much water as nitric acid; filter the liquor through paper, and nitro-muriate of cobalt, or more properly muriate of cobalt, will be obtained.

If letters be traced on clean paper with this solution they will be invisible; but by exposing the paper to a gentle heat, they will appear of a beautiful green: this colour will disappear as the paper cools, and may be made to appear and disappear successively.

The blue sympathetic ink is formed as follows:

Experiment 11. Take one ounce of cobalt reduced to powder, put it into a Florence flask, and pour over it two ounces of pure nitric acid. Expose the mixture to a gentle heat, and when the cobalt is dissolved, add, by small quantities, a solution of potash, until no more precipitate ensues. Let this precipitate subside, decant the super-natant fluid, and wash the residuum repeatedly in distilled water until it passes tasteless. Then dissolve it in a sufficient quantity of pure distilled vinegar, by the assistance of a gentle heat; taking care to have a saturated solution, which will be known by part of the precipitate remaining undissolved after the vinegar has been digested on it for some time. This fluid is a solution of oxyd of cobalt in acetic acid, and forms a beautiful blue sympathetic ink, by the addition of about one-eighth part of muriate of soda. This ink, like that made with the nitro-muriatic acid, is not visible without heat, and disappears when the paper grows cold.

Remark. The cause of this singular change of the solutions of cobalt, Mr. Accum observes (vol. ii. p. 152) has not hitherto been explained in a satisfactory manner. It takes place equally in close vessels, so that it can scarcely be ascribed to the action of the air or of moisture. At present it is supposed (but without any direct proof) to be owing to the partial de-oxydation of the oxyd of cobalt by heat, and its re-absorption of oxygen when cold.

These phenomena arrested particularly the attention of former chemists, and accordingly to their fanciful way they gave them the name of sympathetic inks; an appellation appropriated to all liquids; the characters

of which are colourless and invisible when written on paper, but become visible and coloured by undergoing certain processes, and likewise to those which form, characters upon paper susceptible of changing their colour by artificial methods.

The following miscellaneous experiments with these inks are given by Parke.

Experiment 12. Write with a solution of muriate of cobalt, and the writing while dry will not be perceptible; but if held towards the fire, it will then gradually become visible; and if the muriate of cobalt be made in the usual way, the letters will appear of an elegant green colour.

Experiment 13. Write with acetate of cobalt, or with a muriate of cobalt, previously purified from the iron which it generally contains. When the writing is become dry, these letters will also be invisible. Warm the paper a little, and the writing will be restored to a beautiful blue.

Experiment 14. Draw a landscape with Indian ink, and paint the foliage of the vegetables with muriate of cobalt, the same as that used in Experiment 12, and some of the flowers with acetate of cobalt, and others with muriate of copper. While this picture is cold it will appear to be merely an outline of a landscape or winter scene, but on holding it near the fire it will be transformed to a beautiful summer landscape: this again will appear gradually to lose its verdure, and resume its winter dress, on being removed to a cold situation.

Experiment 15. If oxyd of cobalt be dissolved in ammonia, a red solution will be produced, different in colbur from that of all other metallic oxyds.

Experiment 16. Cobalt ores may be analyzed thus: take 100 grains of the ore, dissolve them in nitrous acid, precipitate the iron by the addition of ammonia, and separate it from the solution by a filter. The nickel, which is always found in these ores, may afterwards be precipitated by the addition of a solution of potash, and separated in the same manner as the iron. The remaining solution may be evaporated to dryness,

and the oxyd reduced by the usual fluxes. A small quantity of cobalt for experiment may be readily procured from smalt.

The oxyd of cobalt forms the most permanent blue colour that we are acquainted with; the oil painters use this oxyd mixed with oil in their paintings, which is the reason why the sky and drapery in some old pictures are of so durable a blue.

Zaffre, which we have long imported from Saxony, is an oxyd of cobalt, mixed with three times its weight of ground silex. There are the black, the brown, and the yellow cobalt ores, all which are oxyds of this metal. The white cobalt ore is a sulphuret of cobalt.

Cobalt seems capable of combining with most of the metals.

SECTION XXI.

OF MANGANESE.

Experiment 1. If the black oxyd of manganese (manganese of the shop) finely pulverised, be mixed with pitch and made into a ball, and introduced into a crucible with powdered charcoal,th of an inch thick at the sides, and 4th of an inch thick at the bottom (filling the empty space with powdered charcoal, and then luting on a cover) and the strongest heat that can be raised applied for one hour, manganese in its metallic state will be produced.

Rationale. As the object is in the reduction of metals to dissipate the mineralising substance, in this case the pitch answers as a flux, and carries off the oxygen from the metal. The metal may also be obtained in the following manner :

Experiment 2. Digest the black oxyd repeatedly, with the addition of th of sugar, in nitric acid; dilute the mixture with three times its bulk of water, filter it, and decompose it by the addition of potash; collect the

precipitate, form it into a paste with oil, and put it into a crucible well lined with charcoal. Expose the crucible for at least an hour to the strongest heat of a forge.

Rationale. The object in using the sugar is to disOxygenise the manganese, so as to render it soluble in the acid; after the solution is formed, the addition of potash precipitates the manganese, not however in a metallic form; but in the subsequent part of the process, the oil disengages the oxygen and leaves the metal behind. Or the following process may be used:

Experiment 3 Prepare a saturated solution of sulphate of manganese, bring it to a boiling heat, and add to it, gradually, a solution of tartrite of potash, until no further precipitate ensues; then filter the solution, and wash the precipitate in water, and, when dry, make it into a paste with oil, and proceed as before.

Rationale. The sulphuric acid unites to the potash, and forms sulphate of potash; and the tartarous acid joins to the manganese, and forms a tartrite of manganese, which is decomposable by heat.

In order to preserve specimens of manganese in a metallic state, it is necessary to varnish them, or to keep them immersed in oil, or ardent spirits.

Remark. Manganese has been in use for many years in the manufacture of glass. A number of experi ments were made on it, the result of which proved, that the black manganese contains a peculiar metal; hence the metal received the name of manganese, and the other the oxyd.

It is

In its metallic state, it is of a grayish white colour, and has a good deal of brilliancy. Its hardness is equal to that of iron. Its specific gravity is about 6.8. extremely brittle. It melts at 160° Wedgwood. The metal becomes tarnished on exposure to the air, which takes place by the absorption of oxygen.

We are furnished with three oxyds of this metal, the white, the red, and the black.

Experiment 4. If the black oxyd be put into nitric acid, with a small quantity of white sugar, a solution of the manganese will take place; and if potash be added,

the metal will be precipitated in the form of the white or prot oxyd. This oxyd may also be formed thus:

Experiment 5. Take the residuum left in the retort after the disengagement of oxygenated muriatic acid gas, dilute it with distilled water, and filter it. Then decompose it by gradually adding a solution of potash, wash the precipitate, and dry it.

Remark. In every case in which manganese is oxydized to the minimum, the prot oxyd is formed; hence all these processes have, for their object, the disengagement of superfluous oxygen.

Experiment 6. If the black oxyd be made to unite with sulphuric acid, as we shall presently notice, and the metal afterwards precipitated by the addition of potash, the red or deut oxyd of manganese will be formed.

Experiment 7. If the product of Experiment 4, 5, or 6th be exposed to the air, the black or per oxyd of manganese will result.

Remark. This takes place by the absorption of oxygen: : the black manganese is the same substance as afforded by nature.

Experiment 8. Let six parts of concentrated sulphuric acid be poured upon one of pulverised black oxyd of manganese, and the mixture heated in a glass retort. A vast quantity of oxygen gas will be disenga ged, and the residuum will be a hard white mass, which must be powdered and boiled in water. The fluid, when filtered, affords, on adding a little sugar, a colourless solution, which, by evaporation, yields large, transparent, quadrangular crystals, called sulphate of manga

nese.

Experiment 9. If the coloured solution of sulphate of manganese be exposed to the rays of the sun, it loses its colour, and regains it when removed into darkness. See Light.

Experiment 10. If one part of the black oxyd of manganese and three parts of nitrate of potass, be melted in a crucible till no more oxygen gas is disengaged, a greenish friable powder is obtained, called mineral cameleon, from its property of changing colour during its solution in water.