of the subject the origin of clay was considered, and it was shown, after M. Ebelmen, that clay, in all cases of its direct production, results from the disintegration and decomposition of igneous rocks, effected by the action of water and of the carbonic acid, and sometimes the oxygen of the atmosphere, on the various silicates, or combinations of silica with bases, entering into the constitution of those rocks. In illustration of this, Ebelmen's recent analyses of the basalt of Crouzet, in Auvergne, (one of the products of the extinct volcanoes of central France,) both in its unaltered and decomposing state, were cited and explained. It appeared from these analyses that in the process of decomposition of this rock, two-thirds of the silica which it originally contains, are removed, together with nine-tenths of the oxide of iron (protoxide in the unaltered, peroxide in the decomposed rock) one-half the lime, ninety-six hundreths of the magnesia, and five-sixths of the potash and soda. In all, fortythree per cent. of the elements of the basalt disappear during decomposition; the alumina alone remaining undiminished, and a quantity of water having become united with that and the residual elements of the decomposed rock. The probability was also shown that the final result of this process of decomposition would be the separation of the remaining bases, together with a further portion of the silica, leaving the alumina combined with the remainder of the silica and with water in the form of a hydrate, constituting, in fact, a variety of clay. A similar process of decomposition, finally resulting in the production of some kind of clay, appears to take place with the silicates of all the igneous rocks, whether related to granite or to trap, whether formed by the agency of heat at great depths within the crust of the earth, or poured forth by submarine or subaërial volcanoes, geologically ancient or modern, upon its surface. Of this process and result, the decomposition of the felspar of granite and other rocks of the same class, and its consequent conversion into Kaolin or Porcelain Clay, is a particular case.

Sir Humphry Davy, it was remarked, had in his Lectures on Agricultural Chemistry, substantially laid the foundation, as Professor Liebig had with full justice admitted, of the modern science of the Chemistry of Agriculture and of Vegetable Physiology; and on the present occasion the merit was claimed for Davy, of also having, in his various works, substantially laid the foundation of the modern science of Chemical Geology. In the instance immediately under consideration he had anticipated Ebelmen and also the French geological chemist, Fournet, in asserting the true agents in the decomposition of felspars and its conversion into porcelain clay to be water and carbonic acid.*

See Philosophical Transactions, 1808, p. 34.

As this substance is the basis of the most important of the fictile wares, and as it may correctly be regarded as forming a type of the clays in general, its natural history was further pursued, nearly as follows.

It had been at first thought by chemists, by Davy, for example, that kaolin differed from felspar merely in consequence of the abstraction of alkali from the elements of the latter, which are in all cases silica, alumina, and a fixed alkali, or an alkaline earth. The French mineralogical chemist Berthier, first proved that a considerable portion of the silica in felspar was removed at the same time with the alkali, in the process of its conversion into kaolin, maintaining that the felspar became separated into a silicate of potash, which was removed by the agency of water, and a silicate of alumina which remained. It has since been maintained also by Dr. Forchhammer, Professor of Geology and Chemistry at Copenhagen, and by MM. Brongniart and Malaguti (both attached to the Royal Porcelain Manufactory at Sèvres), in France, that the removal of the silica in the process by which kaolin is produced, results from the presence of alkali, and the solubility of the alkaline silicate which is separated. The late researches of Ebelmen, however, to whom we are indebted for the first comprehensive investigation of the entire subject of the chemical changes by which the igneous rocks are converted into the materials of many of the sedimentary strata, have shown, that minerals composed of silicates devoid of any alkaline base, are decomposed in nature as readily as felspars, sometimes even more rapidly, and that the removal of the silica in certain cases is much more complete than in that of porcelain clay. The separation of silica from the silicates is, therefore, independent of the presence of alkali.

Adopting and extending the suggestion of M. Fournet (originally made however by Sir H. Davy, as already intimated), that carbonic acid is in reality the agent in the formation of kaolin, Ebelmen has shown that in all the cases examined by him, the decomposition of the silicates of the igneous rocks has been effected by means of oxygen and carbonic acid, the latter decomposing the silicates, and the former converting their protoxide of iron into peroxide. Silica is very soluble in water containing alkaline carbonates; it is sensibly soluble in pure water, and in water charged with carbonic acid. Lime and magnesia are also soluble in carbonated water. Under the prolonged influence then of a liquid charged with carbonic acid, such as are all the waters which filter through the interior of the earth down to great depths, minerals consisting of silicates will be decomposed and dissolved. Accordingly, we find in mineral waters silica itself, and also carbonates, including carbonate of iron, but we never

find silicates in them. But alumina scarcely enters into combination with carbonic acid, nor is it dissolved by carbonated water; while, on the other hand, it has a very strong attraction for silica. Hence, in the decomposition of silicates, it remains, as the final result of the process, united with a certain proportion of silica and with water, in the form of clay, as already exemplified in the final result of the decomposition of the basalt of Crouzet.

The natural process by which clay is produced having thus been reviewed, the particular case of the conversion of felspar into kaolin, and the composition and properties of that substance were considered, various specimens of both minerals being exhibited in illustration. If from 100 parts of the potash-felspar of Halle, analysed by Brongniart and Malaguti, consisting essentially of 62.76 of silica, 19.20 of alumina, and 14.90 of potash, we suppose to be subtracted trisilicate of potash, consisting of 43.95 silica, and 14.90 potash, there would remain 18.81 of silica and 19.20 of alumina, which is almost exactly the composition of the kaolin resulting from the decomposition of that felspar, as determined by the analysis of the same chemists. Tables were exhibited, stating the composition of many other varieties of kaolin, as ascertained by the analyses of different chemists, both English and foreign. The original kaolin of China, from which all other varieties have derived that appellation, contains, according to Malaguti, after the water has been expelled, 76 per cent. of silica, and 17 of alumina, with 6 of potash and soda; the silica being thus the most abundant element, while the alumina is the most characteristic, which is in fact the case with every kind of clay. The kaolin of Japan is very similar in composition, but retains less alkali. That of Seilitz, near Meissen, in Saxony, contains, according to the analysis of Kühn, the water having been expelled, silica 54, alumina 44, with a trace of potash; that of Morl, near Berlin, analyzed by Mitscherlich, silica 71.4, alumina 26, with traces of lime and potash; that of St. Yrieux, in France, according to Berthier, silica 46.8, alumina 37.3, water 13, and potash 2.5. The porcelain clay of Dartmoor, in Devonshire, analysed by Dr. Fownes, is almost identical in composition with the French specimen last mentioned, while Dr. Boase found in that of St. Stephen's, in Cornwall, silica 39.55, alumina 38.05, water 12.50, magnesia 1.45, with an insoluble residue amounting to 8.70 per cent: this was in all probability chiefly silica, and if we add 8.70 to the amount of that earth actually obtained, we shall have as the result proportions of all the ingredients closely corresponding with those stated by Berthier and Fownes in the preceding analyses.

Pure kaolin, in the natural state, would appear to be a compound of single equivalents of silica and alumina, with one or two equivalents of water; and it is remarkable that there is a tendency in the process of decomposition of all silicates containing alumina, to the production, as the final result, of this particular combination.

A view was also taken of the composition of different varieties of pipe-clay and other ordinary clays. The celebrated clay of Stourbridge, in the Staffordshire coal-field, so important in furnace operations, and for the manufacture of glass-house pots, consists, according to Berthier, of silica 63.7, alumina 20.7, water 10.3, and oxide of iron 4. The Devonshire clay, analysed by the same chemist, contains silica 49.6, alumina 37.4, water 11.2. The fine red clay, of which the North-American Indians make their pipes, the locality of which the celebrated American traveller, Mr. Catlin, was the first white man to visit, and which has been named Catlinite from that circumstance, yielded in Dr. Jackson's analysis, silica 48.2, alumina 28.2, magnesia 6 or 7, water 8.4, peroxide of iron 5, with carbonate of lime 2.6, and oxide of manganese 0.6.

The natural history of the principal materials of porcelain and earthenware having now been reviewed, the attention of the Society was directed,-to specimens of the chemical porcelain manufactured at Berlin, and at Meissen in Saxony (the latter termed Dresden ware) which had been kindly contributed for the illustration of the lecture, by Mr. Charles Button, of Holborn, by whom they are largely imported for sale in this country;-and to specimens of stone-ware pottery and porcelain mosaics, exhibited by Messrs. Alfred Singer and Co., of Vauxhall, the manufacturers. The properties and the process of manufacture of the Berlin and Meissen porcelain were also noticed, together with the process recently proposed and patented for the production of an equally perfect description of porcelain in England; but the more complete consideration of this branch of the subject was reserved for the second lecture. Among the characters of the Berlin unglazed or biscuit porcelain, Mr. Brayley mentioned that upon minutely examining it under the microscope, with high powers, it had appeared to him to be perfectly homogeneous, not merely in constitution, but in texture, not the slightest trace of structure being observable in it, but its appearance exactly resembling that which is presented to the naked eye by the jelly called blanc-manger, and by some of the finer varieties of calcedony.*

Ehrenberg's microscopical observations on the structure of porcelain will be noticed in the second lecture.

CONVERSAZIONE,

AT THE HOUSE OF THE PHARMACEUTICAL SOCIETY, November 12, 1845.

INSTEADOf the usual Pharmaceutical Meeting, a Conversazione was held on this occasion, to celebrate the completion of the new Laboratory, which was open for the inspection of the Visitors, Members, and Associates of the Society.

The new Laboratory, which is on the basement, contains accommodation for eighteen pupils, with all the apparatus required for conducting the various processes, the testing of drugs, and analytical or synthetical researches, connected with the science of Pharmacy and its applications.

Several of the above processes were in progress; for instance, the manufacture of sulphuric ether by the continuous process, of pure hyponitrous ether by Professor Liebig's process, of alcohol, carbonate of ammonia, bicarbonates of potash and soda, butter of antimony, nitrate of silver, &c.

The furnaces, sand-baths, stills, drying closets, presses, and other implements, were in operation, in order to give a correct idea of the capabilities of the Laboratory for the education of students in Pharmaceutical Chemistry.

The old Laboratory, which was the only one in use during the last session, was also open for inspection. It contains accommodation for ten pupils, and is furnished with a plentiful supply of apparatus, more particularly for delicate operations on the small scale, not requiring the aid of furnaces. The two laboratories are therefore together fitted for the reception of about thirty students, and the accommodation can be further increased in the event of a demand existing. Sixteen pupils have already entered this season, and several more have signified their intention of entering shortly.

The Curator's room, which is also Mr. Redwood's private laboratory, is a lofty and commodious apartment, provided with an efficient set of furnaces, and every requisite convenience. In this room was exhibited a set of models of crystals, of large dimensions, in wire, ingeniously contrived and made by Mr. Larkin, who was present to explain their object. Here also Messrs. Mayo and Co., Soda Water manufacturers, of 17, Silver Street, Wood Street, exhibited their Patent Syphon Vase and Ice-pail, for Erated Waters, a considerable number of which being charged with soda water, an opportunity was afforded to many of the company present of testifying to the excellence of the beverage, as well as the superiority of this method of supplying rated waters when drank at a party or a dinner-table, over the more