Distillation separates the acid from sul phate of iron, and leaves the brown ox ide of iron, called colcothar.
Vegetable astringent matters, such as nut galls, the husks of nuts, logwood tea, &c. which contain the gallic acid, preci pitate a fine black fecula from sulphate of iron, which remain suspended for a con siderable time in the fluid, by the addi tion of gum arabic. This fluid is well known by the name of ink. See INK. ° The beautiful pigment well known in the arts by the name of Prussian blue, is likewise a precipitate afforded by sul phate of iron.
If two parts of alum, and one of sul phate of iron, be dissolved in eight or ten parts of boiling water, and a solution of prussiate of potash be added as long as any effervescence and precipitation are produced, the precipitate, thoroughly washed by. effusion of boiling water, will have a green colour. This is owing to the yellow oxide of iron thrown down with the prussiate, which must be dis solved by adding muriatic acid. The deep blue powder, insoluble in this acid, is then to be washed and dried for use. According to Professor Proust, the iron in Prussian blue contains .48 of oxygen, and is obtained only from a super-ox) ge nated sulphate ; the precipitate from a pure alkaline prussiate and sulphate of iron with a .minimum of oxygen being white, and containing only .27 of oxygen. This may explain a fact, observed by a French colourman, who, having mixed some Prussian blue and white lead with nut oil, and set it by for some time cover ed with water, found the surface only blue, and all the rest white. On pouring it out on his stone, and beginning to grind it afresh, with intention to add more Prus sian blue, he found the colour gradually returning of itself.. Here it might be supposed that the oxide of the prussiate had parted with oxygen to the oil, or the oxide of lead, or both, thus white, except that on the surface, which was supplied with oxygen from the su perincumbent water ; and that it recover ed its colour by attracting oxygen from the air. But on this supposition it would seem, that light must contain oxygen, since the colour of this paint, spread on wood or paper, returned by exposure to light in vacuo, as well as hi the open air.
The colour of Prussian blue is affected by the contact of iron. Mr. Gill, finding a knife with which he was mixing some Chinese blue acquire a green tinge, spread a little of it, slid afterwards a lit tle Prussian blue, sufficiently diluted, on the blade of a knife, and with a camel hair pencil took off enough to form a tint on paper, and thus continued till he had taken off, in the first instance, thirty-six, and in the second eighty-six, without add ing any fresh colour. These tints dif fered in regular gradation from greenish blue to green, olive-green, yellowish green, yellow, and so on to a buff.
Concentrated nitric acid acts very strongly upon iron filings, much nitrous gas being disengaged at the same time. The solution is of a reddish brown, and deposits the oxide of iron after a certain time, more especially if the vessel be left exposed to the air. A diluted nitric acid affords a more permanent solution of iron, of a greenish colour, or some times of a yellow colour. Neither of the solutions afford crystals ; but both depo sit the oxide of iron by boiling, at the same time the fluid assumes a gelati nous appearance..
Diluted muriatic acid rapidly dissolves iron, at the same time that a large quan tity of hydrogen is disengaged, and the mixture becomes hot. In this, as well as in the sulphuric solution of iron, the same quantity of alkali is said to be required to saturate the acid as before the solution ; whence it is inferred, that the acid is not decomposed, but that the oxidation is ef fected by the oxygen of the water whence also it appears to follow, that the hydrogen must be afforded from the de composed water, and not from the metal.
Carbonic acid, dissolved in water, com bines with a considerable quantity of iron, in proportion to its mass. Vinegar scarce ly dissolves it, unless by the assistance of the air.