Potash, Or Potassa, the name usually given to the hydrated oxide of potassium (potassium hydroxide), or caustic potash. Aristotle is one of the earliest writers who mention it. He says that the Imbrians were in the habit of preparing a lye from the ashes of reeds and plants; and Pliny calls the salt made from the ashes of the oak nitrum. The difference between soda and potash was not recognized by the ancients. The alchemists were of the opinion that the alkali of plants was the product of combustion, the same as the gases, and it was not till 1764 that its presence in the unburned plant was shown by Marggraf. The difference between soda and potash was recognized in 1735 by Duhamel, and the presence of potash in minerals was detected in leucite by Klaproth toward the end of the last century. As soil is formed by the disintegration of rocks, the fact that plants take up the potash from the ground as nourishment soon became known, and the alkali in animals was traced to the food also indirectly derived from the soil. The importance of potash to the growth of plants and animals thus became apparent, and the study of this compound has long occupied the attention of chemists.

Potassium hydroxide, or caustic potash (HKO), is of great importance in the arts, chemistry, and pharmacy, not only as the base of numerous useful salts, but for its own independent properties. It is commonly prepared from the carbonate, a solution of which in 10 to 12 parts of water is boiled with about an equal quantity of freshly slaked lime, made in a paste with water, and gradually added to'the solution. The boiling is performed in a clean iron vessel, and is continued for about half an hour. The'lime seizes the carbonic acid of the carbonate of potash, and leaves the hydrate of potash in solution; and the operation is found to be complete when no effervescence is observed on taking out a portion of the liquid and adding a little hydrochloric acid. It may then be drawn off in part from the calcareous sediment, and the remaining portion be recovered by filtering through a cotton filter, protected as much as possible from the air, by which it might again be partially carbonized. The clear solution is now evaporated rapidly jn a polished iron or silver vessel, till it becomes of an oily consistence and hardens on cooling. Before it is allowed to cool it is usually run into cylindrical moulds, and thus is formed in sticks, which are the caustic potash or potassa fusa of the pharmacopoeia.

It contains impurities, as sulphate and carbonate of potash, chloride and peroxide of potassium, oxide of iron, and alumina, from most of which it may be freed by dissolving in absolute alcohol, evaporating, and again fusing. Hydrate of potash may be economically separated from some feldspars and micas by calcining the minerals with lime and leaving the products for some time in contact with water; but the sources which appear likely to supersede all others are the potash minerals, such as syl-vine, kainite, and carnallite, found in enormous quantities in the salt mines of Stassfurt, Germany. The explorations in one locality have developed the existence of a mass of carnallite equal to 6,000,000 tons of chloride of potassium. The pure hydrate is a white solid substance, of crystalline fracture, and specific gravity 1.7. It is very deliquescent, dissolving readily in water, the solution of specific gravity 1.68 containing 51.2 per cent, of the alkali, and boiling at 329° F. It fuses at a low red heat, and at a white heat it volatilizes without separation of the water. It has an acrid taste and corrosive action upon the cuticle, dissolving and decomposing organic tissues.

It is one of the most powerful bases known, and is hence largely employed for decomposing saline compounds, the acids of which it seizes. It absorbs carbonic acid from the atmosphere, and must consequently be preserved in glass-stoppered bottles, and the glass of these must be free from lead. Mixed with the fat oils, it forms soaps; and in various other ways it is a most useful article in the arts, in chemistry, and to some extent in medicine. The pharmaceutical preparation known as liquor potasses is a solution in water of the hydrate, of specific gravity about 1.05, and containing 4.7 per cent, of po-tassa. Its properties as an antacid, etc, are however as conveniently serviceable in the carbonate of potash. In excessive doses its poisonous action is neutralized by vinegar, the milder acids, or the oils. - Commercial potash, the crude carbonate and hydrate, is chiefly obtained from wood ashes, and is the principal portion of the soluble matters which these contain. The alkalies that exist in the soil are derived from the decomposition of different rocks and minerals.

Feldspar and mica, ingredients of granite, are particularly prolific sources of potash and soda; but they cannot be made to yield these alkalies so economically as the plants, which have taken them up in their sap and hold them in a soluble state, combined with oxalic and tartaric and other vegetable acids, and also with silicic and sulphuric acids. By burning the plants, the salts of the vegetable acids are decomposed, and the potash combines with carbonic acid, remaining with the ashes as a carbonate. The ashes, moreover, contain as soluble ingredients carbonate of soda, the sulphates and silicates of potash and soda, and chlorides of the metals, including chloride of potassium; and besides these, insoluble earthy matters, which are of no value in connection with the production of the alkalies. The proportion of these two classes of ingredients varies in ashes obtained from different plants and their parts, ranging generally from 7/10 to 9/10 insoluble, and leaving 1/10 to3/10 soluble. Berthier found the soluble portion of the ashes of oak wood to amount to 12 per cent., of white beech wood 19.22, red beech 16.3, birch wood 16, fir wood 25.7, fir charcoal 50, pine wood 13.6, wheat straw 10, and potato vines 4.2; and other chemists report the ashes of bean vines to contain 33 per cent, of soluble matter, of pea vines 27.8, of rye straw 19.47, etc.

The branches and bark contain more saline matter than the solid wood, a distribution perhaps dependent on the potash existing chiefly in the sap. The stalks of tobacco, potatoes, beet leaves, tansy, sorrel, etc, contain large proportions of potash, and the removal of such products every year from the soil must cause its impoverishment, unless the potash is 'restored in other ways. - When ashes are treated with water a strongly alkaline solution is produced called a lye, and when this is drawn off and evaporated to dryness the soluble salts remain behind. The evaporation used to be conducted in iron pots, and hence the name potash. The manufacture is largely carried on in several wooded countries, especially where it is desirable to clear off the forests for agricultural purposes; but it appears to be northern countries alone that produce supplies for commerce. These are the northern American states and Canada, and Germany, Bussia, and the other countries of the north of Europe. The method pursued in the American forests is to burn the wood in large heaps to ashes. Barrels sawed across in the middle furnish tubs, which are provided with a false bottom perforated with holes and supported upon cross sticks a little above the real bottom.

Straw is laid upon the false bottom, under which is a cock for letting off the lye. The ashes mixed with about 1/20 of lime are placed in the tubs and drenched with successive portions of water, which are allowed to remain for an hour or two. Those first drawn off, being saturated with the soluble salts, are conveyed directly to the evaporating pans; but the succeeding portions, being weak, are retained to use again upon fresh ashes. The pans are of iron, broad and shallow, and with corrugated bottoms to increase the heated surface. When the liquor becomes of sirupy consistence the heat is checked, and the contents of the pan soon solidify. These when cold are dug out with some difficulty and placed aside as crude potashes. They are intensely alkaline, and reddish brown from the carbonaceous matter they retain. They are afterward purified by heat on the floor of a furnace, losing most of the sulphur that may be present, the excess of water, and other volatile matters, the whole loss amounting to 10 or 15 per cent. The product is white, of a bluish or pearly cast, contains a larger proportion of carbonic acid than the crude article, and is known as pearlash.

The effect of the lime added to the ashes is to decompose the sulphate of potash found among the salts of the ashes and recover the potash, while the sulphuric acid is rendered insoluble by combination with the lime, and is retained with this portion of the ashes. - Crude potash and pearlash are both somewhat variable in composition. The former contains a large proportion of hydrate of potash, which upon exposure continually lessens in quantity from absorption of carbonic acid. Pearlash is principally composed of carbonate, but contains varying proportions of caustic potash. The value of either article, as well as that of sodic hydrate and carbonate, is determined by a kind of analysis called alkalimetry. It is generally accomplished by exactly neutralizing with dilute sulphuric acid such a quantity of pure dry carbonate of the alkali as contains exactly 100 grains of real alkali. A quantity of the acid exactly equal to that used in neutralizing the 100 grains of real alkali is then further diluted till it fills a tall graduated vessel called an alkalimeter or burette, which is divided into 100 equal parts.

A large quantity of acid of this strength being prepared may be readily used as a standard measure, because a volume equal to one division of the alkalimeter will exactly neutralize one grain of real alkali. The instrument may be partially filled to any number of degrees with the standard acid, and the alkali to be tested added. When the liquid is found to be neutral by the use of test paper, the impure article tested will contain as many grains of real alkali as the number of degrees of the instrument which were filled with the dilute acid. An improved form of alkalimeter has been contrived by Dr. Mohr of Ooblentz, in which a tube having a small orifice is attached to the bottom, whereby the amount of liquid may be accurately regulated.

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