590 PORTLAND STONE 



POLYHALXTE. A sulphate of potash, lime, and magnesia, occurring in many 

 salt mines, as at Stassfurt in Prussian Saxony. 



POLYTYPE. A polytype is a cast of a woodcut taken in soft metal by a process 

 now nearly discarded in favour of electrotyping. 



POMADE DIVINE. See BALSAM OF PERU. 



POOLEY MTTNGU. Another name for Musta-paat (Hibiscus cannabinus.) 



POPLAR. (Pcuplier, Fr. ; Pappel, Ger.) The wooden polishing- wheels of the 

 glass-grinder are made from horizontal sections of the stem of this tree. It is used 

 in the manufacture of toys, but not for many other purposes. 



POPLIN. A stuff made of silk and worsted, manufactured in Ireland. The Irish 

 poplins are either watered, brocaded, or tissued : poplins are also made at Norwich. 



POPPY Olli. Much used in painting. See OILS. 



PORCELAIN. See POTTERY. 



PORCELAIN CLAY. See CLAY. 



PORCELAIN JASPER. Clay which has been vitrefied by the igneous rocks. 



PORCELLANOUS SHELLS. See SHHLLS. 



PORPEZITE. A native alloy of gold and palladium, which occurs to some 

 extent in the mines of Gongo Soco in Brazil. 



PORPOISE OIL. See OILS. 



PORPORINO. An Italian glass. 



PORTER is a malt liquor, so called from being for a long period the favourite 

 beverage of the porters of London, and indeed confined exclusively to this class of 

 the workpeople of the metropolis. It is characterised by its dark brown colour, its 

 transparency, its moderately bitter taste, and peculiar aromatic flavour. At first the 

 essential distinction of porter arose from its wort being made with highly-kilned 

 brown malt, while other kinds of beer and ale were brewed from a paler article ; but 

 of late years, the taste of the public having run in favour of sweeter and lighter beve- 

 rages, the actual porter is brewed with a less proportion of brown malt, is less strongly 

 hopped, and not allowed to get hard by long keeping in huge ripening tuns. Some 

 brewers colour the porter with burnt sugar ; but in general the most respectable con- 

 centrate a quantity of their first and best wort to an extract, in an iron pan, and burn 

 this into a colouring stuff, whereby they can lay claim to the merit of using nothing in 

 their manufacture but malt and hops. Porter is now brewed in large quantities in 

 other cities besides London, especially in Dublin. See BEEE. 



PORTLAND ARROWROOT. See ARROWROOT. 



PORTLAND CEMENT is so called because it resembles in colour the Port- 

 land stone. It is prepared by calcining a mixture of the clayey mud of the Thames 

 with a proper proportion of chalk. They make equally good cement in other parts of 

 England and France by mixing chalk or marl with other clays. The materials are 

 reduced to fine powder, and intimately mixed, with the addition of water. The re- 

 sulting paste is moulded into bricks, which are dried and burned. It is of importance 

 that the heat in calcining be sufficiently elevated, otherwise the carbonic acid and 

 water may be expelled, without that reaction between the lime and clay which is re- 

 quired for the production of a cement. It is necessary to employ a white heat, which 

 shall agglutinate and frit the mixture. After this operation the material is assorted, 

 and the portions which are scorified by too much heat, as well as those insufficiently 

 calcined, being set aside, the cement is pulverised for use. It is often advantageous 

 to grind to powder the native mixtures of limestone and clay before burning them, in 

 order to ensure homogeneousness. It will also be seen that calcination at a very high 

 temperature is frequently required to develop the hydraulic character of limestones ; 

 the greater the temperature employed, the more slow is the solidification of the cement, 

 but the harder does it become. See CEMENTS. 



PORTLAND STONE. An oolitic limestone, immediately underlying the Pur- 

 beck strata ; so called in consequence of its development in the island of Portland, 

 situated off the southern coast of Dorsetshire. 



St. Paul's Cathedral, and many of the public buildings of this country, have been 

 built of stone from Portland, and it is still obtained from numerous quarries on the 

 island for transmission to other places, and formerly for the breakwater. 



The quarries from which the stone used for building St. Paul's Cathedral was 

 obtained were situated at the northern extremity of the island, but have been long 

 abandoned in consequence of the stone being somewhat harder and more difficult 

 to work than that met with in other parts of the island. The principal beds of stone 

 quarried in the Isle of Portland are called, in descending order, roach or rochc, 

 rubbly bed, and whit (i. e. white) or best bed. These beds vary much in thickness, but 

 they may be stated to average five and six feet respectively ; some reaching fifteen feet. 



The roach affords large blocks of a hard and durable white stone, particularly 

 adapted for foundations of buildings, docks, breakwaters, and other constructions 





