"Take of solution of potassa, a gallon. Evaporate the water in a clean iron vessel over the fire, until, the ebullition having ceased, the hydrate of potassa melts : pour into proper moulds."

Potassa; olim, Causticum commune acerrimum, Edin. Potassa; formerly, Stronger common Caustic.

"Take of solution of potassa, any quantity. Evaporate in a covered, very clean, iron vessel, until, the ebullition being over, the saline matter flow smoothly like oil, which happens before the vessel becomes red-hot. Then pour it out upon a clean iron plate; cut it into small masses before it hardens, and let it be preserved in well-stopped phials."

Potassa caustica, Dub. Caustic Kali.

"Take of solution of caustic potassa, any quantity. Evaporate it over the fire in a clean iron vessel, until, the ebullition having ceased, the saline matter, on increasing the heat, remains almost quiescent in the vessel. Pour out the melted salt upon a clean iron plate; and while it is concreting, let it be cut into proper pieces, which must be immediately put into a phial closely stopped. During the evaporation the operator must avoid the drops which may be thrown out from the vessel."

Syn. Potasse fondue (F.), Trocknes atzendes Kali (G.), Pietra caustica (I.), Reine Potasche (Dutch).

The concrete potassa procured by these processes is a hydrate, sufficiently pure for medical purposes, but it still contains the same foreign ingredients as the solution. To procure it as pure and free as possible from carbonic acid, the evaporation should be performed in a silver vessel, very quickly; the vessel should be deep, so that the watery vapour which rises may exclude the atmospheric air. It is generally run into moulds, and formed into solid cylinders, which are covered with paper, and kept in well-stopped bottles. The method of Bertholletl for obtaining it in perfect purity, which is usually described in chemical and pharmaceutical works, is too troublesome and expensive to be generally adopted. The following method proposed by Lowitz is more economical.

1 Kali Purum, P. L. 1787. Cauterium potentiate, Lapis causticus, Potassa fusa, P. L. 1824.

A solution of potassa must be evaporated till a pellicle form on its surface, then allowed to cool; and the saline deposit, which consists chiefly of the foreign salts, carefully separated. The evaporation is then to be renewed, skimming off the pellicles that form on the surface of the fluid, which, as soon as these cease to be produced, and the ebullition is ended, must be removed from the fire, and constantly stirred till it is cold. The mass is next to be dissolved in twice its weight of distilled cold water, the solution filtered, and evaporated in a clean iron or silver basin 2 until crystals are deposited. If the heated fluid consolidate into a mass, in any degree, a small portion of water must be added, and the mass again heated to fluidity. The supernatant liquor is left of a brown colour, which, after being kept for some time at rest in well-stopped phials, deposits the colouring matter, and may be again evaporated and crystallized as before. The crystals obtained in the various evaporations, are colourless, pure hydrate of potassa.3

Qualities,-Concrete, pure potassa is a fused hydrate of the salt. It should be a white brittle substance, having the peculiar odour of slaking quicklime, and a degree of causticity which prevents it from being tasted. It is usually, however, of a bluish colour. It attracts water and carbonic acid rapidly from the atmosphere, and is completely soluble in less than its own weight of that fluid at 60°, caloric being evolved during the solution. It dissolves readily in alcohol. When heated to 360°, it melts, and at a red heat is volatilized. Its sp. gr. is 1.706. It unites with sulphur, the acids, many of the metallic oxides, and the fixed oils. Its constituents are, potassa 84.2, and water 15.8, in 100 parts4: or, 1 eq. of potassa (K + O) = 47.15 + 1 water = 9: making the equivalent 56.15. (K + H).

1 Journal de Physique, xxviii. 402.

2 Lowitz orders the evaporation to be performed in a glass retort; but pure potassa, when hot, dissolves glass.

3 Nicholson's Journal, 4to. i. 164.

4 Phillips's Trans, of Pharm. 1837. Potassa is a protoxide of potassium, consisting of 1 potassium = 39.15 x loxygen = 8: making the equivalent 47.15.

Medical properties and uses.- Concrete potassa is used only as an escharotic, for forming issues in diseases of the hip-joint, the spine, and in deep-seated inflammations. It erodes the skin and soft parts beneath it to a certain extent, destroying the life of the part, which is subsequently thrown off as a slough, and an ulcer is left. To prevent inconvenience from its deliquescent nature, the skin should be covered with a piece of calico, spread with adhesive plaster, and having a hole in its centre sufficient to bare the part only where it is intended to apply the caustic. It is much and justly recommended for the removal of strictures of the urethra.