POTTERY, PORCELAIN 603 



at a heat approaching to redness, it begins to boil, is volatilised, and converted into a 

 green-coloured gas, which condenses into globules upon the surface of a cold body. 

 Its specific gravity in the purest state is - 865 at 60. When heated in the air, it 

 takes fire, and burns very vividly. It has a stronger affinity for oxygen than any other 

 known substance ; and is hence very difficult to preserve in the metallic state. At a 

 high temperature it reduces almost every oxygenised body. When thrown upon 

 water, it kindles, and moves about violently upon the surface, burning with a violet 

 flame, till it be consumed ; that is to say, converted into potash. When thrown upon 

 a cake of ice, it likewise kindles, and melts a hole in it. If a globule of it be laid 

 upon -wet turmeric-paper, it takes fire, and runs about, marking its desultory paths 

 with red lines. The flame observed iu these cases is owing chiefly to hydrogen, for it 

 is at the expense of the water that the potassium burns, potassuretted hydrogen being 

 formed. 



POTASSIUM, BROMIDE OF. A compound resembling iodide of potassium, 

 and prepared by similar processes. 



POTASSIUM, CHIiORIDE OF. This salt maybe obtained from sea-water or 

 from kelp, but it is now generally prepared by washing carnallite, a double chloride 

 of potassium and magnesium, from which the latter chloride is readily removed by 

 water. Chloride of potassium occurs among the Stassfurt salts as a distinct mineral 

 known as Sylvine. See CARNALLITE ; POTASH ; SYLVINE. 



POTASSIUM, CYANIDE OF. See CYANIDES. 



POTASSIUM, HYDRATE OF. See POTASH. 



POTASSIUM, IODIDE OF. Iodide of potassiiim is usually prepared by 

 digesting 2 parts of iodine, and 1 part of pure iron filings, in 10 parts of water, till 

 they have combined to form a solution of a pale green colour, which is a solution of 

 the iodide of iron. This solution is decomposed with exactly the requisite quantity of 

 carbonate ' of potash, and iodide of potassium is held in solution, the iron salt being 

 precipitated. The iodide is then crystallised out. 



A more recent process consists in digesting 1 part of amorphous phosphorus in 40 

 parts of warm water, and adding 20 of dry iodine. To the solution, carbonate of 

 baryta is first added, and then baryta-water ; the phosphate of baryta is filtered off, 

 and the solution of iodide of barium treated with sulphate of potash, when iodide of 

 potassium and sulphate of baryta are obtained. 



Iodide of potassium is much used in photography to obtain the iodide of silver ; and 

 for this purpose its purity is of great importance. The iodide of potassium of commerce 

 frequently contains carbonate of potash, caustic potash, and the bromide and chloride 

 of potassium. 



POTASSIUM, OXIDE OF. See POTASH. 



POTATO. (Pommc de terrc, Fr. ; Kartoffel, Ger.) The well-known tuber (under- 

 ground stein) of the Sofatmm tuberosum. 



Many methods have at different times been tried for preserving potatoes in an un- 

 changeable state and always ready to be dressed into a wholesome and nutritious dish, 

 but none with such success as the plan of Mr. Downes Edwards, for which he obtained 

 a patent in August 1840. The potatoes, being first clean-washed, are boiled in water 

 or steamed, till their skins begin to crack, then peeled, freed from their specks and 

 eyes, and placed in an iron cylinder, tinned inside, and perforated with many holes 

 one-eighth of an incli in diameter. The potatoes are forced through these by the 

 pressure of a piston. The pulp is finally dried on well-tinned plates of copper, 

 moderately heated by steam, into a granular meal. When this is mixed into a pulp 

 with hot water, and seasoned with milk, &c., it forms a very agreeable food, like 

 fresh mashed potatoes. See STARCH. 



POTATO-STARCH. English Arrowroot. See STABCH. 



POTATO-SUGAR. See STOAE. 



POTSTOXVE, or Lapis ollaris ( Topf stein, Ger.), is an impure variety of steatite used 

 in Germany for ornamental purposes. See TALC. 



POT METAXi. A metal composed of lead and copper, used for making pots. 



POTTER'S ORE. Picked lumps of the sulphide of lead (galena). See LEAD. 

 POTTERY arid PORCELAIN ; EARTHENWARE, STONEWARE. (Engl. and Fr. ; 

 Steingut, Porcellan, Ger.) The French call the potter's art ceramiquc, from the Greek 

 noun /ce'pa/nos, an earthen pot, or burned clay. In reference to chemical constitution, 

 there are only two genera of ceramic ware. The first consists of a fusible earthy 

 mixture, along with an infusible, which, when combined, are susceptible of becoming 

 semi -vitrified and translucent in the kiln. This constitutes fane porcelain or china- 

 ware ; which is also called hard and genuine, or tender and spurious, according to the 

 quality and quantity of the fusible ingredients. The tender or soft porcelain is an 

 earthy body, which is covered with and penetrated by a transparent glaze. The second 

 kind consists of an infusible mixture of earths, which is refractory in the kiln and 



