in natural history, a genus of earths, the characters of which are these: They are firmly coherent, weighty, and compact; stiff, viscid, and ductile to a great degree, while moist, smooth to the touch; not easily breaking between the fingers, nor readily diffusible in water; and, when mixed, not readily subduing from it. See Chemistry, no 647, &c.

Clay shrinks remarkably when drying; in so much that Dr Lewis observes, the purity of it may be known by the degree to which it shrinks. He made experiments on it pure, and when mixed with various proportions of sand. Pure clay he found shrunk one part in 18 while drying; but, when mixed with twice its weight of sand, only one part in 30.

The common clays are never free from siliceous earth: the best method of obtaining the argillaceous earth in perfect purity is, by dissolving Roman alum in water, filtering the solution, and precipitating it by mild volatile alkali. When procured by this method, its specific gravity is about 1305; it is soluble in acids with a little effervescence; it forms alum with the vitriolic acid; and deliquescent falters with the nitrous and marine. When dry it absorbs water greedily, and becomes soft, and acquires such a tenacity that it may be moulded at pleasure; it contracts, however, greatly in the fire, by which numerous cracks are occasioned. With a certain degree of heat it becomes so hard as to strike fire with steel, and by thus burning it loses its tenacity, the water being excluded by the approach of its particles towards each other. After having lost this property, it cannot be made to assume it again without being dissolved in an acid, and then precipitated from it. Fixed alkalies also dissolve it in the dry way, as acids do in the moist; but of these last the vitriolic is the most proper, as it may be most easily concentrated.

According to Mr Kirwan, the specific gravity of this earth, when pure, does not exceed 2000. It is exceedingly diffusible in water, though scarcely more soluble than magnesia. It is combinable with acids, from whence it may be separated like magnesia, but can scarce be precipitated by the vitriolic acid, with which it forms alum, a salt that always contains an excess of acid, and has an astringent taste. When in combination with any of these acids, it cannot be precipitated by acid of sugar; a criterion by which it is distinguished from all the other earths; every one of which (terra ponderosa alone excepted, which when united to the vitriolic acid, is not affected by any other excepting that of fluor) is precipitated from the vitriolic, nitrous, and marine acids, by that of sugar. The precipitation of these earths, however, does not take place if there be an excess of the mineral acids, nor does it always appear before the liquors are evaporated. Though clay is hardened by a very strong heat, it cannot be made thereby to assume the properties of lime. By a mixture with calcareous earth it readily melts; and hence M. Gerhard has found it fusible in a crucible of chalk, though not in one of clay. Its fusion is not promoted by fixed alkali, but borax and microcosmic salt dissolve it; the former with a very slight effervescence, but the latter with a more perceptible one. It is less affected by calces of lead than the calcareous earths are.

M. Beaume has formed a new hypothesis concerning this earth; supposing the basis of alum, or pure argillaceous earth, to be nothing else than flint; and common clay to be siliceous earth combined with a little vitriolic acid. This opinion has been examined by Mr Scheele, who began by trying, in the following manner, whether the siliceous earth be in reality soluble in vitriolic acid. He took an ounce of mountain crystal reduced to powder, and mixing it with three ounces of salt of tartar, melted the whole by a strong fire. The mass was then dissolved in 20 ounces of water, and as much diluted vitriolic acid poured upon it as was more than sufficient for saturation. The liquor being then filtered and evaporated, yielded a drachm and an half of alum, besides a quantity of sub-acid vitriolated tartar. It now remained therefore to determine whether the precipitated siliceous earth, by a repetition of the same process, would still continue to yield alum. The operation was therefore repeated seven times, and a quantity of alum procured at each operation. But when our author was about to be confirmed in his opinion that M. Beaume was in the right, he happened to inspect his crucibles, and perceived them to be full of little cavities, and everywhere rough and uneven on the inside. Thus he began to suspect that the alkali had dissolved part of the clay of his crucibles, by which means the alum had been produced; and this supposition was verified by his afterwards using a crucible of iron, when he could not obtain a single particle of alum, nor perceive the smallest mark of solution on the siliceous earth.

M. M. Beaume also pretends that clay contains a little vitriolic acid, and is therefore soluble in a large quantity of boiling water. Mr Scheele likewise tried this experiment; but found, that of several kinds of argillaceous earth not the smallest quantity was dissolved; and he likewise made several experiments in order to obtain vitriolic acid from pure clay, but without success: neither was he able to obtain any hepar either by means of alkali or tartar or with charcoal; nor could he obtain with clay a vitriolic neutral salt from the residuum of the distillation of muriatic and nitrous acid.

The contraction of clay by heat has afforded Mr Wedgewood an opportunity of constructing by its means an instrument for measuring those degrees of heat which are above the reach of the scale of common thermometers, as described under the article Thermometer.

Mr Scheele has made several experiments to discover the properties of alum when mixed with other substances. A solution of alum, he finds, is decomposed by lime-water; and if no more of the water be added than is exactly requisite for the precipitation, the earth of alum forms a transparent precipitate like boiled starch; and if the clear water be filtered, it is found to be a solution of gypsum. On adding more lime-water than is necessary for precipitation, the precipitated matter is destitute of the gelatinous appearance just mentioned. If the whole be allowed to stand for a quarter of an hour, and frequently agitated during this time, no gypsum, nor even lime, is found in the filtered liquor, unless too much lime-water has been used. On examining the precipitate in this case, our author found it to consist of earth of alum, selenite, and lime. This was discovered first by treating it with muriatic acid, which dissolved the aluminous earth, leaving the gypsum behind. The addition of caustic volatile alkali threw down a transparent gelatinous mass, which was the earth of alum; and on straining it again, and then adding a fixed alkaline lixivium, the lime was thrown down; whence it appeared, that the lime and gypsum had separated from the water, and united with the earth of alum.

To understand the reason of this uncommon precipitation, Mr Scheele next poured into a solution of alum a quantity of caustic volatile alkali more than sufficient to saturate the acid, in order to be certain of having it all taken off. The precipitated earth was then educated, and mixed with a solution of gypsum, that he might observe whether the gypsum would separate from the water, and precipitate with the earth of alum; which, however, did not take place. On mixing lime-water with the precipitate, he found that the former very soon lost its caustic taste, and that the earth of alum became opaque. Some part of the water was strained, and lixivium tartari dropped into it; but it remained clear, nor was any precipitate formed by a solution of corrosive sublimate. He afterwards added muriatic acid to the last precipitate, which it dissolved entirely without leaving any gypsum behind; whence our author concludes, that the earth of alum had united with the lime into a peculiar kind of compound.

Lastly, he now imagined, that this compound of earth of alum and lime might be capable of separating gypsum from water. To try this, he prepared a large quantity of the compound earth, mixed it with a solution of gypsum, and let it rest for a quarter of an hour; when he found, to his surprise, that the gypsum still remained suspended in the water, and that the precipitate was entirely soluble in muriatic acid. He now mixed a solution of gypsum with lime-water, adding earth of alum at the same time; when he found, that the whole was precipitated as before, the lime and aluminous earth having fallen to the bottom along with the gypsum, leaving the water pure. On the whole, our author concludes, 1. That the vitriolic acid in gypsum is capable of combining with more lime than is necessary to an exact saturation. 2. That calcareous earth is capable of forming an union with the earth of alum. 3. That gypsum cannot combine with the earth of alum; but that if a superfluous quantity of lime be united with vitriolic acid, it will then serve as a bond of union to combine gypsum with the earth of alum, and thus form a new compound consisting of three earths. Pure clay has no effect upon limewater.

Cronstedt is of opinion, that common clay, especially the blue, grey, and red kinds, may derive their origin from mud; and as the mud proceeds from vegetables, it will thence follow, that the varieties of clay just mentioned are nothing else but the common mould altered, after a length of time, by means of water. This opinion, he thinks, receives considerable strength from the following circumstances; viz. that a great quantity of sea-plants rot every year in the lakes, and are changed into mud; very little of which, however, is seen upon the shores after the water is dried in the summer-time; and that the clay begins where the mud ceases. Professor Bergman has likewise hinted, that pure clay may be a calcareous earth combined with some acid not yet discovered; "but (says he) compositions of this kind ought to be considered as primitive substances, with respect to our knowledge of them, till they shall be experimentally decomposed: for no sound knowledge in natural philosophy can be obtained from the consideration of mere possibilities; since daily experience shows, that even the most probable suppositions have proved false, when the means of putting them to the test have afterwards been found out."

"Now, therefore, (says M. Magellan), that the argillaceous is acknowledged to be a simple primitive earth, which cannot be decomposed into any other principles, nor formed by the combination of any other simple substances we know, we ought to rest satisfied at present without endeavouring to account for its formation."

The principal species of the argillaceous earths or clays are,

1. The argilla aeruta, or lac luna. It is generally found in small cakes of the hardness of chalk; like which, also, it marks white. Its hardness is nearly like that of the fleatites, and it feels less fat than clays commonly do. It is of a snow-white colour, and about the specific gravity of 1.669. When examined with a microscope, it is found to consist of small transparent crystals; and, from Mr Schreber's experiments, appears to be an argillaceous earth saturated with fixed air, in consequence of which it effervesces with acids. It contains also a small quantity of calcareous earth, and sometimes of gypsum, with some slight traces of iron. It is found at Halles.

2. The argilla apyra, porcelain clay, the kaolin of the the Chinese, is very refractory in the fire, and cannot in any common strong fire be brought into fusion farther than to acquire a tenacious softness without losing its form. When broken, it has then a dim shining appearance, and is of a solid texture; strikes fire with steel; and has consequently the best chemical properties of any substance whereof vessels can be made. It is found of an excellent quality in Japan, and likewise in different parts of Europe. In Sweden it is met with in coal-pits between the strata of coal. Cronstedt informs us, that he has seen the root of a tree entirely changed into this kind of earth.

M. Magellan remarks, that we must be careful to distinguish between the pipe-clay of which there is plenty in Devonshire, and that used in the porcelain manufactures. The former, in a strong fire, burns to a bluish grey or pigeon colour, the latter remains white. The porcelain clay, according to our author, seems to be only a decayed feldspar; and, consequently, according to Mr Bergman, contains magnesia. Our porcelain clay contains likewise quartz, crystals, and mica, parts of the granite which it originally composed. Before it is used, the quartz must be separated, but the mica remains.

3. Combined with phlogiston, and including the white tobacco-pipe clay, with others of a grey, black, or violet colour. Mr Kirwan observes, that many of the white clays become grey in a low degree of heat, because the mineral oil with which they are mixed burns to a kind of coal, and tinges them; but this being consumed in a stronger heat, they again become white. The other clays evidently contain phlogiston; in consequence of which, they become quite black internally on being exposed to a quick and strong fire, assuming the appearance of common flints both in colour and hardness; but if heated by degrees, they are first white, and afterwards of a pearl colour. They contain a larger quantity of the inflammable principle in proportion to their apparent fatness; which may be judged of both by their smoothness and unctuousness, and by their shining when scraped with the nail. "It is difficult (says M. Magellan) to determine whether this strongly adherent phlogiston is the cause of the above-mentioned pearl-colour, or prevents them from being burned white in a strong fire; yet no heterogeneous substance can be extracted from them except sand, which may be separated from some by means of water, but does not form any constituent part of clay. If they be boiled in aqua-regis in order to extract their iron, they lose their viscosity." In the least unctuous clays, our author has found pure quartz in greater and smaller grains, and he has likewise found that clays of this kind sometimes attract phlogiston in the fire.

4. The lithomarga, or stone-marrow, when dry, feels as fat and slippery as soap, but is not wholly dissoluble in water. When mixed with this fluid, it falls to pieces either in larger or smaller masses, so as to assume the appearance of curds. In the fire it readily melts into a white or reddish frothy flay; which, in consequence of its internal vacuities, is then of a larger volume than it formerly was. In the mails it breaks into irregular scaly pieces. This kind is called fuller's earth (vasklera) in Sweden. In Crime Tartary it is called kefekit; and is said to be used there instead of soap, for washing. It is found also in the Austrian Flanders in the barony of Hierges, near Niverle, belonging to the Duke of Arenberg. It was showed to M. Magellan by the Duke's chancellor; who, from the uprightness of his behaviour, has obtained the honourable appellation of Jean de Bien. At present it is only found in separate masses; but M. Magellan is of opinion, that some considerable strata of it might be met with, if properly searched for on the spot, by digging the ground to a considerable depth.

To this species also belongs the yellowish-brown earth called terra lemmia; which is of a shining texture, and falls to pieces in water with a crackling noise. According to Mr Bergman, this is a compound of the argillaceous, siliceous, and magnesian earths. Its component parts are the same as those of the talc, but looser, and in different proportions. M. Cronstedt remarks, that "the terra lemmia cannot properly be called a fuller's earth, as it is never used in the fulling business, nor is likely to be applicable to it, as being besides very scarce. The true fuller's earth of England agrees entirely with the description of the stone-marrow already given, and in colour and texture resembles that from Sweden, which is composed of coarse particles. The Hampshire fuller's earth is of a dusky brown, inclining to green, with veins of a faint yellow; and contains a small portion of muriatic acid, and of a yellow oily matter. Every fine clay that does not communicate a colour, is in general fit for the business of fulling; even the excrements of hogs, mixed with human urine, are used for this purpose in various woollen manufactures. The properties required in a good fuller's earth are, that it shall carry off the oily impurities of the woollen cloth, and at the same time thicken it by causing the hairs or fibres to curl up. The best is composed of fine siliceous earth with argilla, and a little calcareous earth without vitriolic acid; a little martial calx, however, is not hurtful, if unattended with any active menstruum.

The terra lemmia is so called from the island of Lemnos, now Statimane, in the Aegean Sea, from whence it is procured. It is likewise called the Turkish earth, on account of its being impressed with the seal of the Grand Signior.

The Swedish fuller's earth is found in a mountain named Ojmund at Ratwick in East Dalecarlia. The stratum is three feet thick, and the mountain itself is chiefly calcareous. It is of an ash colour; harder, and of finer particles, than the Lemnian earth.

"All these substances (the fuller's earths)," says M. Magellan, "are akin to zeolites, and likewise resemble some marles. But in the Ojmundian earths, the connection of the parts is not merely mechanical, as in marles; which on that account effervesc strongly with acids, though they often contain a smaller quantity of calcareous earth or magnesia than the litho marga."

The following table shows the proportion of ingredients in each of the fuller's earths.

| Terra lemmia | Ojmund fuller's earth | Hampshire fuller's earth | |--------------|-----------------------|-------------------------| | Siliceous earth, 47.0 | 60.0 | 51.8 | | Chalk, 5.4 | 5.7 | 3.3 | | Magnesia, 6.2 | 0.5 | 0.7 | | Argilla, 19.0 | 11.1 | 25.0 | | Calx of iron, 5.4 | 4.7 | 3.7 | | Water or volatile matter, 17.0 | 18.0 | 15.5 |

No. 82. 5. Bole, bole, or iron clay, is a fine and dense clay of various colours, containing a large quantity of iron, so that it is very difficult, or even impossible, to know the natural and specific qualities of the bole itself. It is not so easily softened in water when indurated as the porcelain and common clays; but either falls to pieces in the form of small grains, or repels the water, and cannot be made ductile. In the fire it grows black, and is then attracted by the loadstone.

M. Kirwan thinks the term bole a word of such uncertain signification, that it ought to be banished from common use, or at least from every mineralogical treatise. "Some (says he) bestow this name upon very smooth compact clays, consisting of the finest particles: others require besides, that their colour should be red, yellow, or brown, and that they should contain iron." The red generally blacken in the fire; but, according to Rinman, without becoming magnetic. The yellow, when heated, become first red; and, in a strong heat, brown or black. What the Italians call Calamita Bianca, according to Ferber, is a white bole striated like asbestos. The true figillata rubra contains calcareous earth; and, according to Rinman, becomes magnetic after torrefaction. The yellow, red, and brown clays contain most iron, sometimes dispersed through their substance, and sometimes united to the siliceous part: in this case they are fusible with greater difficulty. The yellow calx of iron is more dephlogisticated than the red, and the red than the brown. These clays do not become magnetic after calcination, unless they contain 14 or 15 per cent. of iron.

The soft boles are of various colours, as red, yellow, green, grey, and bluish grey. The red kind is that used in medicine under the name of Armenian bole; an indurated kind of which affords the material for the red pencils. Formerly, when the terra figillate were esteemed in medicine, the druggists endeavoured to have them of all different colours; for which reason they not only sealed up all the natural sorts of clay, but such as had been mixed and coloured artificially; whence the clays of boles was supposed to be much more numerous than it really is. Cronstedt concludes, that "since the greatest part of these terra figillate contain iron, the bole must be a martial clay; and as such it seems to be more fit for medical uses than other clays, if any dead earth must be used internally, when there is such an abundance of finer substances.

The indurated bole or slate is of a reddish brown or grey colour, and is found in most coaleries between the seams of coal. It is met with frequently in pieces like nuts of various sizes; which, when broken, exhibit impressions of plants as the nodules of copper flat: from Ilmenaus contain fish.

6. With facy particles, the horn-blende of the Swedes. This is called horn rock-stone by Wallerius, who places it among the apyrous stones; but Linnaeus has put it among the calcaceous stones by the name of horn-flint, talcium cornueum. It is named talcium friatum by Rinman, and has the following properties:

1. Its specific gravity is never less than 2.660, and frequently rises to 3.880. 2. It has a strong earthly smell, which is particularly sensible on breathing upon it, or pouring hot water on it. 3. A toughness or viscidly is perceived on pounding it in a mortar, as is the case with mica and horn; from which last it derives its name. 4. When pounded it affords a greenish-grey powder. 5. It is said to be fusible per se; though Mr Kirwan informs us, that he could never melt this stone even by the assistance of a blow-pipe. This stone is frequently mixed with pyrites. It is distinguished from the martial glimmer or mica by the flecks being less shining, thicker, and rectangular. It is of two kinds, black, and greenish. The former, when rubbed fine, affords a green powder. It is the cornuum nitens of Wallerius, and is either of a lamellated or granular texture; the former being sometimes so soft as to be scraped with the nail, and its surface frequently as glossy as if it had been greased: the specific gravity being from 3600 to 3880. It does not detonate with nitre, but becomes of a fluff-colour when heated, and then slightly effervesces with diluted nitrous acid; the solution assuming a greenish colour.

In order to discover the principle on which the smell of this stone depends, Mr Kirwan boiled its powder in water; but could not discover, either by the taste or by any other method, that any thing had been communicated to the fluid. An hundred parts of the lamellar part contain 37 of siliceous earth, 22 of pure argillaceous earth, 16 of magnesia, and two of calcareous earth, both in a mild state, together with 23 of calx of iron not much deplogisticated. The greenish kind is of a granular texture, or striated; the specific gravity of a specimen examined by Mr Kirwan was 2683. The common pale, greenish-grey whitestone seems to belong to this species.

7. The zeolite was first discovered by Cronstedt, and by him reckoned a genus distinct from every other; but on a proper chemical analysis, both Kirwan and Bergman have reckoned them among the argillaceous earths; and here M. Magellan observes, that, "it is not so much the quantity as the intensity or predominancy of property that should in general direct us in the classification of mineral bodies; not to mention, that if the rule respecting quantity were rigorously adhered to, the two primitive earths, magnesia and argil, would not be found among the earths; which would doubtless be an absurdity, as Bergman has rightly observed."

The properties of zeolite are,

1. It is a little harder than the fluors, and other calcareous spars; but is forached by steel, and does not strike fire with it. 2. It melts easily in the fire, with an ebullition like borax, into a white frothy flag, which cannot without great difficulty, be brought into a solid transparent state. 3. It dissolves more readily in the fire by the help of mineral alkali, than that of borax or microcosmic salt. 4. It does not ferment with the latter as lime does, nor with the former as those of the gypseous kind. 5. It dissolves very slowly, and without effervescence, in acids, as oil of vitriol and spirit of nitre. With the former a great heat arises, and the powder unites into a mass. By distillation with nitrous acid, some fixed and deplogisticated airs are procured. Some sorts of zeolite, however, found in Sweden, do not melt by themselves in the fire, but are readily dissolved by the acid of nitre into a kind of jelly.

7. The fusible kinds, in the very moment of fusion, emit a phosphoric light. With regard to the component parts of zeolite, M. Bayen is of opinion that it consists of equal parts of siliceous and argillaceous earths, which is also confirmed by M. Guettard; but according to Mr Bergman's analysis, the red zeolite of Adelfores contains 80 per cent. of siliceous earth; 9.5 of argillaceous; 6.5 of pure calcareous earth; and four of water. The white, oval, radiated zeolite of Feroe in Iceland, contains, according to M. Pelletier, 50 of flax; 20 of argillaceous earth; 8 of pure calcareous earth; and 22 of water. According to Mr Mayer's analysis, a radiated zeolite yielded 58.33 per cent. of flax; 17.5 of argill; 6.66 of lime; and 17.5 of water. In general the crystallized kind contain more water than the other. At any rate, though the proportions of ingredients are various, flax always seems to predominate.

In general the zeolites are of a crystalline form, composed of imperfect pyramids turned towards a common centre; their form is sometimes globular, but seldom prismatic. Messrs Faujas and Rome de Ville mention zeolites, of a cubic and other forms, found in Iceland, the Cyclops Islands near Etna in Sicily, the island of Bourbon, &c., their specific gravity is from 2.00 to 3.150; but this last is very rare. Fabroni mentions a semitransparent zeolite from Garphyttan in Sweden, which has an electric power. To the species of zeolite also belongs the lapis lazuli, from which ultramarine is made. See Lapis Lazuli, and Ultramarine.

The sparry zeolite resembles a calcareous spar; but is of a more irregular figure, as well as more brittle. It is found in Sweden of a light red or orange colour.

The crystallized zeolites are met with in greater plenty than the other kinds; and are found in Sweden of various forms and colours. Brunich informs us, that in the north, the countries of the zeolites and of the chalcedony and catoholong pieces are shown as curiosities, in which the zeolite is inclosed in the chalcedony; but this is not sufficient to prove that the one was produced from the other.

Cronstedt observes, that the zeolites have nearly the same qualities in the fire as the boles. The property of swelling in the fire, like borax, is peculiar to the crystallized kind; the others rise only in some small blisters, which are of a white colour at their edges, and instantly cover themselves with a white glairy skin, after which they become quite refractory. According to Bergman they have a great affinity to the scherls; but their component parts are not so strongly connected as to hinder the action of acids, which can destroy their combination, without being previously treated with fixed alkali; its last being a necessary requisite for analyzing scherls. Mr Pazumot is of opinion that the zeolites cannot be a volcanic production, but only a secondary one formed by the decomposition of volcanic earths. Pure basaltes and volcanic lavas have indeed the same component parts with the zeolites; and these last have not yet been found but among volcanic matters: but, as M. Faujas observes, there are many instances of true zeolites being quite buried within the bodies of solid basaltes, some being only fragments, and others complete zeolites; "which, (says M. Magellan), undeniably proves, that the latter have been completely formed before these volcanic masses were produced by subterranean fires.

8. Tripoli used in polishing hard substances. See Tripoli.

9. The common or brick clay, has the following properties. 1. It acquires a red colour, more or less deep, in the fire. 2. It melts pretty easily into a greenish glass. 3. It consists of a mixture of pure clay, siliceous and marl earth, containing also a small quantity of vitriolic acid. It is found in a state of purity of various colours, as red, pale-red, grey, and blue. In some provinces of Sweden a white kind is met with, often in a flaky form, with fine sand between its strata; which when burnt is of a paler colour than any of the preceding, and does not cake well in the fire; it is also more fusible than any of them. In this country also is found a species called, by Cronstedt, fermenting clay, argilla intumescens. It is very like the preceding as to the external appearance and other qualities; but, when both are found in the same place, they seem to be different in regard to the fermenting property of this variety. "This fermentation (says our author) cannot be the effect of the sand mixed with it, because sand is found in them both: and besides, this kind ferments in the same manner when it is mixed with gravel or stones; and then it ferments later in the spring than the other, since by the stones, perhaps, the frost is longer retained in it.

This kind of clay is also found mixed with calcareous earth, in which case it is called Marle. It is also found in an indurated state, and that either pure or mixed with phlogiston and a large quantity of vitriolic acid; in which case it constitutes the ores of alum. It is also found in this state mixed with calcareous earth, forming stone marle.

10. Argillaceous flaky flints. The most remarkable of these are, 1. The schistus regularis, or common house-flint. It is of a bluish purple colour, does not strike fire with steel, and may be slightly scraped with the nail. It is very brittle, of a lamellar texture, and of the specific gravity 2.876; giving a clear sound when in pieces of half an inch thick. It is never transparent, but has a moderately fine grain, effervescing slightly with acids when powdered, but not otherwise. In the fire it loses upwards of 2 per cent. of its weight; detonates slightly with nitre; and then assumes a brownish red colour; however, it is not rendered magnetic by calcination. By a vehement heat it is fusible per se, and melts into a black scoria. It melts with difficulty in the dry way with mineral alkaline, but more easily with borax and microcosmic salt, with little effervescence; and it melts with equal ease in chalk or clay vessels. By digestion for two months in dephtlogificated spirit of nitre, the menstruum assumes a green colour. According to Mr Kirwan, it contains 26 parts of argillaceous earth; 46 of siliceous; 8 of magnesia; 4 of calcareous earth; and 14 of iron. Part of the iron seems to be phlogisticated by a mineral oil united with it; and part dephtlogisticated, or in a red calx. This last is united to the argillaceous part as well as to the siliceous, and cannot be separated without great difficulty. The colour of this flint varies to the pale, to the slightly purple, and to the bluish. The laminae of the last are thicker, their texture coarser, and they contain more siliceous earth. and less iron than the foregoing. Other stones are also made use of for covering houses; but their laminae are much thicker, their surface more uneven, and their texture coarser. They belong chiefly to the sand-stones, or to the calcareous kinds. The dark blue *schilus scriptorius* contains more magnesia and less iron than the foregoing, and therefore effervesces more briskly with acids. Its specific gravity is 2701. 2. The *pyritaceous schilus*, to which also belongs that from which alum is made, is of a grey, blue, brown, or black colour; and is more or less decomposable by its exposure to air, according to the quantity of the pyrites, and the state of the iron in it. When the iron is in a ferruginous state, the schilus will be easily decomposed; but much more slowly, if at all, when the calx is much dephlogisticated. 3. The *bituminous schilus* is generally black, of a lamellar texture, and various degrees of hardness. It never gives fire with steel, but emits a strong smell when heated, and sometimes without being heated. When scraped it does not produce any white mark like the other schilus. M. Magellan mentions a specimen found in Yorkshire which burned like coal, with a strong smell of bitumen.

There are various other species of argillaceous earths, as the flag-stone, sand or free stone, toad-stone, &c. for a description of which see these articles.

Clays are of very extensive use in common life. Some varieties of the porcelain clay become perfectly white in the fire; and it is not to be doubted but they are used in the porcelain manufactories. The indurated porcelain clay, however, cannot be easily heated without cracking; and therefore we can go no great length in hardening it. The boles have almost lost their value as medicines; but are still employed to make bricks, potter's ware, &c. Tripoli is of indispensible use in the business of polishing, and is likewise, on many occasions, used for making moulds to cast metals in.

In agriculture, clay is indispensably necessary; excepting, however, according to Cronstedt, the white and fermenting clays above mentioned, for which no use has yet been discovered. By its coherency clay retains humidity; on which perhaps its chief power of promoting vegetation depends.

Dr Black observes, that clay, when mixed with a large proportion of water, and kneaded a little, becomes a remarkable ductile adhesive mass, which is not easily dissolved in more water, and to render it thin and fluid requires great trouble. Hence it is employed for confining large quantities of water, as in making canals and dykes; but the soil must either contain a great quantity of clay naturally, or some quantity of it must be spread on the bottom; or the water itself must deposit a quantity of clay sufficient to render it tight. Hence also we see the bad effects of allowing cattle to tread much in clay-grounds when wet; for the clay is reduced to such an adhesive mass as not to admit the roots to penetrate the soil, or the water to enter to the roots.

Clay is used in the refining of sugar; for which no other property is requisite than that it may not dry too soon; but that species used in fulling must, if we were to judge *a priori*, besides the benefits of its particles, be of a dry nature, or such as attracts oils; tho' this quality perhaps may not be found in all those clays that are now employed in the business. According to Fabroni, the pure white clay being calcined in a strong heat, acquires a phosphorescent quality.

**Clay**, a town of Norfolk in England, seated on an arm of the sea between two rivers, in E. Long. 0° 30' N. Lat. 47° 28'.

**Clay-Lands**, those abounding with clay, whether black, blue, yellow, white, &c. of which the black and the yellow are the best for corn.

All clay-foils are apt to chill the plants growing on them in moist seasons, as they retain too much water; in dry seasons, on the contrary, they turn hard and choke the plants. Their natural produce is weeds, goose-grass, large daisies, thistles, docks, poppies, &c. Some clay-foils will bear clover and rye-grass; and, if well manured, will produce the best grain: they hold manure the best of all lands; and the most proper for them are horse-dung, pigeon's dung, some kinds of marle, folding of sheep, malt-duft, ashes, chalk, lime, foot, &c.