PLATED MANUFACTURE 581 



PIPESTONE. A variety of clay slate. See CATLINITE. 



PISOLITE. The peastone. See LIMESTONE. 



PISOLITIC IRON ORE. An ore made up of small nodules, like peas. See 

 IRON. 



PITA HEMP. The fibre of the American aloe (Agave Americana), used for tex- 

 tile fabrics, and in paper-making. 



PITCHBLENDE. An ore of uranium. See UEANITTM. 



PITCH, MINERAL, is the same as BITUMEN and ASPHALT, which see. 



PITCH of wood-tar (Poix, Fr. ; Peck, Ger.) is obtained by boiling tar in an open 

 iron pot, or in a still, till the volatile matters are driven off. Pitch contains pyro- 

 ligneous resin along with colophony (common rosin), but its principal ingredient is 

 the former, called by Berzelius pyretine. It is brittle in the cold, but softens and 

 becomes ductile with heat. See TAB. 



PITCHSTOWE. A volcanic rock resembling obsidian, but having a pitchy 

 rather than a glassy lustre. 



PIT-COAX. See COAL. 



PITTA.CA.lt, from two Greek words, signifying fine pitch, is one of the principles 

 detected in wood-tar by Eeichenbach. It is obtained by adding baryta-water to a 

 solution of picamar, or of oil of tar deprived of its acid, when the pittacal falls. It is 

 a dark blue solid substance, somewhat like indigo, and assumes a metallic lustre on 

 friction. It is void of taste and smell, not volatile ; carbonises at a high heat without 

 emitting an ammoniacal smell ; is soluble or rather very diffusable in water ; gives a 

 green solution, with a cast of crimson, in sulphuric acid, with a cast of red-blue in 

 muriatic acid, and with a cast of aurora-red in acetic acid. It is insoluble in alkalis, 

 and in alcohol and ether. It dyes a fast blue iipon linen and cotton goods with tin 

 and aluminous mordants. 



PLAGIOCLASE. See FELSPAR. 



PLANE TREE. The Platanus occidentalis, one of the largest of the American 

 trees. The wood of the plane tree is much used for quays ; it is also employed for 

 musical instruments, and for other works requiring a clean light-coloured wood. The 

 False Plane, or Sycamore, is one of the Maple family (Acer pseudo-platanus'). 



PLANTAIN. See BANANA. 



FXiASIMCA. A translucent chalcedony, of a greenish colour and a glittering lustre. 



PLASTER. See MORTAR. 



PLASTER OP PARIS. See ALABASTER and GYPSUM. 



PLASTIC CLAV. Any clay which, when in a moist state, may be kneaded 

 between the fingers, and admits of being moulded into a definite form. 



Plastic clay is not confined to any particular strata, but is found in secondary and 

 tertiary formations, and also in deposits derived from the decomposition of other 

 rocks. In geological nomenclature, however, the term Plastic Clay is applied to those 

 portions of the Lower Tertiary or Eocene strata which intervene between the Chalk and 

 the London clay, in consequence of some of the beds of clay of which they are com- 

 posed being of a plastic nature. Some of the earliest pottery made in the country was 

 manufactured from these clays, dug up at Crendle Common, near Cranborne, in Dorset- 

 shire, where, as well as at Newport in the Isle of Wight, Fareham in Hants, &c., the 

 clay is still dug and converted into pottery. The clay from the Plastic Clay series is 

 generally of a bright brick-red colour, frequently mottled with white, but sometimes 

 (as at Crendle) it is dark purple or nearly black towards the lower part, and this clay 

 is said to be the best as regards quality. The clays of the Plastic Clay burn to a 

 red colour, and are manufactured into bricks, tiles, flower-pots, and other coarse 

 pottery. H. W. B. 



The Plastic Clay series, between the Thanet Sand and the London Clay, is now 

 generally known by Mr. Prestwich's name of the Woolwich and Eeading beds. 



PLATE-CLEANING. Boil 30 grms. of finely-powdered and calcined hartshorn 

 in a quart of water, and while on the fire put as many silver articles in the vessels 

 used for boiling as it will hold, and leave them there for a short time ; then withdraw 

 them, and dry them over the fire; continue this until all the articles have been 

 treated in the same manner ; then introduce into the hartshorn-water clean woollen 

 rags, and allow them to remain until saturated, after which dry them, and use them 

 for polishing the silver. This is also the best substance for cleaning locks and brass 

 handles of room-doors. When the silver articles are perfectly dry, they must be care- 

 fully rubbed with a soft leather. This mode of cleaning is excellent, and much pre- 

 ferable to the employment of any powder containing mercury, as mercury has the effect 

 of rendering the silver so brittle as to break on falling. 

 PLATE CLASS. See GLASS. 



PLATED MANUFACTURE. (Fabrique de plagiie, Fr. ; Silber Plaitirung, 

 Ger.) The silver in this case is not applied to ingots of pure copper, but to an 



