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nearly allied to fome of the clays, and whichbecome plaftic by decompofition. Clay how-ever is by no means a mineral fpecies ftridtlyfpeaking, being in moil cafes, perhaps in all,the refult of the decompofition of other mine-rals. It feems advifable therefore to confiderthe property of plafticity as an eflential cha-racter, and to exclude from the clafs of clays allearthy bodies that are deftitute of it.

Mineralogifts have hitherto arranged all theplaftic clays under two fpecies, rather from theeconomical ufes to which they are applied thanaccording to their external characters, compo-fition or geological fituation: the firft fpecies isthe white infufible porcelain clay, and the fecondcontains all the reft confounded together, underthe general appellation Potters’ clay. That thismode of arrangement is very defective will bereadily allowed, and the reader it is hoped willreceive with candour the following attempt ata more fcientific claffification.

Eflential character. Plajtic by intimate mix-ture iirith 'water*

1. Porcelain clay .

Its colour is generally reddiih-white, alfogreyifh and yellowilh-white: it has no luftrenor tranfparency. It occurs either friable orcompaCt; ftains the fingers; adheres to thetongue; is foft but meagre to the feel: is eafilybroken. Sp. gr. about 2.3. It falls to piecesin water, and by kneading becomes duCtile,though not in a very great degree. The Cor-nilh porcelain clay certainly originates from thedecompofition of felfpar, and contains particlesof quartz, mica, and talc, from which it is fepa-rated by elutriation. The Chinefe kaolin alfocontains mica, and is probably of the fameorigin as the Cornifh. The fame remark maybe applied to the French , &c. It is howeverby no means certain that all porcelain clay isderived from felfpar, as it varies confiderablyin its compofition and fufibility; all the kindsindeed are infufible at any temperature lefs thana white heat, but fome, efpecially the Japanefe,are refractory in the moft powerful furnaces.The Cornifh clay, according to Wedgewood,confifts of 60 per cent, alumine, and 40 filex.That from Limoges in France , according toHaflenfratz, contains 62 filex, 19 alumine, 12magnefia, 7 fulphat of barytes. Another fpe-cimen of the fame, analyfed by Vauquelin, gave55 filex, 27 alumine, 2 lime/0.5 iron, 14 water.A porcelain clay analyfed by Rofe, gave 52filex, 47 alumine, 0.33 iron. s.

2. Steatitic clay .

Its colour is a light flefh red paffing into

cream colour; its texture is minutely foliated;it has a flight fomewhat greafy luftre and takesa polifh from the nail. It ftains the fingers, isvery friable, and has a fmooth unCtuous feel.When laid on the tongue it diflolves into afmooth pulp without any gritty particles ; it isvery plaftic and has a ftrong argillaceous odour.It occurs in nodules, in a hard cellular horn-ftone that forms large mountainous mafles nearConway in North Wales, and originates fromthe decompofition of indurated fteatite-

3. Clay from Slate.

Its colour is afh-grey paffing into ochre-yel-low : its texture is foliated: it has a fmoothunctuous feel, and its filiceous particles are fofmall as to occafion fcarcely any grittinefs be-tween the teeth. It occurs in thin beds on thetops of the fofter kinds of flate-rock, and fromits impervioufnefs to water is always foundlining the bottoms of the peat modes withwhich this kind of mountains is generally co-vered, and in thefe fituations it is of a whiteafh colour, being deprived of its iron and car-bon by the acid of the peat. It alfo occurs inthicker beds at the foot of the mountains, but isof a darker colour and lefs plaftic.

4. Clay from Shale.

Its colour varies from greyifh-blue to bluifh.black: its texture is foliated: it has a fmoothunCtuous feel, takes a polifh from the nail, isexceflively tenacious and duCtile, and has but a.flight degree of grittinefs. When burnt it ac-quires more or lefs of an ochre-red colourthofe varieties which are of the deepeft redufually contain before burning a portion of ful-phuric acid. It is for the moft part difficult offufion. It occurs abundantly in all collieries,and is produced by the fpontaneous decom-pofition of the (hale with which the beds ofcoal are covered. A fandy clay of a greyercolour and more refractory nature is procuredfrom the decompofition of the indurated claythat forms the floor of the coal, and is previn-cially called clunch. The Sturbridge clay fromwhich crucibles, glafshoufe-pots, &c. are made,,is of this kind.

5. Clay from Trap .

At the foot of the fofter rocks of trap-for-mation, fuch as wakke, clay-porphyry, and fomevarieties of grunftein and hornblende rock, arefound beds of clay evidently originating fromthe gradual difintegration of thefe by the wea-ther. The colour of this clay is generallybrowniffi grey verging to ochre yellow; it occursin mafs, is of a compact texture, unctuous, te-nacious, gritty and probably is not very infufible-