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potassium Chem.|pəʊˈtæsɪəm|
[In form, mod.L. (Davy 1807), f. potass or potash (see Note there), in accordance with the names of metals in -ium.]
a. One of the elements, an alkaline monad metal, the basis of potash; it is a highly lustrous white metal with a slight tinge of pink, soft at ordinary temperatures, of specific gravity 0·865, being the lightest solid body known except lithium; when exposed to the air it at once tarnishes or oxidizes, and when thrown upon water instantly decomposes it, uniting with the oxygen and causing the liberated hydrogen to burn with a characteristic violet flame. Symbol K (for Kalium); atomic weight 39·1.
1807Sir H. Davy in Phil. Trans. XCVIII. 32 Potassium and Sodium are the names by which I have ventured to call the two new substances. 1812― Chem. Philos. 321 Small metallic globules will appear at the negative surface, which consist of potassium. I discovered this metal in the beginning of October 1807. 1839Ure Dict. Arts, Potassium..is a metal deeply interesting..from its having been the first link in the chain of discovery which conducted Sir H. Davy through many of the formerly mysterious and untrodden labyrinths of chemistry. 1864H. Spencer Princ. Biol. I. ii. x. §92. 276 Potassium alone melts at 136°, sodium alone melts at 190°, but the alloy of potassium and sodium is liquid at the ordinary temperature of the air. 1881Med. Temp. Jrnl. XLVIII. 176 Bromide of potassium in large doses..has a beneficial effect [in dipsomania].
b. attrib. in names of chemical compounds, as potassium carbonate (also carbonate of potassium, of potassa; or of potash, potassic carbonate), K2CO3; so potassium chlorate, potassium chloride, potassium cyanide, potassium hydrate, potassium iodide, potassium oxide, etc.; potassium salt.
1865Mansfield Salts 257 Its Potassium compound. 1869Roscoe Elem. Chem. (1871) 17 Formed by the action of strong sulphuric acid upon a salt called potassium permanganate. 1873Watts Fownes' Chem. (ed. 11) 319 Potassium Bromide is a colourless and very soluble salt. Ibid. 320 Potassium Hydrate, commonly called caustic potash or potassa, is a very important substance, and one of great practical utility. Ibid. 324 Potassium-salts are always most abundant in the green and tender parts of plants.
c. Special comb.: potassium-argon, used attrib. to designate a method of isotopic dating, or results obtained from it, based upon measurement of the relative amounts in rock of potassium 40 and its decay (electron capture) product, argon 40.
1953Bull. Geol. Soc. Amer. LXIV. 1473 (heading) Potassium argon studies at the University of Toronto. 1955Ibid. LXVI. 1711 (heading) Potassium-argon ages of metamorphic and igneous rocks from the Southern Appalachians. 1968Times 3 Oct. 13/5 The duration of the various magnetic reversals is known from potassium-argon dating of land rocks. 1969Bennison & Wright Geol. Hist. Brit. Isles xvi. 362 Earlier pleistocene deposits can only be dated, as yet, by the use of the potassium-argon method..which, because of the long half-life of potassium, is not suited to dating such relatively recent events. 1977Time 7 Nov. 52/2 The age of a fossil can often be determined by analyzing the layer of rock or soil in which it was found and determining, often by the so-called potassium-argon method, just how old the layer is. |
单词 slalom 释义 slalom n 1 ski-race along a zig-zag course marked out by poles with flags......