Central Heat of the Earth.

temperature of the water above 32 degrees Fah. If, then, the heat of the earth's centre amount to 450,000 degrees Fah., as Mr. Cordier deems highly probable-that is to say, about, twenty times the heat of melted iron, even acTHE theory which assumes that at a certain cording to Wedgewood's scale, and upwards depth below the surface of the earth, a vol- of 160 times according to the improved pyrocanic influence is existant, sufficiently power-fluid mass, could not long have a temperature meter-it is clear that the upper parts of the ful to preserve matter in a state of igneous only just sufficient to melt rocks. There must fluidity, is well known to every one acquainted be a continual tendency towards a uniform with the science of geology. This theory, heat; and, until this were accomplished by the however, although it might have appeared interchange of portions of fluid of different probable at a period when but little attention densities, the surface could not begin to consolidate. Nor, on the hypothesis of primitive was paid to the subject, is soon dispelled upon fluidity, can we conceive any crust to have the application of calm reason to its considera-been formed until the whole planet had cooled tion. Necessarily confined as are geological down to about the temperature of incipient researches to a comparatively small section of fusion. the earth, there are but few data upon which "It may be said that we may stand upon the even a plausible conjecture can be founded as hardened surface of lava current while it is to the state of its centre, but everything tends still in motion-nay, may descend into the materially to upset the doctrine of Leibnitz crater of Vesuvius after an eruption, and stand with regard to the above-mentioned circum-on the scoria while every crevice shows that stance. Thus we have in the sea, for instance, the rock is red-hot two or three feet below us; a conclusive proof that the temperature of and at a somewhat greater depth, all is, perwater diminishes from the surface downwards haps, in a state of fusion. May not, then, a at the mean rate of one degree in twenty-eight much more intense heat be expected at the fathoms in temperate latitudes, and one degree depth of several hundred yards or miles? The in twenty-five fathoms in the tropics. Why, answer is, that, until a great quantity of the then, are we to suppose that the contrary is heat has been given off, either by the emission the case with regard to the earth? "The of lava, or in a latent form by the evolution of upturned strata of the earth," says a writer in steam and gas, the melted matter continues to the Westminster Review, "expose rocks which, boil in the crater of a volcano. But ebullition when in their original horizontal position, are ceases when there is no longer a sufficient supin some instances supposed to have been ply of heat from below, and then a crust of buried upwards of twenty miles below the pre-lava may form on the top, and showers of sent surface; but a rupture even of this extreme depth is, as compared with the distance of the centre from the surface (3,956 miles) but as the scratch of a pin on the rind of an orange." That there are many volcanoes which have their origin at a very great depth below the earth's surface, we by no means intend to deny, but these, it should be remembered, are but merely isolated cases of unfrequent occurrence, and in no instances of sufficient magnitude to warrant us in the belief that they are fed by an inexhaustible fire, pervading the very heart of nature.

In Lyell's "Principles of Geology" we find the following most conclusive argument ad

duced:-
:-

scoriæ may then descend upon the surface and remain unmelted. If the internal heat be raised again, ebullition will recommence, and soon fuse the superficial crust. So, in the case, of the moving current, we may safely assume that no part of the liquid beneath the hardened surface is much more above the temperature sufficient to retain it in a state of fluidity.

"It may assist us in forming a clearer view of the doctrine now controverted, if we consider what would happen where a globe of homogeneous composition placed under cirtion of heat, to those above stated. If the cumstances analogus, in regard to the distribuwhole planet, for example, were composed of water, covered with a spheroidal crust of ice "In Mr. Daniel's recent experiments, for 50 miles thick, and with an interior ocean, obtaining a measure of the heat of bodies at having a central heat about 200 times that of their point of fusion, he invariably found that the melting point of ice, or 6,400 degrees Fah.; it was impossible to raise the heat of a large and if, between the surface and centre, there crucible of melted iron, gold, or silver, a was every intermediate degree of temperature single degree beyond the melting point, so between that of melting ice and that of the long as a bar of the respective metals was kept central nucleus, could such a state of things immersed in the fluid portions. So in regard last for a moment? If it must be conceded in to other substances, however great the quan- this case, that the whole spheroid would be tities fused, their temperature could not be instantly in a state of violent ebullition-that raised while any solid pieces immersed in the ice (instead of being strengthened annually them remained unmelted, every accession of by new internal layers) would soon melt, and heat being instantly absorbed during their form part of an atmosphere of steam-on what liquefaction. These results are, in fact, no principle can it be maintained that analogous more than the extension of a principle pre-effects would not follow in regard to the earth, viously established, that so long as a fragment under the conditions assumed in the theory of of ice remains in water, we cannot raise the central heat?"

Painting in Oil.

THE origin of painting in oil has usually been attributed to the Van Eyck, or early Flemish school, Antonella da Messina, who studied under Van Eyck, having first introduced it. into Italy; but recent critical writers have awarded the honour of the invention to the Greeks, while others, proceeding still farther, have stated that the Egyptians were probably acquainted with the advantage of oil as a vehicle for pigments-referring for their authority to stone sculpture figures in the British Museum, which are painted with an unctious vehicle appearing to be oil.

tially Roman:"-and again, "The treatise of Eraclius likewise proves the existence of the art of mixing colours with oil, and of the preparation of canvasses, skins, or panels, with colours ground in linseed oil for the purpose of paintings which were afterwards to be executed in colours ground in the same oil."

Linseed oil varnish composed of simpler materials-as sandarac and mastic resin boiled in the oil-is found to have been used for some time after this in all known recipes. An unsuccessful attempt was made in the fourteenth century to introduce nut oil. That vehicle, so long discarded, was attempted to be restored by the Van Eycks at the commencement of the fifteenth.

"Thenceforward the (nut or linseed) oil varnish, as distinguished again from varnishes composed of resins dissolved in essential oils, still continued to be exclusively used till the close of the fifteenth or beginning of the sixteenth century; when the Italians, who had already adopted a different system from the first improvers of oil painting, began to employ essential oil varnish."

Many of the ancient medical writers seem to have been acquainted with the peculiar properties of walnut, linseed, poppy, and other oils, while Galen mentions the drying properties of linseed and hempseed-and on nut oil is more decisive; and Pliny observes that all resins may be dissolved in oil. Aetins, who wrote at the end of the fifth and the beginning of the sixth century, mentions a drying The foregoing authors form the connecting oil in reference to works of art. The passage links of a history which brings us down to the is curious, and is as follows:-"The medicinal time of the earliest known authority in modern uses of walnut oil are the same with linseed Italian Art, Cennino Cennini. He received and other oils; but it has a use beside these, from his master, Agnolo Gaddi, the scholar of being employed by gilders and encaustic pain- Giotto, such traditions of processes or methods ters; for it dries and preserves gildings and as Cimabue himself had adopted from the encaustic paintings for a long time;" but with Greek practice. That the men in Cennini's all these we do not find oil mentioned as being time availed themselves of such information as applied to paintings; and Mr. Eastlake re- the early writers abound with is shown by marks that it was only used at this early period Vasari's account of Antonio Venziano, a felas a preservative varnish for the gilded low pupil of Cennini-who quitted painting for articles, and not as a mordant for the applica- medicine in consequence of having studied tion of the gilt. Cennini, however, speaks of Dioscorides.-All these notices, then, go to the application of oils combined with other in- show that the Van Eycks were not the ingredients as mordants for gilding-but later ventors of the art of oil painting. Cenwriters only speak of glutinous mordants. It nino Cennini, the first to treat of modes of is, therefore, clear, says Mr. Eastlake, that an manipulation, tells us how to paint in oil on oil varnish composed of inspissated nut oil, or walls, on panels, on iron, or on any other of nut oil combined with a dissolved resin, surface-how to prepare walls for painting in was employed on gilt surfaces and pictures oil-how to prepare an oil fit for tempering with a view to preserve them at least as early colours, and also fit for mordants, by boiling it as the fifth century. It may be added, that a over the fire-again, how to prepare good and writer who could then state, as if from his own perfect oil by baking it in the sun, and how to experience, that such varnishes had the effect grind colours in oil. Such statements do not, of preserving works for a long time, can hardly however, show that this material was applied be understood to speak of a new invention. to the painting of the human form. Oil inLeonardo da Vinci, writing a thousand years corporated with varnish is seen to be in Cenafter Aetius, recommends as a varnish nut oil nini's time applied as a coating to protect thickened in the sun. The Lucca manuscript,gilding; and to be employed as a varnish for published by Muratori-placed by Mabillon in the time of Charlemagne-describes a varnish composed of linseed oil (lineleon) and a needless variety of resins, with which gums even appear to be intermixed, while nut oil is nowhere mentioned in it. The age of Charlemagne was an era in the arts; and the addition of linseed oil to the materials of the varnisher and decorator may thus, on the above evidence, be assigned to it.

Between the tenth and twelfth centuries Eraclius, or Heraclius, is supposed to have written his treatise "De Artibus Romanorum." Mr. Hendrie observes of him, "The art of Eraclius is the school of Pliny, increased, it is true, by Byzantine invention, but yet essen

tempera pictures, while it was used in draperies and ornamented portions of the work. The flesh always appears dry and flat-the consequence of the tempera; and the inspissated or thickened bil makes the surface of either draperies or ornaments of such early pictures seem embossed. This varnish, "vernice liquida," not particularly described by Cennini, was composed according to Mr. Eastlake, of amber or sandarac and linseed oil, with and without concrete turpentine, pece Greca. In England, in the thirteenth and fourteenth centuries, the notices in the account-rolls are numerous of the employment of oils and varmishes. That the great improvement in oilpainting (ultimately leading to its general use)

was first made in Flanders is naturally acDELICATE TEST FOR GOLD.-W. Maugham, counted for "by the necessity of counteracting Esq., in a communication made to the Editor the effects of a humid atmosphere on painted of the Pharmaceutical Times, states that he has surface by hydrofuge or oleoresinous prepara- found the following method of determining tions."-That the Van Eycks carried the pro- the presence of gold of considerable service cess of oil-painting farther, in point of delicacy in examining ores supposed to contain that and refinement, than had been previously done metal: He first digests about five grains of may be inferred from a reference to the picture the ore, finely powdered, in strong nitric acid, by one of these artists in our own National for the purpose of removing all the sulphurets; Gallery compared with such specimens of an- that portion of ore which is insoluble in nitric terior times as exist here or abroad. The acid is then to be well heated on a piece of Strasbourg MS. of the fifteenth century, but platinum foil, so as to drive off any remaining recording practice of an earlier date, must not sulphur, and is next to be digested in aqua be forgotten amongst the documents tending regia. From the solution thus obtained, when to show the early use of oil; but the mention it has become clear, take a drop or two by of its employment-in the different public means of a glass rod, and place it upon a piece buildings at Westminster as well as in Italy- of white porcelain, so as to form a streak. is so constant that attempt at further enumera- Warm the porcelain over the flame of a spirittion here would be mere prolixity, Horace lamp, and, when the liquid portion of the Walpole, the Rev. Mr. Bentham, Mr. Smith solution has evaporated, throw on the spot Raspe, and others, have all shown that paint-where it was placed a jet of flame by means of ing in oil was practised in England from very a blow-pipe: if any gold be present it will be early time up to the fifteenth century. indicated by a beautiful and characteristic purple colour, which, to a certain extent, will disappear when the heat is withdrawn. The same colour may be made to appear and disappear at will by heating and cooling the spot in question. This spot in some instances is not to be distinguished whilst the porcelain remains cold, except by its being duller than the of gold may be discovered by this method. glaze of the porcelain. A very minute portion To accustom the eye to judge of the purple colour produced in the manner described, first practise with what is known to be a solution it from time to time with water, for the purof chloride of gold of a given strength, diluting flame the presence of gold by the peculiar pose of observing to what extent the dilution may be carried, so that under the blowpipecolour produced shall be rendered mani

(To be concluded in our next.)

fest.

THE PATENT ELASTIC INFANT GYMNASIUM; OR, BABY JUMPER.-Under the above title there has lately been introduced to the notice of the public an invention made by an ingenious American (Mr. G. W. Tuttle). Its object is the very laudable one of supplying a nurse free from all the vices to which mortal nurses are heir, and into whose care an infant may be confided, its safety and pleasure being counted upon as a matter of course. But all the panewould hardly suffice to exhibit the good qualigyrics we could pen from this till doomsday ties of this piece of mechanism so well as a brief description. Imagine to yourself, then, gentle reader, suspended from a book affixed to the ceiling, or any other convenient place in WATER-COLOUR PAINTING. There is no an apartment, a strong elastic spring, tastefully style of art so interesting to the mass of the concealed by means of a covering of silk or people of this country as the truly English any other material which fancy may dictate school of water-colour painting, and in no for the purpose; from this are dependant two other country is there any approach to the silken ropes to which a loop is fastened by excellence of our own artists. We are, theremeans of four others, which is intended to pre-fore, delighted to find that the National Galvent the least compression of the chest or lery has received an addition to the pictures at restraint on the motions; and a dress or coat, present exhibiting in the shape of one of the inside of which is affixed a strong band serving pictures by Turner selected by the trustees for a seat, terminates the whole. This, then, is from the Vernon collection. The room approthe machine. The infant, being properly se-priated for the exhibition is the small one on cured within the dress, is left to itself; its the left of the entrance; the picture, the wellslightest movement serves to set the "jumper" in motion, and to produce an agreeable gymnastic exercise at once conducive to the health and amusement of the child. In cases of curvature of the spine, and the various bodily malformations to which infants are peculiarly liable, this invention is truly invaluable, while the pleasure it is sure to afford to young children must be sufficient recommendation for its adoption in the nursery. In point of price, too, its economy forms an important feature. In conclusion, we advise all who are blessed with the fruits of connubial love to pay a visit to the proprietors' establishment, which is at No. 137, Strand, London.

known one of Venice, being hung between two of those recently bequeathed by Mr. R. Simmons. Beneath the picture the trustees have placed a small tablet, on which is inscribed "Venice. By J. M. W. Turner, R.A. Presented by Robert Vernon, Esq., being one of the extensive and valuable collection of the works of British artists given by him to the trustees for the public benefit." The trustees have also selected Mr. Haghe's large watercolour drawing. We sincerely hope that these acts on their part may but merely prove the prelude to the establishment of a collection of the British school of water-colour paintings in the new gallery-when we get it.

FROM

Notices to Correspondents.

NOTICE.

To HOUSE DECORATORS, GRAINERS, CABINET MAKERS, &c.-Beautifully coloured Specimens of the various Woods and Stones will be presented to our Subscribers monthly. The first Plate is in the hands of our Engraver, and will be) shortly ready for delivery. Each Plate will contain four Specimens, while its size will adapt it for binding in the Work.

department of this publication, must be addressed (post paid) as follows:-"To the Editor of the DECORATOR'S ASSISTANT, 17, Holywell-street, Strand, London." No attention will be paid to any addressed otherwise.

METHOD OF REMOVING VARNISH MODERN PAINTINGS.-In removing varnishes of a recent date, pumice stone powder may be employed, and a very soft and fine bottle cork will save the fingers; but nothing will answer so well as the finger on the more delicate tints. For the removal of a strong varnish, as copal, &c., a mixture of spirit of wine and spirit of turpentine will be required. To make these two spirits unite, a small quantity of the salt of tartar (tartrate of potash) is to be added. Every time this is used, the bottle is to be well shaken, very little poured on the picture, and rubbed on with a small piece of flannel; then All communications relating to the literary lay on the part rubbed a few drops of oil of olives to retard the action of the spirits. These operations are to be repeated over the whole picture, frequently changing the pieces of flannel, and as frequently applying the olive oil, in order to see what progress has been made. The picture, lastly, is to be washed with a sponge, soap, and water, afterwards with clean water, and then covered with a fresh varnish. If any stains should be found on the picture so unconquerable as to remain after the above process, a little oil of spike lavender will certainly remove them; but the greatest care must be taken in using this essential oil; it softens old paint so quickly, that there is scarce time to apply it and the olive oil before it has gone too far; it is better to reduce its strength with spirit of turpentine if it should happen to be too genuine. Many use lancets and small scrapers, but this operation has also its risks from scratches, &c.Fielding on the Knowledge and Restoration of Old Paintings.

THE DECORATIVE ART UNION.-Some few weeks since (see p. 30, ante) we cursorily noticed a project announced under the above title. At that period, we could not, of course, be expected to enter deeply into the matter, having scarcely any other information at hand respecting it than that afforded by a prospectus. Since then, however, we have regarded its progress and prospects with especial interest, and have, after mature consideration, come to the conclusion that it is eminently deserving of both the patronage and support of every lover of art. The wide sphere of operation which opens itself before such an institution is only equalled in magnitude by the positive benefits its operations within that sphere are calculated to produce to the community at large. By its means a new channel will be opened for those various productions of the head and hand which have hitherto been restricted in their production by the precarious chance which existed of their disposal after being manufactured, while it will afford an opportunity to all for possessing themselves of valuable works of art at an almost insignificant outlay. The idea of establishing the "Decorative Art Union" originated with the Editor of the Critic, a journal of respectability and influence, and has by his exertions been carried out up to the present time. We see from the last weekly list published up to the time of our writing this, that 178 members have already joined the association. The subscription is half-a-guinea.

Part IX. of the DECORATOR'S ASSISTANT, in a beautifully embellished Wrapper, is now ready, price Sevenpence. Parts 1. to VIII. still continue on sale. The First Volume, beautifully bound in scarlet cloth, in a style designed expressly for this Work, gilt and lettered, price 5s. For those parties who have taken in the Work in Numbers or Parts, Cases have been prepared for binding it in, price 18. 3d. each; or the Publisher will undertake to get it bound, including the caes. Early orders are respectfully requested for Back Numbers.

ANSWERS TO QUERIES.

PAINTING MAGIC LANTERN SLIDES, &c.-Sir, I beg to inform your intelligent correspondent "F. E." that water colours are those used for painting magic lantern slides although very thin oil colours may be employed. There is no peculiar preparation necessary for either when applied for this purpose. All colours may, I believe, be used; at least I have always used whatever colours my fancy dietated with good effect; but the brightest ones are decidedly the best to be employed, black being merely used for giving full relief to the other colours. Your humble servant, PHILOSOPHICUS, Chatham.

J.

A

M.-You may stain holly, pear-tree, and beech black by means of the following preparation; and we think it will answer for deal:-Take a troy pound of logwood, and add a sixth part of its weight of galls; form a stain of these; and apply one coating to the work; then add one part of verdigris to the stain, and give the work another coat; after which add one part of sulphate of iron, and add one or more additional coat, as may be deemed necessary. You may conclude by polishing with linseed oil.

DECORATOR AND SUBSCRIBER (Manchester).-We never intend to alter the price of this work; its limits will grow with the amount of patronage bestowed on it. The recommendation of our subscribers, as we have before stated, is to us of the utmost importance;-they have already, we flatter ourselves, seen what we can do; for what we will perform under more genial auspices our honour and the ardent desire we possess for the extension of useful information amongst the industrial classes must be our sufficient guarantee. We have had already some thoughts of commencing a series of biographies of painters, and we will, perhaps, do so shortly. For your good wishes we are much obliged, and shall receive with pleasure any extracts &c., suitable to the nature of our work, with which you may feel disposed to favour us. MARY QUEEN OF SCOTS. By A. Dumas. London: Geo. Peirce, 310, Strand.-Mr. Peirce is well known as the publisher of a series of translations of the most approved works of fiction extant in French literature. The fidelity of their rendering and the taste with which they are got up are only equalled by the cheap rate at which they are offered to the public. The present work is a peculiarly interesting story written in Dumas' happiest vein, and we strongly recommend our readers to judge of its merits by a perusal; for although the supposition on which it is founded may be erroneous, it cannot fail of leaving an impression on the mind which romances do not often produce.