Alum has also been used for the purpose of checking the rapidity of combustion, in some particular fire-works. In one of the formulæ for the preparation of fire-balls, to be thrown with the hand, or fired from a gun, given in the Memoir on Military Fire-works, as taught at Strasburg, in 1764, there is, besides sulphur, mutton suet, saltpetre, and antimony, nitre of alum, equal to one-fourth of the weight of the compound. That this salt, the supersulphate of alumina and potassa, is used to make paper, as cartridge paper, &c. incombustible, is a fact, with which every one is acquainted.
We might, also, enumerate the uses of glue, isinglass, gum arabic, &c. for similar purposes; and also of wood-ashes, in the composition of the, so called, blind fuse. Light twisted white rope, when soaked in strong ley, or a strong solution of potash, we are informed, will form a slow match that will burn only three feet in six hours.
Potash is obtained from wood-ashes, by lixiviation with water, and evaporation. It contains more or less impurities; and always carbonic acid, from which it is separated by quicklime, the alkali being rendered caustic. Some of the foreign ingredients are burnt off by exposing it to heat in an oven. It then assumes a white, somewhat pearly appearance, and takes the name of pearl-ash, but is still the same alkali.
Wood-ashes, when mixed with quicklime, and lixiviated, produce caustic ley, the strength of which depends on the quantity of alkali held in solution. It is this ley, when boiled with oils, fat, &c. that produces soft soap. Hard soap is a combination of oil or fat, and soda. The quantity of real alkali in potash may be known by the proportion of acid required to saturate a given weight of it. Potash, pearl-ash, salt of tartar, and salt of wormwood are all carbonates of potassa. This alkali is called the vegetable alkali, because it is obtained from vegetables. It is considered to be the hydrated deutoxide of potassium, and when decomposed will furnish potassium.
Table of the saline or soluble products of one thousand pounds of ashes of the following vegetables.
| SALINE PRODUCTS. | ||
| Stalks of Turkey wheat, | 198 lbs. | |
| Stalks of sun-flower, | 349 | |
| Vine branches | 162.6 | |
| Elm | 166 | |
| Box | 78 | |
| Sallow | 102 | |
| Oak | 111 | |
| Aspen | 61 | |
| Beach | 219 | |
| Fern, cut in August, | 116, | or 125 according to Wildenheim. |
| Wormwood | 748 | |
| Fumitory | 360 | |
| Heath | 115 | |
The observations of Mr. Kirwan on potash may be seen in Aikin's Chemical Dictionary.
When a piece of hydrated potassa is placed between two disks of platinum, which are brought in contact with the poles of a galvanic battery, consisting of upwards of 200 pairs of plates, four inches square, the oxygen will separate at the positive surface, and small metallic globules of potassium will be formed at the negative surface. The potassa, in the mean time, will undergo fusion.
Sir H. Davy discovered potassium, in 1807. It may be obtained by means of iron turnings, in the following manner: Heat the iron turnings to whiteness in a curved gun barrel, and suffer potassa, in a state of fusion, to fall upon them very gradually, air being excluded: potassium will form, and collect in the cool part of the tube. For the different facts respecting this metal, consult Sir H. Davy's communications on the subject, and the memoirs of Gay-Lussac and Thenard, Curadeau, &c. See also, Davy's Chemical Philosophy, and Thenard's Traité de Chimie.
Potassa unites with, and neutralizes, acids, and forms salts; the principal of which are the sulphate, muriate, and nitrate of potassa. It unites also with sulphur, phosphorus, &c.