POTASSIUM

This extraordinary metal was discovered by Davy in the year 1807, and was one of the first fruits of his masterly researches into the chemical powers of electricity.Its properties are so remarkable, that it was for a time doubted whether it could with propriety be placed among the metals; but the progre of discovery has removed all difficulty upon that point, by making us acquainted with other metallic substances, the properties of which are, as it were, intermediate between those of pota ium on the one hand, and the common metals on the other. One of the striking peculiarities of pota ium is mechanical rather than chemical, namely, its low specific gravity, it being the lightest known solid; another is its intense affinity for oxygen, and its consequent energetic action when placed upon water, where it immediately takes fire. The specific gravity of pota ium is -865 at the temperature of 60°; it is solid at the ordinary temperature of the atmosphere; at 80° it becomes soft, and at 150° is perfectly liquid; at 32° it is brittle, and has a crystalline texture. In colour and lustre it resembles mercury. Its attraction for oxygen is such that it immediately loses its brilliancy on exposure to air, and becomes converted into pota a; heated in the air it burns with a purple flame. The equivalent of pota ium is 40, and that of pota a or protoxide of pota ium is 48. When pota ium is heated in oxygen it absorbs a larger quantity of that element and becomes a peroxide, which, however, is immediately converted into protoxide by the action of water. Protoxide of pota ium exists in the state of hydrate in what is called caustic potash, which is a ' compound of 48 pota a -f- 9 water. This substance fuses at a dull red heat: it is very soluble and deliquescent, and powerfully corrodes almost all animal textures. It is the lapis caustic us of old pharmacy. Di olved in water it forms soap ley, or the liquor pota a of the Pharmacopoeia. This solution is obtained by pouring water upon a mixture of equal parts of quicklime and carbonate of pota a; the lime abstracts carbonic acid from the carbonate, and becomes converted into an insoluble carbonate of lime, whilst the evolved pota a is taken up by the water. The solution thus obtained is powerfully alkaline and caustic; and, as it soon absorbs carbonic acid when exposed to air, it should be carefully preserved in well-stopped phials. Free pota a is easily detected and recognised, when in solution, by its alkaline reaction upon proper vegetable colours,' rendering, for instance, the yellow of turmeric brown, and the blue of violet green; when exce of tar- [s. 986]