PUTREFACTION 669 



three, one over the other, as above ; and do this once more, making five times in all, after 

 which the paste is ready for use. Care must be taken, during the rolling, continually 

 to dust the board and the paste with a little flour, to prevent sticking. The paste 

 may now be placed in the dish, or tin, in which it is to be baked, taking care to cut 

 the protruding edges with a pointed and sharp knife, so as to leave the paste all round 

 with a clean cut edge, for otherwise it will not puff up or swell. The thick edges of 

 pies and tarts are made by cutting strips of the paste with the knife, and carefully 

 laying them on all round, taking care to leave the edges quite sharp. The prepared 

 articles are then put in an oven, previously brought to a good heat, and the elastic 

 vapour disengaged from the butter and water will at once cause the paste to swell 

 into parallel layers of great tenacity, and apparently light, but really very heavy, 

 since each of these thin laminae is compact and distinct. It is essential to the success 

 of the operation that the floor of the oven should be hot. 



PUMICE-STONE (Pierre-ponce, Fr. ; Bimstein, Ger.) is a spongy, vitreous- 

 looking mineral, consisting of fibres of a silky lustre, interlaced with each other in all 

 directions. It floats upon water, is harsh to the touch, having in mass a mean 

 sp. gr. of 0'914 ; though brittle, it is hard enough to scratch glass and most metals. 

 Its colour is usually greyish white ; but it is sometimes bluish, greenish, reddish, or 

 brownish. It fuses without addition at the blowpipe into a white enamel. According 

 to Klaproth, it is composed of silica, 77'5; alumina, 17'5; oxide of iron, 2; potassa 

 and soda, 3 ; in 100 parts. The acids have hardly any action upon pumice-stone. The 

 chief localities of pumice are, the Islands of Lipari, Ponza, Ischia, Vulcano, Andernach, 

 upon the banks of the Rhine ; in Teneriffe, Iceland, and Auvergne, &c. 



PUMP. See HYDRATJLIC MACHINERY. 



PUREECK. IKAXtBIiX:. A hard bluish-grey limestone, so called from its being 

 found in the Isle of Purbeck, where it occurs in the upper bods of the formation of that 

 name. Like the Sussex marble, it is susceptible of a fine polish, and is crowded 

 with the remains of a species of freshwater snail (Paludina carinifera), and the 

 beauty of the marble is the result of the pattern produced by the sections of the 

 included shells. These latter are of a much smaller species than those which occur 

 in the Sussex marble, and the difference in the size of the shells affords an easy means 

 of distinguishing between the two marbles. 



Many old sepulchral monuments are partly composed of Purbeck marble ; as are 

 also the slender shafts and columns of many of the Gothic churches of this country, 

 of which there are examples in the Temple Church in London, Westminster Abbey, 

 Winchester and Salisbury Cathedrals, &c. 



Fine blocks of this marble are still quarried in the Isle of Purbeck, but, except for 

 ecclesiastical purposes, it is little used, in consequence probably of its inferiority to 

 other marbles with regard to colour. H. W. B. 



PURPLE OP CAS5XUS, Gold purple (Pourpre de Cassius, Fr. ; Goldpurpur, 

 Ger.). See CASSIUS, PURPLE OF. 



PURPLE OP MOLLUSC A. A viscid fluid, secreted by the Buccinum lapillus, 

 and some other shell-fish. The Tyrian dye of the Greeks, and Imperial Purple of the 

 Romans, was in all probability obtained from the same source, the mollusca of the 

 Mediterranean Sea. See MUREXIDE. 



PURPLE DYES. The purple dyes now obtained by more or less complex 

 processes from coal-tar are so incomparably superior to any others, both in brilliancy 

 and permanence, that their production has opened up a new era in dyeing and calico- 

 printing. See ANILINE, &c. 



PURPURXC ACID is an acid obtained by treating uric or lithic acid with dilute 

 nitric acid. It has a fine purple colour. See- MUREXIDE. 



PURPURIW is the name of a colouring principle, supposed by Robiquet and 

 Colin to exist in madder. See MADDER. 



PURREE. A yellow colouring-matter, imported into Europe from India 

 and China. It is said to be formed from the urine of camels, elephants, and 

 buffaloes, after the animals have eaten the fruit of the mangosteen. Stenhouso, 

 however, believes it to be a vegetable extract mixed with magnesia. It is much 

 used in the manufacture of Indian yellow, which is employed in oil and water- 

 colour painting. 



PUTREFACTION", and its Prevention. (Fdulniss, Ger.) Putrefaction is the 

 spontaneous decomposition of albuminoid or protein and gelatine compounds, when 

 exposed to a limited amount of air. It is the decomposition of bodies containing 

 nitrogen, called by some persons azotised bodies, although they are produced only 

 by life, are the principal means of producing life, and more fitly called zoogens. 



Conditions of the Prevention of Putrefaction. The circumstances by which putre- 

 faction is counteracted, are: 1, the chemical change of the azotised juices; 2, the 

 abstraction of water ; 3, tho lowering of the temperature ; and 4, the exclusion of 



