S ER

SHE

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jute or gneifs; the noble Terpentine belongs al-moft wholly to this formation. The otherformation is more recent, is lefs penetrated byveins, and confifts of common ferpentine.

The older ferpentine is found in Corfica,Italy, and Siberia , and of uncommon beautyat Portfoy in Scotland . The more recent fer-pentine occurs in Cornwall, at Zoblitz inSaxony, and various other parts of Germany .

This mineral is capable of being readilyfafhioned in a lathe, and of receiving a verygood polifh, on which account it is formedinto various articles of ornament and ufe, fuchas vafes, chimney-pieces, &c. many of whichare exquifitely beautiful.

SERUM. See Blood.

SHELL.

The {hells of marine animals and of all eggshave long been known to confift principally ofcarbonat of lime, fo that when burned theyafford a purer quicklime than that procuredfrom moft of the common lime-ftones, and onthe fea-coaft {hells are often collefted in quan-tity, and employed for making quicklime. Allfhells therefore effervefce ftongly with acids.

But as they are organized parts of living ani-mals they have fome peculiarities of texturedependent on the mode of their formation, andthe earthy part of which they are compofedmuft have been held together by fome animalcement, and not merely cohering by Ample ag-gregation. Befides this they moftly containfome portion of phofphat of lime, though invery fmall quantity compared with the car-bonat, and alfo in comparifon with the quan-tity entering into bone, and the hard parts ofland animals.

Mr. Hatchett has given a very valuable me-moir on the various fpecies of marine {hells,which is ufeful not only as a regifter of chemicalanalyfes, but as {hewing a diltinction betweenfeveral large claffes of {hell and elucidatingthe mode of their formation. 1

The method of analyfis was fo Ample that itmay be defcribed at firft as applying' to all.

■ The fhell was firft immerfed in dilute nitric orfometimes in acetous acid, and a folution was■effected, without heat, of all the calcareouspart, which fometimes required a digeftion ofa few days. The acid here-; diffolved both thecarbonat and phofphat of lime if prefent, butJ eave the cementing animal membrane,which being of the nature of condenfed albu-men would remain undiffolved. It would alfoleave untouched any animal charcoal produced

* Phil. Tranf.

by previoufly roaftmg the {hell m a refor a {hort time, and charring the cementinggluten, which, when not aCtually in the dateof denfe membrane, would otherwife diffolvein the acid. To die folution pure ammoniawas firft added as long as it produced anyprecipitate, which in thefe circumftances wasphofphat of lime undecompofed. The remain-ing clear liquor was then fully faturated withcarbonat of ammonia, which precipitated allthe carbonat of lime, and thus thefe tvs o cal-careous falts were feparated, and their quantityafcertained. Sometimes however the fubftancewhen of a cellular or porous ftructure, wouldcontain fome flime and argil, too clofely adher-ing to be feparated properly, in which cafe thealumine diffolved along with the calcareousfalts would be precipitated by ammonia as wellas the phofphat, and would be blended with it.In fuch cafes the mixed precipitate was redif-folved in nitric acid and acetite of lead droppedin, which would feparate the phofphoric acidin the form of an infoluble phofphat of lead,and the quantity of acid, (and from this,that of lime united to it) could be thus eafilyeftimated.

There are two great claffes of marine {hells,the teftaceous and the cruftaceous. The formerare by much the moft numerous, and includeall the marine {hell-animals that have not legsand the power of tranfporting themfelves fromplace to place: the cruftaceous {hells are thofeof the lobfter, crab, prawn, and fuch animalsas readily fhift their place, and who carry the{hell merely for protection from external injury,and not alfo, as in the teftaceous, as a place ofrefidence.

The teftaceous fhells again are divifible intotwo claffes, the Porcelldnoits and thofe com-pofed of Nacre or Mother of Pearl , which differfrom each other in the quantity of connectinggluten or membrane, and the mode in which itis difpofed.

The Porcellanous fiells are fo called becaufethey have an afpect like porcelain, with an ena-melled glofly furface, are fmooth, hard, andfonorous, and when broken often exhibit a{lightly fibrous texture. All the univalve fhells,fuch as whelks, limpets., cowries, and moft ofthe beautiful convoluted fhells of tropicalcountries are of this defeription.

Thefe fhells when expofed to a red heat in acrucible for a quarter of an hour, crackled andloft the colours of their enamelled furface, butdid not emit any apparent fmoke, nor any fmellrol. 89 and 90.

20 Z