PUT

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pruffiat does not readily cryftallize, and appearsto be extremely foiuble in water. It is notexaClly determined, whether muriated or ni-trated ftrontian decompoies pruffiat of potafli.Pure barytes added to pruffiated ftrontian,caufes a precipitate of prulnated barytes, but invery fmaff quantity, fo that it appears to decom-poie it only partially, and hence the affinity ofbarytes for this acid is not much ftronger thanthat of ftrontian.

The affinities of the pruffic acid are ex-tremely difficult to be made out on account ofthe peculiar tendency which it has to formtriple and even quadruple combinations.Among the metals, mercury appears to ftandfirft and iron next, and among the earthy andalkaline bafes the order is probably barytes,ftrontian, potafh, and lime.

PUDDING STONE.

This term, invented by the Englifh lapidaries,is applied by them to defignate one particularmineral aggregate, confifting of oblong and•founded pebbles of flint, about the fize of al-monds, imbedded in a hard filiceous cement:the pebbles are ufually black, and the cementis a light yellowifh brown. It is capable ofreceiving a very high polifh, and ufed to bemuch employed in inlaying and other orna-mental works. It is chiefly procured from thecounty of Effex. Many of the French rnine-ralogifts have naturalized this term (Poudingue)and have applied it to all rounded ftones im-bedded in a cement, fo as to make it nearlyfynonimous with the German Agglomerat, and.the Englifh Rubble-Jlone.

PUMICE. Bimftein, Wern. Pierre Pomce,Br.ch.

The colour of this mineral is greyifh white,bluifh or yellowifh grey, greyifh black and red-difh brown. It occurs in mafs and diffemin-ated, being always more or lefs cellular or ca-rious. It is gliflening, fometimes even fhining,with a filky luflre. Its fraCture is fibrous. Its.fragments are indeterminate moderately fharpedged. It is opake, fometimes a little tranflu-cent on the edges. It is foft, that is, is readilyfcratched or pulverized, but its particles whenin powder have a confiderable hardnefs. It isbrittle and very meagre to the touch. Sp. gr.0.91.

It is fufible without addition before the blowpipe into a white enamel. It is compofed ac-cording to Klaproth ®f

) PUT

77.5 Silex

17.5 Alumine1.75 Oxydoflron

96.753.25 lofs

100.

It is generally looked upon as a volcanicproduct. It is wrought in confiderable quarriesin the Lipari illands, which are almoft entirelycompofed of this mineral. It alfo occurs inSicily and Iceland .

Pumice ftone is extenfively ufed for grindingdown metals, glafs, ivory, &c. previous to po-lifhing; it is alfo employed for fmoothing lea-ther, and other fimilar purpofes.

PUTREFACTION.

Putrefaiftion is that fpontaneous procefs ofdecompofition which takes place in all the foftparts of animals, and fome vegetables underproper circumftances of temperature and moif-ture, by which they are finally diforganized,and are chiefly refolved, when with accefs ofair, into a variety of gafeous and volatile com-pounds, which mingle with the atmofphere.

The fubject of putrefaction is ftill very ob-feure, that is to fay, though we know the circum-ftances required for it, and many of the pro-ducts, we are very far from poffeffing exaCtideas concerning the mode in which the confti-tuent parts of the animal body break up theirformer combinations, and form new ones alto-gether different.

PutrefaCtion is naturally a very flow procefs,and the infuperable difguft and loathing attend-ing it feem to forbid a very minute enquiry intothis important phenomenon of nature.

We fhall only mention a few of the faCtsknown concerning it. The circumftances re-quifite to the putrefaction of animal matter,are a temperature from about 40° to the higheftnatural heat, and a conftant moifture in thefubftance putrefying. The operation of a freez-ing temperature in flopping putrefaClion iswell known, by the conftant cuftom in coldcountries of bringing victuals frozen to market,in which ftate they wall keep for any length oftime without any other preparation ; by bodiesof men or other animals remaining unalteredunder ice for many weeks, and the like. Onthe other hand, a fcorching heat alfo preventsputrefaClion, which it does however, appa-rently, fimply by expelling the moifture whichis effential to the procefs, for by any inferior