POTASSIUM (symb. K, equiv. 39, sp. gr. 0.865) is one of the alkaline metals. The letter K is selected as its symbol, as being the first letter of kali, the Arabic word for potash, the letter P being preoccupied as the symbol for phosphorus. The following are the chief characters for this metal. It is of a bluish-white color, and presents a strong metallic luster. At 32° it is brittle, and has a crystalline fracture; at a somewhat higher temperature it is malleable; at 60° it is soft, and of the consistence of wax; at 130° it is completely liquid; and at a red heat it becomes converted into a beautiful green vapor. Its affinity for oxygen is so great that on exposure to the air it imme diately becomes covered with a film of oxide. When heated it burns with a violet flame. Its intense affinity for oxygen is well shown by throwing it into water, on which, from its low specific gravity, it floats. The metal abstracts oxygen from the water and forms oxide of potassium (potash); while the liberated hydrogen carries off a small portion of the volatilized potassium, and, taking fire from the heat evolved by the energetic chem ical action, burns with a brilliant violet flame. The experiment is a very beautiful one, the burning metal swimming about rapidly on the water, and finally disappearing with an explosion of steam, when the globule of melted potash becomes sufficiently cool to come in contact with the water. At an elevated temperature this metal removes oxygen from almost all bodies into the constitution of which that clement enters; and in the laboratory it is often employed to remove any traces of oxygen from hydrocarbons, by distilling the latter with a small quantity of the metal. From the above facts it is obvious that potassium must always be kept in some fluid, such as purified rock-oil or naphtha, which contains no oxygen.
Potassium does not occur in the native state, and can only be obtained by the reduc tion of its oxide, potash. There are three principal modes of reduction, all of which deserve a brief special notice, either on historical grounds or for their practical value.
1. Davy, in 1807, decomposed a fragment of hydrate of potash, by the current of a strong voltaic battery, into potassium, which separated as globules at the negative pole, and oxygen, which was evolved at the positive pole. This mode of procuring potassium
yields only very small quantities, and is expensive; but the experiment was a most important one for the progress of chemistry, as showing for the first time that potash is not, as was previously supposed, a simple body.
2. Stimulated by Davy's discovery, Gay-Lussac and Thenard, in the following year (1808), succeeded in obtaining the metal by purely chemical means in greater abundance, by decomposing potash by means of metallic iron at a white heat. The oxygen of the potash combines with the iron, and the potassium in a gaseous form is condensed in a receiver filled with naphtha, and kept cool.
3. The method now usually adopted consists in the distillation of a mixture of car bonate of potash and charcoal at a white heat, in an iron retort. If proper proportions are taken, the mixture is wholly converted into carbonic oxide and potassium, as is shown in the equation: Carbonate Carbonic of Potash. Oxide.
+ 2C = K 300.
Potassium forms two compounds with oxygen, viz., a protoxide, KO, which consti tutes potash, and is strongly basic, and a teroxide, which does not combine with acids, and of which it is unnecessary to say more than that it is a yellowish-brown sub stance, which is found when the metal is burned in an excess of oxygen gas.
Potash can be procured in the anhydrous form by oxidizing thin slices of the metal in air perfectly free from moisture or carbonic acid. It is white, very deliquescent, and caustic. When moistened with water, it becomes incandescent, and the water cannot be expelled by any degree" of heat. A far more important substance is the hydrate of potash or caustic (K0,110). This is commonly prepared by dissolving carbonate of potash in ten times its weight of water, and gradually adding to the boiling solution a quantity of slaked lime, equal in weight to half the carbonate of potash used. The resulting compounds are carbonate of lime, which falls as a precipitate, and hydrate of potash, which remains in solution; the changes being expressed by the equation: Carbonate of Carbonate of Hydrated Potash. Slaked Lime. Lime. Potash.