The terms " potash," "potashes " are very commonly given to this substance, from one of the methods of its preparation, e. pot ashes. Formerly its only source was the ashes of plants, aud about one-third of the total potashes now imported into this country are still the product of the old processes. Tho treatment of tho ashes of plants to obtain carbonate of potassium is a time-hououred institution, and the industry is still an important one in districts where wood is plentiful—in Canada, North America, Russia, Hungary, &c. As the old forests became extinct, other sources of an article of every day use had to be found, and, as will be noted hereafter, the methods of prepa paration are now exceedingly various.
Carbonate of potash is a hard white solid—sometimes a white granular powder—with a strong alkaline reaction and taste. At a temperature somewhat over 800° it melts, losing a portion of its carbonic acid. At a still higher temperature it slowly volatilizes. All the acids, even in a dilute state, decompose it with evolution of carbon dioxide, leaving a salt of the acid employed. It is exceedingly deliquescent, absorbing moisture from the air, and forming a thick oily liquid, some-. times kown as "oleum tartari per deliquium." It is soluble in about its own weight of water at the ordinary temperature, the solubility increasing with the heat applied :— thus, At 0° 100 parts of water dissolve parts of carbonate.
0 112 „ )7 27 )) 040 I) 117 „ 140 „ 77 135° 205 ..
These solutions are always accompanied by the evolution of heat. The following table gives the densities and corresponding percentage composition of various carbonate solutions at 15°.
When a concentrated solution is cooled, crystals of carbonate, having the composition + and belonging to the monoclinic system, separate out. When these are heated to 100° they fall into a powder, losing two-thirds of their water of crystallization. At 130°, the whole of the water is driven off, and an anhydrous carbonate, in the form of a white powder, left.
It has been stated already that there are many methods of preparing carbonate of potash. For laboratory purposes, or where a particularly pure srticle is required, the crystals of cream of tartar (bitartrate of potassium) may be calcined, the residue treated with water, evaporated and dried at a low red heat. Or a stream of carbon dioxide may be passed into a solution of pure hydrate of potassium, and the resulting carbonate evaporated to dryness, and ignited. Or one part of salt petre may he heated with two parts of oxalic acid and a little water, evaporated to dryness, and ignited. Or pure saltpetre may be fused in a porcelain or earthenware crucible, and small pieces of charcoal dropped in till all deflagration ceases.
Upon a manufacturing scale, the following are the principal sources of carbonate of potassium : About one half of the total produce is still made from the ashes of land and marine plants ; one-fourth from sulphate of potassium, produced by the decomposition of the chloride by sulphuric acid, and from various potassium compounds ; the remainder from " Suint," or the wool of sheep impreg nated with the sweat exuded from the skin, felspar and other silicates. Although the last named
source is as yet comparatively untried ground, it should be noted that in reality felspar and the other silicates yield, in the first instance, their potash salts to all the other sources. Many powerful agencies —the oxidizing action Qf the air, the wearing and disintegrating force of water, and especially water containing carbonic acid, the very carbon dioxide in constant expiration by animals—are continually at work breaking up the hardest rocks, and dissolving and carrying away the alkalies contained therein. One Table will suffice to sbow the changes induced by this process of decomposition— felspar being the example.
Manufacture from Wood-ashes.—Potassium salts abound in many land and marine plants, the ashes obtained by calcination showing great diversities in quantity and composition. The carbonates are found usually in the largest proportion, also the chlorides, sulphates, phosphates, and silicates. The following Table will sufficiently exhibit the remarkable diversity in composition ; the proper tions given are of 1000 parts.
A great deal of this diversity is of course due to the soil in which the plants have been grown. The vegetable salts are contained in the juice, therefore the more succulent tie plant the greater the yield of these substances. As a rule, the twigs and leaves yield more potash than the larger growths. Potatoes, chestnuts, heather, broom, furze, sorrel, beet leaves, and tobacco stalks may bo mentioned an yielding upon incineration large quantities of potassium sults. Asa wimple of the composition of timber, the following analysis (Berthier) of the ash of pine wood may be given :— It will he seen what an enormous amount of material has to be treated to obtain, say, one ton of commercial carbonate of potassium. The wood of the pine yields about 1.25 per cent. of ash, and of this not one-fifth consists of carbonate. Large timber is usually selected in preference the smaller growths or herbaceous plants, although these are far richer in potash salts, because the soils in which the trees are grown is not impoverished by a too rapid utilization of their potassium compounds. Without an altogether unremunerative system of artificial preparation of the land, it would be impossible to keep up a supply of the woods which are richest in potash.