FOR 



FOR 



manufacture of the finer kinds of porce- 

 lain, the ingredients are carefully washed, 

 dried, and ground by a mill to a very fine 

 powder, which is passed through a sieve. 

 This is made into a paste with water, 

 which is well kneaded, so as to be uni- 

 form in composition. The vessels shaped 

 from this paste are baked in earthen pots, 

 to render them tolerably hard and com- 

 pact : they are then covered with the ma- 

 terials tor glazing-, which, in the better 

 kinds of porcelain, consist of a mixture 

 of earths, which form a compound more 

 vitrifiable than the porcelain itself. 



These materials are diffused in a very 

 fine powder in water, into which the 

 baked vessels are dipped : the surface is 

 thus covered with a thin crust, the water 

 being- absorbed. When dry, they are 

 again placed in the earthen pots, and ex- 

 posed to a very intense heat. The solid 

 matter of the porcelain undergoes a semi- 

 vitrification, whence it possesses all the 

 hardness of glass, and has an additional 

 value, in being less brittle, and much 

 more able to bear sudden alterations of 

 temperature : it derives also much beauty 

 from its semi-transparency and white co- 

 lour. The glaz/mg on the surface is, 

 from its greater fusibility, more com- 

 pletely vitrified, arid is, of course, more 

 smooth and impervious. See GLAZING, 

 ENAMELLING, &c. 



PORCH, in architecture, a kind of ves- 

 tibule supported by columns ; much used 

 at the entrance of the ancient temples, 

 halls, churches, Sic. See ARCHITEC- 

 TURE. Such is that before the door of 

 St. Paul's, Covent Garden. When a porch 

 had four columns in front, it was called a 

 tetrastyle ; when six, hexastyle ; when 

 eight, octostylej ike. See TETRASTYLE, 

 &c. 



PORCUPINE. SeeHisTuix. 

 PORE, in anatomy, a little interstice or 

 space between the parts of the skin, 

 serving for perspiration. SteCui'is and 

 PHYSIOLOGY. 



PORES, are the small interstices be- 

 tween the particles of matter which com- 

 pose bodies; and are either empty, or filled 

 with some insensible medium. 



Condensation and rarefaction are only 

 performed by closing and opening the 

 pores. Also the transparency of bodies 

 is supposed to arise from their pores be- 

 ing directly opposite to one another. And 

 the matter of insensible perspiration is 

 conveyed through the pores of the cutis. 

 Sir Isaac Newton shows, that bodies 

 are much more rare and porous than is 

 commonly believed. Water, for exam- 



ple, is nineteen times lighter and rarer 

 than gold ; and gold itself is so rare, as 

 very readily, and without the least op- 

 position, to transmit magnetic effluvia, 

 and easily to admit even quicksilver into 

 its pores, and to let water pass through 

 it : for a concave sphere of gold hath, 

 when filled with water, and soldered up, 

 upon pressing it with a great force, suf- 

 fered the water to ooze through it, and 

 stand all over its outside, in multitudes 

 of small drops, like dew, without burst- 

 ing or cracking the gold. Whence it 

 may be concluded, that gold has more 

 pores than solid parts, and consequently 

 that water has above forty times more 

 pores than solid parts. Hence it is that 

 the magnetic effluvia passes freely 

 through all cold bodies that are not 

 magnetic ; and that the rays of light pass, 

 in right lines, to the greatest distances 

 through pellucid bodies. 



POR1SM, in geometry, has been de- 

 fined a general theorem, or canon, de- 

 duced from a geometrical locus, and 

 serving for the solution of other general 

 and difficult problems. But Dr. Simson 

 defines it a proposition, cither in the 

 form of a problem or theorem, in which 

 it is proposed either to investigate or 

 demonstrate. Euclid wrote three books 

 of Porisms, which are lost; and nothing- 

 remains in the works of the ancient gco 

 metricians on this subject, besides what 

 Pappus hus preserved in his mathemati- 

 cal collections. Dr. Simson, among the 

 moderns, left behind him a considerable 

 treatise on the subject of Porisms, which 

 was printed at the expense of the late 

 Earl Stanhope, who was himself a very 

 able mathematician, and the patron of 

 several persons who had distinguished 

 themselves in that branch of science. 



PO'ROSTEMA, in botany, a genus of 

 the Polyadelphia Polyandria class and 

 order. Natural order of Lauri, Jussieu. 

 Essential character : calyx six-parted, un- 

 equal; corolla none ; filaments nine, with 

 four anthers on each ; capsule covered, 

 four or six-celled, many-seeded. There 

 is but one species, viz. P. guianensis, a 

 native of the woods of Guiana. 



PORPHYRY, in mineralogy, a name 

 appropriated to that rock where grains or 

 crystals of felspar are imbedded in a cer- 

 tain basis, as in horn-stone, pitch-stone, 

 or indurated clay. There are five species 

 of rocks belonging to the porphyritic 

 formation, viz. 1. Horn-stone porphyry ; 

 the horn-stone, which serves as the base of 

 this substance, is generally red or green, 

 and incloses crystals of quartz and fel- 



