FOR 



106 



P O R 



Porcelain, with any other substances, but those of which the Chinese 

 1 form their celebrated manufacture. M. Reaumur was 

 the first who examined this subject scientifically. He 

 analyzed the Chinese and European porcelains ; he 

 tried to ascertain their real and inherent qualities by 

 the action of heat ; and the result of his experiments 

 was, that the properties of the two manufactures were 

 essentially different j that European porcelain, when 

 subjected to the fire, underwent perfect fusion ; that 

 the eastern porcelain was unaltered by the fire, and 

 was a half-vitrified substance, in a middle state between 

 the common baken earthenware of vulgar manufac- 

 tures, and true glass. Other philosophers have since 

 made this subject the object of scientific investigation, 

 particularly the late Mr. Wedgewood, though the result 

 of his experiments was not exactly the same as those of 

 Reaumur. Mr. Wedgewood ascertained the fusibility of 

 most of the European porcelains ; that several kinds 

 manufactured in England underwent perfect vitrifica- 

 tion at temperatflres from 90 of his scale to 120 ; that 

 one kind, manufactured at Bristol, showed no symptoms 

 of vitrification at 135 ; and that the Dresden porcelain 

 was still more obstinate in resisting the heat. He also 

 ascertained that common Chinese porcelain did not 

 vitrify completely by any heat, but began to soften at 

 120, and at 156 became so soft as to lose their shape ; 

 that the real King-te-tching porcelain did not soften in 

 any heat that could be applied ; but that, on the con- 

 trary, its internal substance, entirely unaltered, still re- 

 tained its granular texture. In a former part of this 

 article, it was mentioned that earths have recently 

 been discovered in different parts of Europe, possessing 

 similar properties as those of China ; and hence that 

 our European porcelain has of late attained to very 

 considerable perfection. The true and real porcelain 

 seems to be formed when the fusible part requires the 

 greatest degree of heat for that purpose. This is the 

 case with the petuntse of the Chinese, which is allowed 

 to be similar to, if not the same as the feldspar of Corn- 

 wall. The other ingredient, to which the porcelain 

 mass owes its ductility, is called porcelain-clay or 

 earth, which is a substance composed of alumina and 

 silex, in various proportions, and which corresponds in 

 no inconsiderable degree with the kaolin of the east. 

 In Europe, the substance known by the name of soap- 

 rock or steatite, is employed with the kaolin or porce- 

 lain clay, with the view of giving firmness to the in- 

 fusible part of the manufacture. Such being our ad- 

 vantages, we may hope soon to rival the celebrated 



porcelain manufacture of the east ; though probably, Porcelain, 

 even allowing we have the materials, centuries may Porchester. 

 elapse ere our workmen attain to that surprising degree ^ ~~ y ~ m 

 of delicacy, ingenuity, and qp curacy, in conducting the 

 different steps of the progress, for- which the Chinese 

 labourers have, for time immemorial, been so celebrated; 

 and without which all the other advantages we possess 

 can be of no avail. 



The foregoing discussions, however, have had a re- 

 ference almost entirely to the porcelain of the Chinese, 

 and to the art of manufacturing it as practised by that 

 ancient and celebrated people. European porcelains, 

 and the ingredients of which they are made, have been 

 mentioned but collaterally, and as by no means ex- 

 hausting the subject. The materials in this portion of 

 the world being naturally different substances, and ex- 

 hibiting different features from those of China (though 

 they are not widely different in inherent radical pro- 

 perties) necessarily undergo a process of preparation 

 and management considerably dissimilar from those 

 that obtain in the east. For this reason, and because 

 every kind of porcelain manufactured in Europe, par- 

 ticularly what is denominated soft porcelain, are in 

 every respect so analogous to pottery (of which indeed 

 they may be regarded a species) both in internal in- 

 gredients, and in the mode in which they are formed, 

 baked, glazed, and printed, that we beg leave to refer 

 our refers to that article for suitable information on 

 this subject. 



For Reaumur's articles on this subject, see Mcmoires 

 de V Academic des Sciences, 1727 1729 ; for Scheffer's, 

 Swedish Transactions, 1753; for Guettard's, Mem. de 

 t'Acad. des Sciences, 1 765. See Macquer's Chemical Dic- 

 tionary ; Nicholson's Journal, vols. iii. xii. ; Philoso- 

 phical Magazine, vol. iii. ; Annals of Philosophy, vol. 

 iii. ; Murray's System of Chemistry, vol. iii. See also in 

 this work the articles POTTERY and WKDGEWOOD. (&) 



PORCHESTER, a village of England, in Hamp- 

 shire, is about a mile long, extending along the road 

 towards Fareham. It is celebrated chiefly for its an- 

 cient castle, built on a neck of land in the middle of 

 Portsmouth harbour, and consisting of a series of walls 

 and towers, covering an area of about five acres. It 

 served during the late war as a prison for between 

 8000 and 5000 prisoners. The castle exhibits speci- 

 mens of Roman, Saxon, and Norman architecture, and 

 it seems to have been in the possession of the Roman 

 General Vespasian. A full account of it will be found 

 in the Beauties of England and Wales, vol. vi. p. 303. 



P O R I S M S.* 



Pousms. A ORISMS are a species of proposition in geometry much 

 employed by the ancients : they appear to have been 

 highly valued, and to have assisted them greatly in 

 their geometrical researches. The name is almost all 

 that remains to us of their labours, and although, from 

 the singularly curious nature of the subject, and the 

 facilities they afford in the cultivation of every branch 

 of geometrical science, it is improbable that the sub- 

 ject should have occupied the attention of only a single 

 author, yet we are acquainted with but one geometer 

 among the ancients who composed a work expressly 

 on the subject, and, unfortunately for the science,' the 

 whole of that work is lost, if we except a small frag- 



ment preserved by Pappus, in such an imperfect man- 

 ner, as almost to have rendered hopeless any divination 

 of its meaning. The three books on Porisms, by the 

 author of the Elements of Geometry, form the only work 

 expressly devoted to this subject, whose title has been 

 handed down to us ; and even the meaning of the word 

 Porism was involved in considerable obscurity. Pap- 

 pus Alexandrinus, through whom alone we derive any 

 information on this subject, has given two definitions 

 of the word Porism ; the first he blames as insufficient, 

 because it might include some loci, and the second, 

 which he adopts, is so general and indefinite, as to con- 

 vey to us no precise knowledge of their nature. 



The Editor has been indebted for this interesting article to CHAIILES BABBAGE, Esq. F.B.S. L. and E. &c. 



