Red Heat Research Articles

On a new register-pyrometer, for measuring the expansions of solids, and determining the higher degrees of temperature upon the common thermometric scale

In the year 1821, the author published in the Journal of the Royal Institution an account of a new pyrometer, and of some determinations of high temperatures, in connexion with the scale of the mercurial thermometer, obtained by its means. The use of the instrument then described was, however, limited; and the author was subsequently led to the invention of a pyrometer of a more universal application, both to scientific researches and to various purposes of art. Fie introduces the subject by an account of the late attempt of M. Guyton de Morveau, to employ the expansions of platina for the admeasurement of high temperatures, and for connecting the indications of Wedgwood’s pyrometer with the mercurial scale, and verifying its regularity. The experiments of that philosopher were by the contraction of porcelain, and by actual comparison with those of the platina pyrometer, at no higher temperature than the melting point of antimony; but they are sufficient to establish the existence of a great error in Wedgwood’s original estimation of his degrees up to that point. This he carries on by calculation, on the hypothesis of uniform progression of expansion, up to the melting point of iron; the construction of his instrument not admitting of its application to higher temperatures than a red heat, in which platina becomes soft and ductile. Mr. Daniell shows, by an examination of M. Guyton’s results, that he has failed in establishing the point he laboured to prove; namely, the regularity of the contraction of the clay pieces. The pyrometer of the author consists of two distinct parts; the one designated the register , the other the scale .

Experiments and observations on the neivry pitch-stone, and its products, and on the formation of pumice

After describing the geological locality and the external character of the above mineral, and adverting particularly to its oily smell, Mr. Knox proceeds to show, by its chemical analysis, that, exclusive of the constituents of this substance ascertained by Klaproth, it contains a considerable but variable proportion of a peculiar bitumen, separable from it by distillation at high temperatures. The author also succeeded in detecting some volatile principle in the pitch-stone of Meisser, analysed by Klaproth, as also in that of Arran; but it exists in them in smaller quantities than in the pitch-stone of Newry. After having separated the water and bitumen from the mineral by heat, Mr. Knox found that by subjecting the residue to a bright red heat, it assumed not merely the appearance, but the properties of pumice; and he attributes this appearance to the slow escape of the bituminous matter, producing a vesicular structure.

Observations on naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar, by exposure to a red heat

By passing coal tar through a red-hot iron tube, a portion of an aqueous fluid, and of a substance like tar, was obtained; the latter is black, soluble in ether, and partially in alcohol, of an aromatic odour, and sweetish taste. It was submitted to slow distillation, and among other products afforded naphthaline, a white concrete substance of an aromatic odour and taste, fusible at 180º, and scarcely soluble in water, but readily so in ether, alcohol, and oils. Of the various characters of this substance, detailed by the author, its tendency to crystallize appears the most remarkable; its vapour condenses in rhombic plates, which are sometimes modified into hexagonal plates, by the incomplete development of the smaller angles of the usual rhomb.

On the measurement of high temperatures

The author, after adverting to the many abortive endeavours of former experimentalists to obtain instruments for the accurate ad-measurement of high temperatures, and after suggesting doubts as to the confidence to which Wedgwood’s pyrometer is entitled, describes several attempts of his own to effect this very desirable object. In the course of his inquiries, a remarkable fact presented itself to his notice in the change which occurred in an index constructed on the compensation principle, and formed by two slips of metal, the one of silver and the other of gold, originally quite pure, and united without any alloy. In the course of a few years, although it had never been subjected to a heat above that of melting lead, the whole surface of the gold became converted into an alloy of silver, the impregnation extending gradually to a considerable depth in the gold, and destroying the sensibility of the instrument to changes of temperature. After trying various plans, he gave the preference to one founded on the following principles: namely, that the fusing points of the pure metals are fixed and determined; that those of the three noble metals, namely, silver, gold, and platina, comprehend a very extensive range of temperature; and that between these three fixed points in the scale, as many intermediate ones as may be required may be obtained by alloying the three metals together in different proportions. When such a series of alloys has been once prepared, the heat of any furnace may be expressed by the alloy of least fusibility which it is capable of melting. The determinations afforded by a pyrometer of this kind will, independently of their precision, have the advantage of being identifiable at all times and in all countries: the smallness of the apparatus is an additional recommendation, nothing more being necessary than a little cupel, containing in separate cells the requisite number of pyrometric alloys, each of the size of a pin’s head. The specimens melted in one experiment need only to be flattened under the hammer in order to be again ready for use. For the purpose of concisely registering the results, the author employs a simple decimal method of notation, which at once expresses the nature of the alloy, and its correspondence with the scale of temperature. Thus G. 23 P would denote an alloy of gold with 23 per cent. of platina. As the distance between the points of fusion of silver and of gold is not considerable, the author divides this distance on the scale into ten degrees; obtaining measures of each by a successive addition of 10 per cent. of gold to the silver, the fusion of which, when pure, marks the point of zero; while that of gold is reckoned at ten degrees. If minuter subdivisions were required for particular objects of research, these might easily be made, following always the decimal series. From the point of fusion of pure gold to that of pure platina, the author assumes 100 degrees, adding to the alloy which is to measure each in succession 1 per cent. of platina. Whether these hypothetical degrees represent equable increments of temperature is a question foreign to the purpose of this paper, and must be the subject of future investigation. The author then enters into a detailed account of the method he employed for insuring accuracy in the formation of the requisite series of alloys, and of various experiments undertaken to ascertain their fitness as measures of high temperatures. The determinations of the heats of the different furnaces adapted to particular objects, are given in a tabular form. The remaining portion of the paper contains the recital of the author’s attempts to determine by means of an apparatus connected with an air thermometer, the relation which the fusing point of pure silver bears to the ordinary thermometric scale. An extensive series of experiments, of which the results are given in a table, were made with this apparatus. From the data thus afforded, after making the necessary corrections, the author deduces the following results in degrees of Fahrenheit: viz. A full red heat 1200°; orange heat 1650°; melting point of silver (which had been estimated by Wedgewood at 4717°, and by Daniell at 2233°,) 1830°; of silver alloyed with one tenth gold 1920°. The paper is accompanied with drawings of the apparatus employed.

On the action of finely divided platinum on gaseous mixtures, and its application to their analysis

In the first section of this paper the author describes the action of finely divided platinum, at common temperatures, on mixtures of hydrogen and olefiant gas with oxygen; of hydrogen and carburetted hydrogen with oxygen; of hydrogen and carbonic oxide with oxygen; of hydrogen and cyanogen with oxygen; of carbonic oxide and carburetted hydrogen with oxygen; of hydrogen, carburetted hydrogen and carbonic oxide with oxygen; and of the same with the addition of olefiant gas. From the experiments detailed under these several heads, it appears that when the compound combustible gases mixed with each other, with hydrogen, and with oxygen, are exposed to balls of platinum sponge, the several gases are not acted upon with equal facility; but that next to hydrogen, carbonic oxide is most disposed to unite with oxygen, then olefiant gas, and lastly carburetted hydrogen. By due regulation of the proportion of hydrogen, the author remarks, that it is possible to change the whole of the carbonic oxide into carbonic acid, without acting on the olefiant gas or carburetted hydrogen; he observes, however, that with respect to olefiant gas this exclusion is attended with some difficulty, and it is generally more or less converted into carbonic acid and water. The second section of this paper relates to the action of finely divided platinum upon gaseous mixtures at increased temperatures. In these experiments the gases, mixed with oxygen enough to saturate them, were severally exposed in small retorts containing a platinum sponge, and immersed in a mercurial bath, to a temperature which was gradually raised till the gases began to act on each other. It was thus found that carbonic oxide began to be converted into carbonic acid at about 300º; olefiant gas was decomposed at about 500º; carburetted hydrogen at a little above 555º; and cyanogen appeared to require a red heat.

Some researches on flame

This communication is subdivided into four sections, of which the first treats of the effect of rarefactions of the air, by diminished pressure, upon flame, and explosion. An inflamed jet of hydrogen was placed in the receiver of an air-pump, and the flame was observed to enlarge during exhaustion, till the gauge indicated a pressure of one fourth or one fifth; it then diminished in size, but was not extinguished till the pressure was reduced to between one seventh and one eighth. A somewhat larger jet burned until the rarefaction amounted to one tenth, and rendered the glass tube whence the gas issued white hot. To this circumstance the author refers the long-continued combustion of the gas, and thinks the conclusion confirmed by the following experiment. A platinum wire was coiled round the jet tube, so as to reach into and above the flame, and it became white hot during the exhaustion, and continued red hot even when the pressure was only one tenth. The lower part of the flame was now extinguished, but the upper part in the contact of the wire continued to bum till the pressure was reduced to one thirteenth. The flame, therefore, of hydrogen is extinguished in rarefied atmospheres, whenever the heat it produces is insufficient to communicate visible redness to platinum wire. Sir Humphry Davy was thus led to infer, that those combustibles which require least heat for combustion would burn in rarer atmospheres than those requiring more heat; and that bodies which produce much heat in combustion would burn in rarer air than those producing little heat, and experiments are detailed proving this to be the case: thus, an inflamed jet of light carburetted hydrogen, which produces little heat in combustion, and requires a high temperature for its ignition, was extinguished whenever the pressure was below one fourth, even though the tube was furnished with a wire. Carbonic oxide burned under a pressure of one sixth; sulphuretted hydrogen of one seventh. Sulphur, which burns at a lower temperature than any other ordinary combustible, except phosphorus, had its flame maintained in an atmosphere rarefied 15 times, and phosphuretted hydrogen was inflamed when admitted into the best vacuum of an excellent air-pump. The author next proceeds to consider the influence of rarefaction, produced by heat, upon combustion and explosion. A volume of air at 212° is expanded to 2·25 volumes. At a dull red heat its probable temperature then is 1032°, provided it expand equably for equal increments of heat.

On the magnetic attraction of oxides of iron

Mr. Lane having observed that hardened iron is not so readily attracted by the magnet as soft iron, was proceeding to make some experiments on the subject, when he was led, by Mr. Hatchett’s paper on Magnetical Pyrites, &c., to examine what magnetical properties iron possessed when free from inflammable matter. For this purpose he procured some of the precipitate sold at Apothecaries’ Hall under the name of Ferrum precipitatum , and which is prepared by adding purified kali to a solution of sulphate of iron. This precipitate, the author says, has no magnetic particles; nor, when exposed to a clear red heat, does it acquire any, except when smoke or flame have access to it. The solar heat, when concentrated to the degree at which glass melts, does not render this oxide magnetic, provided it be protected by glass from the dust floating in the air; if not so protected, many of the particles become magnetic. Mr. Lane then rubbed various portions of the oxide, in a glass mortar, with different combustible substances, namely, coal, sulphur, charcoal, camphor, ether, alcohol, &c., but found the oxide was not thereby rendered magnetic, without the assistance of a degree of heat equal to that of melting lead; with that degree, however, it became magnetic. Hydrogen, when aided by a red heat, had the same effect. Charcoal and cinders, well burnt, were found to require a longer continuance of the heat, to have their full effect on the oxide, than dry wood, coal, or sulphur. A single grain of camphor, dissolved in an adequate portion of alcohol, was found sufficient, when assisted by a red heat, to render all the particles of 100 grains of the oxide magnetic. But such substances as are easily sublimed, will, by a continued application even of a low heat, quit the oxide, leaving it, as at first, unmagnetic. Hence we may understand why Prussian blue, sulphurets, and ores of iron, containing inflammable matter, become magnetic by the agency of heat, and revert to their unmagnetic state if the heat is continued long enough to drive off the inflammable matter.

Results of some recent experiments on the properties impressed upon light by the action of glass raised to different temperatures, and cooled under different circumstances. By David Brewster, LL. D. F. R. S. Edin. and F. A. S. Ed. in a letter to the Right Hon. Sir Joseph Banks, Bart. K. B. P. R. S

The author, being engaged in making a variety of experiments on resinous and other bodies that could be fused between plates of glass, remarked a partial depolarization while the subject of examination was hot, but which diminished on cooling, and consequently could not be ascribed to incipient crystallization. He therefore tried a plate of glass alone; and having previously raised its temperature almost to a red heat, he found that a ray of polarized light became completely depolarized by its passage through it: and he further thence infers, that glass brought to a certain temperature forms two images, and polarizes them like all doubly refracting crystals, only that the two images are, in fact, coincident, instead of being separated. Since in the formation of the glass-tears, called Prince Rupert’s drops, which are made by dropping melted glass into cold water, it is probable that in consequence of the sudden consolidation at the surface, the interior part is prevented from contracting, and consequently retains, in some measure, that relative distance of its particles which obtained in the fluid state, the author conceived these drops to be a fit subject for an interesting experiment; and having procured several such drops, made of white flint-glass, he cut and polished one of them by two planes at right angles to the axis, and a second by two planes parallel to its axis and to each other. When polarized light was transmitted through a drop in either of these directions, it was found to be depolarized; but there was not any position in which the transmitted ray would retain its polarization, as is found in corresponding experiments with crystallized substances.

On a method of analysing stones containing fixed alkali, by means of the boracic acid

The method of analysis here described by Mr. Davy is founded on the attraction of the boracic acid for the simple earths, which is considerable at the heat of ignition, and on the ease with which the compounds formed with them are decomposed by the mineral acids. The process is as follows: 100 grains of the stone to be examined must be fused for about half an hour, in a strong red heat, with 200 grains of boracic acid: an ounce and a half of nitric acid, diluted with seven or eight times as much water, must be digested upon the mass till the whole is decomposed; and the fluid must then be reduced, by evaporation, to an ounce and a half or two ounces.

On the composition of zeolite

Although the substance called Natrolite by Mr. Klaproth has lately been found, under a crystalline form, perfectly similar to that of mesotype, M. Haüy has not thought himself warranted, from this circumstance, to consider these two bodies as of the same species, on account of the difference of their chemical composition; because, according to the analysis of M. Vauquelin, there was not observed to be any soda in zeolite, but, on the contrary, a considerable quantity of lime, which is not a constituent part of natrolite. Mr. Smithson, on the contrary, is inclined to consider them as the same substance, by the agreement of their chemical properties, as well as crystallographical form. Although he had, many years since, found that the zeolite of Staffa contained soda, as has since been observed by Dr. Hutton and Dr. Kennedy, he still felt uncertain whether these were of the same kind as those analysed by M. Vauquelin: but having lately received from M. Haüy a cluster of mesotype in rectangular prisms, terminated by a quadrangular pyramid, Mr. Smithson took this opportunity of ascertaining whether this substance and natrolite did or did not differ in their composition. The method pursued by the author in the analysis of zeolite was first to expose ten grains of it to a strong heat, for the purpose of ascertaining the loss; and he found it to be ·95 grain. An equal quantity was then dissolved in muriatic acid; and after the solution had been evaporated to dryness, the residuum was exposed to a red heat. Water was then poured upon it, and dissolved a portion, which, upon evaporation, was found to be muriate of soda, weighing 3·15 grains. From the solution of this salt, neither carbonate of ammonia nor oxalic acid occasioned any precipitate; by which it appeared that this zeolite contained no lime.

I. On the Existence of Two New Fluids in the Cavities of Minerals, which are immiscible, and possess remarkable Physical Properties

In the year 1818, my attention was accidentally directed to the subject of water in crystallised bodies, by the explosion of a crystal of Topaz, which I had exposed to a red heat, for the purpose of expelling its colouring matter. This violent disruption of the specimen, which was shivered into a thousand films, of extreme tenuity, arose from the expansion of the imprisoned fluid, and induced me to institute a series of experiments, for the purpose of determining the nature of the fluid, the form of the cavities which contained it, and the arrangement of these cavities in reference to the crystalline form of the mineral.

XIV. On the anomalous magnetic action of hot iron between the white and blood-red heat

In consequence of certain theoretical results relative to the magnetic action of iron, obtained by Mr. Charles Bonnycastle, I was desirous of ascertaining the relative attraction which different species of iron and steel had for the magnet; and with this view I procured two bars of each of the following descriptions of metal, 24 inches in length, and 1 inch and a quarter square, which being placed successively in the direction of the dip, at a certain distance from the compass, the disturbance occasioned by each was carefully noted; first with one end upwards, and then with the other; and assuming the tangents of the angles as the measure of the disturbing power, I obtained the following specific results, viz. Mag. Pow. Mag. Pow. Malleable iron - - 100 | Shear steel soft - - 66 Cast iron - - - 48 | ------------- hard - - 53 Blistered steel soft - 67 | Cast steel soft - - 74 ---------------- hard - 53 | ------------ hard - - 49 As it was obvious from these experiments, that the softer the iron the greater was its power, and the contrary, I was desirous of determining how nearly these different kinds of metal would approximate towards each other in their magnetic action, when rendered perfectly soft by being heated in a furnace. With this view, bars of equal size of cast iron, malleable iron, shear steel, &c. were rendered white hot, and being placed in the direction of the dip, as before, their powers, as was anticipated, agreed nearly with each other; but still the cast iron, which was weakest while the metal was cold, exceeded a little in power all the others when hot, and the malleable iron which had the greatest power cold, had the least when hot; but the difference was not very great, and might probably arise from some accidental circumstance. While carrying on these experiments, it had been observed, both by Mr. Bonnycastle and myself, that between the white heat of the metal, when all magnetic action was lost, and the blood-red heat, at which it was the strongest, there was an intermediate state in which the iron attracted the needle the contrary way to what it did when it was cold, viz. if the bar and compass were so situated that the north end of the needle was drawn towards it when cold, the south end was attracted during the interval above alluded to, or while the iron was passing through the shades of colour denoted by the workman the bright red and red heat.

II. Observations on naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar by exposure to a red heat. Communicated by W. H. Wollaston, M.D. F.R.S.

II. <i>Observations on naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar by exposure to a red heat. Communicated by</i> W. H. W<scp>ollaston</scp>, <i>M.D. F.R.S.</i>

XVI. Observations on naphthaline, a peculiar substance resembling a concrete essential oil, which is apparently produced during the decomposition of coal tar, by exposure to a red heat

Although the existence, and many of the properties of the substance above-mentioned, have been already noticed in two of the Philosophical Journals of this country, there has not yet appeared, as far as I can discover, any systematic description of the mode by which it may be obtained, or of its relation to the substance from which it is produced; on which account I have been induced to offer to the Royal Society the following observations respecting these points of its history. In the experiments which led, in the present instance, to the detection of the substance in question, it was proposed to effect the decomposition of coal tar by passing its vapour through an ignited iron tube; and, in order to increase to the utmost the extent of the ignited surface, that portion of the tube which was constantly kept up to a red heat, was filled, in the first instance, with a series of hollow iron cy­linders open at both extremities, and successively decreasing in diameter, so as to be included one within another. In other instances these cylinders were removed, and their place supplied by sand, or by pieces of well burnt coke, or by pieces of brick; but it was found that the interstices between the cylinders, or between the particles of sand, &amp;c. were so soon choaked up with carbon from the decomposition of the tar, as to be rendered absolutely impervious to the gas produced during the decomposition; so that it became necessary to pass the vapour of the tar simply through the tube itself.

XX. On the magnetic attraction of oxides of iron

Having found by experiment, that hardened iron is not so readily attracted by the magnet as soft iron, and that needles are inferior to iron wire as indexes to Six's thermometer, I was proceeding to other comparative experiments, when I received the Second Part of last year's Philosophical Transactions, in which I saw an Analysis of magnetical Pyrites, with Remarks on Sulphurets of Iron, by Mr. Hatchett. This Paper led me to examine what magnetical properties iron possessed, when free from inflammable matter. For this purpose I obtained a precipitate of iron, prepared and sold at Apothecaries' Hall by the name of Ferrum præcipitatum Mr. Moore, the chemical operator, informed me, that he prepared it by dissolving twelve pounds of sulphate of iron in twenty-four gallons of distilled water, and then adding eight ounces of sulphuric acid to render the solution more complete. Twelve pounds of purified kali were mixed with the solution: the precipitate was well washed with hot distilled water, and then carefully dried. This precipitate is similar to the sediment of chalybeate waters, and affords no magnetic particles; nor, when exposed to a continued clear red heat, does it suffer any alteration beyond the acquirement of a darker colour. But if any smoke or flame has access to it, then magnetic particles are evident. Heat, by the converging rays of the sun, equal to that at which glass melts, blackens the oxide, but does not render it magnetic, if free from any inflammable matter. It is requisite, in this experiment, to protect the oxide, by glass, from the dust floating in the air, which otherwise will render many of the particles magnetic. I attributed this effect to the deoxidising property of light, till by employing a protecting glass, the result proved it to proceed from the dust in the atmosphere.

XV. On the composition of emery

The substance called emery, which, from its great hardness, has been long used in various manufactures, for grinding and polishing other bodies, has not, it appears, been hitherto correctly analyzed. In books of mineralogy, it is considered as an ore of iron; an opinion probably derived from its great specific gravity, as well as from the iron which it frequently contains. But, where this metal is most abundant, it could not be extracted from it with advantage, and ought rather to be regarded as an impurity, as it does not contribute to produce the peculiar hardness for which this substance is distinguished. In Mr. Kirwanʼs mineralogy, he mentions an examination of emery made by Mr. Wiegleb, from which he inferred that 100 parts consisted of 95, 6 of silex, and 4, 4 of iron. Mr. Kirwan, however, justly suspects the correctness of this account, and observes that he had no doubt but some other stone was imposed upon Mr. Wiegleb for emery. When powder of emery is boiled in acids, it becomes of a lighter colour, from the loss of part of the iron; after which, it does not seem to undergo any further alteration. As acids produce so little effect on it, I exposed it to a pretty strong red heat, with carbonate of soda, in a crucible of platina. On adding water to the mass contained in the crucible, the greater part of the emery was found in powder; having only become of a light colour, from the extraction of part of the iron. Though this process was twice repeated with the remaining powder, and in a stronger heat, a great proportion of it remained undissolved.

XI. On the action of nitre upon gold and platina

Gold, which cannot be calcined by exposure to heat and air, has been also considered as incapable of being affected by nitre. But in the course of some experiments on the diamond, an account of which has been communicated to this Society, I observed, that when nitre was heated in a tube of gold, and the diamond was not in sufficient quantity to supply the alkali of the nitre with fixed air, a part of the gold was dissolved. From this observation I was induced to examine more particularly the action of nitre upon gold, as well as to inquire whether it would produce any effect upon silver and platina. With this intention I put some thin pieces of gold into the tube together with nitre, and exposed them to a strong red heat for two or three hours. After the tube was taken from the fire the part of the nitre which remained, consisting of caustic alkali, and of nitre partially decomposed, weighed 140 grains; and 60 grains of the gold were found to have been dissolved. Upon the addition of water about 50 grains of the gold were precipitated, in the form of a black powder. The gold which was thus precipitated was principally in its metallic state, the greater portion of it being insoluble in marine acid. The remaining gold, about 10 grains in weight, communicated to the alkaline solution, in which it was retained, a light yellow colour. By dropping into this solution diluted vitriolic or nitrous acid, it became at first of a deeper yellow, but if viewed by the transmitted light, it soon appeared green, and afterwards blue. This alteration of the colour from yellow to blue arises from the gradual precipitation of the gold in its metallic form, which by the transmitted light is of a blue colour. Though the gold is precipitated from this solution in its metallic form, yet there seems to be no doubt that while it remains dissolved it is entirely in the state of calx. Its precipitation in the metallic state is occasioned by the nitre contained in the solution, which having lost part of its oxygen by heat, appears to be capable of attracting it from the calx of gold; for I found that if the calx of gold is dissolved by being boiled in caustic alkali, and a sufficient quantity of nitre which has lost some of its air by heat is mixed with it, the gold is precipitated by an acid in its metallic state.*

XVII. Experiments and observations to investigate the nature of a kind of steel, manufactured at Bombay, and there called Wootz: with remarks on the properties and composition of the different states of iron

Doctor Scott , of Bombay, in a letter to the President, acquaints him that he has sent over specimens of a substance known by the name of wootz; which is considered to be a kind of steel, and is in high esteem among the Indians. Dr. Scott mentions several of its properties, and requests that an inquiry may be instituted to obtain further knowledge of its nature. This gentleman informs the President, that wootz " admits of a harder temper than any thing known in that " part of India; that it is employed for covering that part of " gun-locks which the flint strikes: that it is used for " cutting iron on a lathe; for cutting stones; for chizzels; for making files; for saws; and for every purpose where " excessive hardness is necessary. ” Dr. Scott observes that this substance " cannot bear any thing beyond a very slight red heat, " which makes it work very tediously in the hands of smiths; " and that " it has a still greater inconvenience or defect, that " of not being capable of being welded with iron or steel; " to which therefore it is only joined by screws and other " contrivances." He likewise observes, that when wootz is “ heated above a slight red heat, part of the mass seems to “ run, and the whole is lost, as if it consisted of metals of " different degrees of fusibility. " We learn also from Dr. Scott's letter, that “ the working with wootz is so difficult, that it is a " separate art from that of forging iron." It will be proper also to notice his observation, that “ the magnetical power in "an imperfect degree can be communicated to this substance." § 2. Mechanical and obvious Properties The specimens of wootz were in the shape of a round cake, of about five inches in diameter, and one thick; each of which weighed somewhat more than two pounds. The cake had been cut almost quite through, so as to nearly divide it into two equal parts. It was externally of a dull black colour; the surface was smooth; the cut part was also smooth, and, excepting a few pinny places and small holes, the texture appeared to be uniform. It felt about as heavy as an equal bulk of iron or steel. It was tasteless and inodorous. No indentation could be made by blows with a heavy hammer; nor was it broken by blows which I think would have broken a like piece of our steel. Fire was elicited on collision with flint. Under the file I found wootz much harder than common bar steel not yet hardened, and than Huntsman' s cast steel not yet hardened. It seemed to possess the hardness of some kinds of crude iron, but did not effectually resist the file like highly tempered steel, and many sorts of crude iron: for although the teeth of the file were rapidly worn down and broken, the wootz. was also reduced to the state of filings The filed surface was of a bright bluish colour, shining like hardened steel; but some parts were brighter than others; and the most shining places seemed to be the hardest parts : hence perhaps the reason of the surface being uneven, and a little pinny . Notwithstanding this uneven and pinny appearance of the filed surface, a polish was produced, which was I think at least equal, if not superior, in brilliancy and smoothness to that of any steel I ever saw. The wootz filings were attracted by the magnet like common iron filings.

XIX. An attempt to make a thermometer for measuring the higher degrees of heat, from a red heat up to the strongest that vessels made of clay can support

A measure for the higher degrees of heat, such as the common thermometers afford for the lower ones, would be an important acquisition, both to the philosopher and the practical artist. The latter must feel the want of such a measure on so many occasions; particularly when he attempts to follow, or apply to use, the curious experiments of Mr. Pott, related in his Lithogeognosia, and other modern writers upon similar subjects.