth, ponderous spar, and flint. It is found either loose or semi-indurated, in a matrix of calcareous spar, on tacky schistus, or on hematites or other iron ores. It is found likewise in heavy hard masses of lamellar, radiated, or equable texture, or crystallized in pyramids, rhomboids, or short brittle needles.

Manganese is also met with in a state of union with iron. This is black, with a metallic splendour, and is the kind commonly employed in glass-houses and potteries. There are several varieties of this stone in the mountains round Bath named Mendip-hills, of which the Bristol potters consume great quantities. The black ores of manganese differ but little from the brown ones. They are both found either crystallized as the red ores, or in solid masses, some of which have a metallic appearance; but others are dull, earthy, and mixed, or embodied in quartz, or in a loose earthy form. Their specific gravity is about 4.000. The black manganese is met with either solid and of a flaggy texture, fleck-grained, radiated, or crystallized.

The Perigord stone belongs to this species of manganese. It is of a dark grey colour like the basaltes, and may be scraped with a knife, but cannot be broken without difficulty. It is very compact, heavy, and as black, internally, as charcoal. It has a glittering appearance of a striated kind, like the ore of antimony; and its particles are disposed in the form of needles crossing one another without any agglutination, in such a manner that some are loose in a manner similar to iron filings when stuck to a lodestone, and resembling on the whole the scoria from a blacksmith's forge. By calcination this substance assumes a reddish brown colour, and becomes harder, but is not magnetic. It does not melt per se, but affords an amethyst-coloured glass with borax. Nitrous acid has little effect upon it without sugar. It seems to contain clay and some iron, and is of considerable specific gravity. It is found in some parts of England as well as in Gascony and Dauphiny in France. The French potters and common enamellers sometimes employ this substance in the glazing of their ware.

Black-wadd is likewise an ore of manganese. It is found in Derbyshire, and is of a dark brown colour, partly in powder, and partly indurated and brittle. If half a pound of it be dried before the fire, and afterwards suffered to cool for an hour, and two ounces of linseed oil afterwards added, mixing the whole loofly like barm with flour, little clots will be formed, and, in something more than half an hour the whole will grow hot, and at last burst into flame. The heat of the room in which this experiment was tried might be about 30° of Fahrenheit, and the heat to which it was exposed in drying about 130°. According to Wedgwood's Analysis, this ore contains 43 parts of manganese, as much iron, 4½ of lead, and near 5 of micaceous earth.

Besides the ores mentioned above, Mr Scheele informs us, that he has found manganese existing in potashes. Chemists, he tells us, have often observed, that alkaline salts, when calcined, assume a bluish or greenish colour. The cause of this has been said to be a quantity of phlogiston present in the alkali; but to this he objects, that such a colour is not destroyed by fusion with nitre. When fixed alkali is made to run over the crucible by too strong a fire, the part that attaches itself to the outside acquires a dark-green colour in consequence of the ashes uniting with it. If one part of alkali of tartar be mixed with one-fourth of fine sifted ashes and one-eighth of nitre, a dark-green mass is obtained, which, by solution in water, affords a beautiful green solution, and, when filtered, turns red on the addition of a few drops of vitriolic acid. Some days afterwards a small quantity of brown powder falls to the bottom, which discovers the same chemical properties as manganese. On dissolving a quantity of sifted ashes in muriatic acid by digestion in a sand-pan, the same smell of aqua-regia arises that is perceived on mixing manganese with spirit of salt. Adding some hours afterwards a certain quantity of vitriolic acid, in order to precipitate the greater part of the calcareous earth, the liquor had a yellow colour when filtered, and by means of fixed alkali let fall a yellow precipitate, which by calcination turned of a dark-grey, and showed signs of containing manganese. Hence it appears that manganese really exists in the ashes of vegetables, but not equally in all; for Mr Scheele observes, that wood-ashes contain much more than those of thyme (thymus serpyllum). Mr Scheele has laboured exceedingly to decompose this substance, and to discover its component parts. He candidly acknowledges, however, that he did not succeed in this investigation according to his wish, and therefore cannot be certain that his conclusions are altogether just. The following experiments, however, he tells us, were made with the greatest accuracy as well as expense of time and trouble.

Half an ounce of phlogisticated manganese, purified from all foreign particles, was calcined upon an iron-plate till it grew black. It was then dissolved in diluted vitriolic acid, with the addition of a little sugar, in a sand bath till the solution became limpid. On cooling, a fine shining powder precipitated, which proved to be selenite. Having separated this by filtration, and then diluted the solution with six ounces of distilled water, precipitating it afterwards by vegetable alkali, the powder was edulcorated, and again exposed to calcination (a). The manganese, when deprived of its phlogiston, was again dissolved by means of sugar in diluted vitriolic acid; by which means as much selenite was obtained as before. The filtered solution was treated exactly in the same manner, and the operation repeated eleven times, yielding to appearance as much selenite as before. On weighing the results of all these calcinations, the manganese was found to be reduced to three drachms and five grains, and the quantity of selenite had increased to 49 grains; the whole seems therefore to be convertible into calcareous earth. On attempting to invert the experiment, and to produce manganese by combining phlogiston with calcareous earth, he found it impossible to unite the two substances by any means he could devise.

This analysis of manganese was undertaken at the desire of Mr Bergman; who having informed him that Mr Sage supposes manganese to be nothing else than a mineralised mixture of cobalt and zinc, he afterwards made several experiments with a view to detect these substances, but in vain. "Manganese (says Mr Bergman) has been classed by all mineralogists among the ores of iron; but Mr Pott supposed the iron to be only accidentally mixed with it; and at last Mr Cronstedt, in his Essay on Mineralogy, 1758, placed it among the earths. For my part, however, I must own that there are several circumstances which make me think that it is a metallic substance. No pure earth colours glass, but all metallic calces have this property. Manganese, therefore, in this respect, shows a great resemblance to the latter; which is further increased by its specific gravity, and its strong attraction for phlogiston." Having then mentioned its precipitation by the Prussian alkali as an additional proof of its metallic nature, he proceeds thus: "But what kind of metal it is which manganese contains is not so easily ascertained. The solution of cobalt does not lose its colour on adding sugar or any other phlogistic substance, and zinc does not impart any colour to acids. These two substances consequently differ from manganese, which does not indeed entirely agree with any other of the known metallic earths. I have, however, great reason to conjecture that it must be platina, the earth of which is not yet known; or a new metal, which at least would agree with platinum in the great difficulty with which it fuses."

It has already been observed, that manganese is used in glass-works, and is capable both of destroying the colour of glass, and of giving a new colour to it, viz. that of violet. Mr Scheele deduces its operation from the properties related under the article Chemistry, n° 1359 &c. He enumerates its effects on glass-fluxes as follows: 1. A colourless glass-flux becomes constantly more or less red on addition of manganese, according to the quantity. 2. If the flux be a little alkaline, the colour will approach to violet. 3. Arsenic, gypsum, and calx of tin, destroy the red colour in these glasses, and thus render them clear. The action of arsenic is easily explained from Chemistry, n° 919, where it is shown that manganese decomposes arsenic by uniting with its phlogiston, and that arsenic itself is composed of an acid of a peculiar nature united with phlogiston. On mixing manganese, therefore, with glass in fusion, the phlogiston of the arsenic unites with the manganese, while the acid of arsenic unites with the alkali of the glass. This experiment succeeds in a covered crucible, though never when gypsum or calx of tin are made use of; but on adding powdered charcoal, an effervescence ensues, the red colour disappears, and the glass becomes colourless. The phlogiston of the charcoal is therefore the cause of the destruction of the colour, and the effervescence is a necessary consequence of the emission of the phlogiston. 4. If glass coloured red by manganese be fused in a crucible with powdered charcoal, the colour disappears during the effervescence without the addition of gypsum or calx of tin; but on keeping the glass a long time in fusion upon charcoal, by means of the blow-pipe, the colour does not disappear. Nay, if the colourless glass be kept in this state for a short time upon charcoal, it grows red again. 5. By adding a little sulphur, the colour disappears; and the same thing takes place on the addition of any metallic calx or any neutral salt containing the vitriolic acid. But here it must be observed, that all metals whose calces colour glass, while they deprive it of that which it has received from the manganese, will not fail to communicate their own peculiar colour to it. If to such a colourless glass-globe, nitre, even in the smallest quantity, be added, it presently grows red again; and the same thing happens if such a colourless glass-globe be kept in fusion for a few minutes upon an iron plate; and thus the red colour may be made to appear and disappear as often as we please.

From this explanation it appears how manganese purifies glass. When the colour of it depends on a quantity of coaly matter, it is improper to add more than is just sufficient to saturate the phlogiston. With regard to the green colour of common bottle-glass, Mr Scheele made the following experiment to determine whether it proceeded from iron or not. Having melted green glass by the blow-pipe upon a piece of the same substance, left in using a crucible he should have been deceived by the iron it contained, he poured

(a) As in this process a quantity of fixed air is always expelled from the alkali, it was necessary, in order to prevent any of the manganese from being dissolved by it, to place the whole for some hours upon hot sand, to expel the aerial fluid. Manganese upon a large quantity of muriatic acid; and having extracted a tincture, and poured into it a few drops of the solution of Prussian alkali, it assumed a bluish colour. Hence he concludes, that iron, nearly in its metallic form, is present in common green glass; for its calx always gives a yellowish colour to glass, and manganese added to a solution of iron in acids destroys the green colour, substituting a yellow one in its room; and in like manner, nitre added to green glass in fusion takes away its colour. The same effect is produced by manganese if added in proper quantity; though, according to the experiments of Mr Scheele, somewhat of a yellowish colour ought to have been communicated by it; and he is of opinion that it was really so, though the quantity of iron was too small to render it distinctly visible. It is also remarkable, that the rays of light passing through glass of this kind, when nearly red hot, appear of a yellowish colour.

Mr Engestrom's experiments on this subject are somewhat different from those of Mr Scheele. Having melted manganese and borax together upon a piece of charcoal, the glass at first assumed the common colour of manganese; but this was repeatedly destroyed, and made to appear without adding any thing. During the operation he took notice of the following phenomena: 1. When a small quantity of manganese was taken, the colour was light, but with a larger it became nearly black; and whatever colour it assumed on the first fusion was manifested also at the second, when it was made to reappear. 2. Manganese, on being melted with borax, effervesces violently; which ceases, however, as soon as the manganese is dissolved. 3. To make the colour of the glass disappear, it was necessary only to direct the blue flame of the candle upon the glass, and that equally and constantly, but not very violently. On blowing more faintly, and allowing the brown flame to touch the place, the colour returned. 4. About the time that the glass becomes colourless, a kind of section or partition is observed in it; and as soon as the colour disappears, the blowing must be immediately discontinued, so that the brown flame shall not afterwards touch the glass. When it is taken out with the forceps, it appears perfectly colourless. 5. This destruction of the colour seems not to happen suddenly, but by degrees; for when the blowing was now and then discontinued before the true mark had appeared, the glass was generally lighter than before, though not quite colourless.

Though our author had been able to discharge the colour thus from glass, and to make it reappear, it seemed doubtful whether this could be done frequently; for having blown the blue flame violently against some glass, the colour of which he had already twice discharged and made to reappear, he found that it could not again be discharged even by constant blowing for an hour. In another experiment, having added a large quantity of manganese, he found that the glass retained its colour even in the utmost heat it could give it, though it always became colourless when warm, but regained its colour in the cold.

In both these experiments the violence of the flame had dispersed and driven off some small globules, which always remained colourless: the reason of this he thinks is, that manganese, or its colouring part, has a strong attraction for a small part only of borax; and that, by means of a violent heat, the superfluous part may be separated, and the rest unite more closely with the earthy particles. The same thing happened likewise with the small globules, which sometimes remained after the mass was taken away, fixed to the charcoal by the violence of the flame. "If this is really the case (says he), it would follow, that by repeating the experiment some of these particles would always separate if a sufficiently strong flame was applied, and it would be impossible to expel the red colour afterwards. I dare not, however, advance this conjecture, though it is grounded on some experiments as a matter of certainty."

Cronstedt observes, that manganese communicates a colour both to glasses and saline solutions. Borax, which has dissolved it, becomes transparent, and assumes a reddish brown or hyacinthine colour; the microcosmic salt becomes transparent, of a crimson colour, and moulders in the air. In compositions for glass it becomes violet with the fixed alkali; but if a great quantity of manganese be added, the glass is in thick lumps and looks black; by scorching with lead the glass obtains a reddish brown colour. Manganese deflagrates with nitre; and the residue, when thus deflagrated, communicates a deep-red colour to its lixivium. The calx, when reckoned to be light, weighs as much as an iron ore of the same texture. It ferments with vitreous compositions, and still more when melted with the microcosmic salt. The colours communicated by manganese to glasses are easily destroyed by the calx of tin or arsenic, and likewise vanish of themselves in the air.

According to Dr Brumich, manganese, when melted with nitre, assumes a green colour. Tin unites very readily with manganese; but zinc not without great difficulty, perhaps on account of its volatility and inflammable nature. White arsenic adheres to it, and, by means of the phlogiston, reduces it to a metallic form. By simple calcination a blackish powder is produced; but if the ignition be continued for twelve days, it acquires a dark-green colour; producing also, sometimes, one of a white or reddish colour. All these various calces, by means of a sufficient degree of heat in a common crucible, run into a yellowish-red glass, which is pellucid, unless from too great thickness.