274

ELEMENTS OF MATERIA MEDICA.

as potassa fusa, or hydrate of potash, is usually more or less coloured(brownish, greyish, or bluish), and not completely soluble in water andalcohol, in consequence of the presence of foreign matters; for purehydrate of potash is white, and dissolves in both water and alcohol.During the solution in water heat is evolved. Its solubility in alcoholenables us to separate it from the carbonate and bicarbonate of potash,both of which are insoluble in this liquid. It has a strong affinity forwater, which it rapidly attracts from the atmosphere, and in consequencebecomes liquid. At a low red heat it fuses, and at a higher temperatureis volatilized. It is odourless, but has a caustic, urinous taste. Itrapidly decomposes organic substances. It possesses the properties ofan alkali in an eminent degree.

(. b.) Of liquor potassm .—This liquid is limpid, colourless, transparent,and inodorous. Its taste is acrid: when prepared according to the Lon-don Pharmacopoeia, its sp. gr. is 1'063. It has a soapy feel when rubbedbetween the fingers. It strongly attracts carbonic acid from the atmo-sphere, and, therefore, should be kept in close vessels. It corrodes flintglass, and is, on that account, directed to be kept in green glass bottles.

Characteristics. —Potash free or combined is recognized by the fol-lowing characters:—The hydrosulphurets, ferrocyanides, and carbonates,produce no precipitate with it. Tartaric (in excess), perchloric, and car-bazotic acids, occasion precipitates of the bitartrate, perchlorate, andcarbazotate of potash respectively. Chloride of platinum throws down ayellow precipitate with potash or its salts. Lastly, the potash saltscommunicate a violet tinge to the flame of alcohol.

The causticity of potash is shewn by its communicating a greencolour to the infusion of red cabbage or syrup of violets; by its reddeningturmeric, and restoring the blue colour of litmus reddened by an acid ;by its not whitening lime water, or effervescing on the addition of an acid;by its soapy feel; by its solubility in alcohol; and by its dissolvingalumina.

Impurities.— Liquor potass ® usually contains a small quantity ofcarbonate of potash. This may be recognized either by lime-water,which renders the liquid turbid, or by a dilute mineral acid, which causeseffervescence. When pure liquor potass ® has been saturated with nitricacid it gives no precipitate on the addition of carbonate of soda, chlorideof barium, or nitrate silver: if the first cause a precipitate it would indi-cate some earthy or metallic impregnation; if there be a precipitate withthe second, which is insoluble in nitric acid, we infer the presence of asulphate ; if the third cause a precipitate, soluble in ammonia, but inso-luble in nitric acid, a chloride is present.

Potassa fusa of the shops contains various impurities, such as sesqui-oxide of iron, carbonate of potash, and silica. These, however, do notmaterially affect its medicinal value, and will not, therefore, requirefurther notice.

Composition. — Pure anhydrous potash has the following com-

position :—

Eq. Eq.Wt. Per Cent. Berzelius .

Potassium .... 1 . 40 . 83'34 . 83'05

Oxygen.1. 8. 16'6'6. 16’95

Potasli. 1 . 48 . 100-00 .lOO'OO