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.p. ,Pescription of the Plate. In No. XIII. we described two air thermometers, or instruments for indicating, by the expansion of air, variations of temperature. The poWer of heat is, however, so great, that not only mercury may be made to boil and fly off in vapour, l>uf the vessels in which it is usually contained, when employed as an instrument, may be melted and destroyed. It, consequently, can afford no measure of the expansion of other bodies by heat, when its own expansion cannot be either perceived or determined. As a substitute for it, therefore, when a higher temperature than ordinary is to be measured, recourse is had to instruments called pyrometers; and by their means chemists have created a scale to determine the different points at which metals melt. One of the first of these wasi contrived by Dr. Cromwell MoMitner; and though it has now been; superseded by the more accurate instrument invented by Mr. Wedgewood, its principles and mode1 of construction are worthy of thetiotice of our readers. We have; therefore, given it here ; and it is'to be remarked, that the fusing points bf several metals in books of chemistry are^ Still given according

to the indications of this pyrometer. This instrument consists of a rod of steel or brass, which, by its expansion, moves a lever turning on an axis, and points out the degrees on a plate. A U is a round rod of steel or brass, one quarter of an inch thick, and three feet one inch in length. When the rod is made of brass the point of A is still made of steel, as lasting longer, and is made to screw on and oil'. C D and b are two iron supporters, joined by a flat cross bar. at bottom, Di, two inches long, in the middle of which is a point, oneeighth of an inch high, under B, and which enters the rod at B, serving to keep it in its place. In like manner the point, A, goes into a small hole at the under side of the lever, and tjie bar is further kept in its place by cross bars afc the upper part of the supporters. The latter are flat or parallel to the front of the machine,' from C to X, where they are twisted half round, so that the lower parts, X D, stand at-right angles with the upper part. This contrivance gives freer access to the rod, for the sand or fluid into which the machine is set to measure its heat; and in making experiments care must be taken always to immerse the rod to the same height iu the substance to be examined. E F a lever turning on the axis, C ; and at F and E two weights, I L, are suspended, the latter being not only heavy enough to counterbalance the longer arm, C F, but also to keep the lever steadily pressed down on the point, A. MNPO is the back of a dial plate which hns been previously graduated, and has a hand or index on the other side. This instrument, in point of fact, merely shows the expansion of the brass or steel rod when immersed in certain melted or melting metals or other substances. Mr. Wedgewood 's' pyrometer proceeds on totally different principles. It has been found that cUy, after being once exposed to a red heat, progressively contracts its dimensions as it is subjected to more heat. On this principle Mr. Wcdgewood constructed his pyrometer. He selected a particular species of tine clay, which was diffused through water, and then the liquid was passed through lawn. After the clay had subsided it was kneaded into a paste, and then formed into small pieces in little metal moulds, which are baked in a dull red heat. For measuring the contraction which these pieces undergo, a gauge is made of two pieces of brass fixed on a brass plate, and being half an inch asunder at one end, and three-tenths of an inch at the other. The pieces of clay are of such a size as exactly to fit at the wide extremity of the scale, which is marked 0, the narrow end being marked 240°, and the scal^ being 24 inches .long. Each of these 240 degrees^ has been supposed to equal one degree of Fahrenheit's thermometer, and the extremity of the scale to correspond to no less than 32.277° of that instrument. We scarcely need remind our readers that there is no connexion whatever between the two; and that the pieces of clay in point of fact indicate nothing but their own contraction and expansion. They have) however, been observed in conjunction with the expansion of mercury, the fusing' of various metals, the operations

of bakingpoltery, and other things; and thus their expansion has become an index and a mark for other facts. They are made use of by exposing the pieces of clay to the heat of the body to be experimented on; and that heat is said to be a certain number of degrees, according to the point in the scale which the pieces of clay occupy when contracted.



Only one of these metals is extensively diffused, or in its metallic state put to any important use. For any interest which they possess, therefore, we might pass them by without any notice; but as substances with which, the researches of chemists have made us acquainted, but which have not yet been decompounded, we must necessarily briefly describe them. The first of them presents a curious illustration of the mode of applying names. Nickel signifies, in German, from which it is derived, false, dirty, or foul; and in this sense is applied to the very worst description of females. Now nickel is found in different parts of Germany, and was supposed by the miners to be copper. As they; could not, however, extract any copper from it, they gave it the. name of kupj'er-nkkel, or false eopper. When a scientific chemist of the name of Cronstedt came to examine it, about 1750, and found that it was a metal different from all others, he retained the - Iat« ter part of the name, given it in reproach, and called it ntckel,, which has since been universally adopted; and thus an element of nature is stigmatized by a term, than which no more opprobrious name can be given to a woman. For any others people but Germans this is certainly of no consequence; but it is rather a curious specimen of the manner in which names, that in a scientific point of view ought to express the qualities of substances, are Applied. Nickel is rather a scarce mineral, and is always found combined with some other metals, which for a long time occasioned its separate exist* eriee to be denied. The brittle metal that is usually sold under this name always contains iron, arsenic, copper, cobalt, and bismuth. When pure it is of a fine white colour, resembling silver,-and is rather softer than iron. It is: malleable both when cold and hot; it is attracted by the magnet, and, like steel, may be converted into a magnet, pointing, when suspended, to the north, like a common magnetic needle. It is put to no use; and is, perhaps, most remarkable on account of its forming a part of almost all the stones which have fallen from the sky (meteoric stones) in every part of the world.

Since the year 1560, a heavy mineral, called cobalt, found in the mines of Germany, has been employed in most parts of Europe to tinge glass blue. From this substance, in 1733, Brandt, a celebrated Swedish chemist, obtained a new metal, to which he gave the name of cobalt. All subsequent attempts to resolve it into other 'metals, or make it out of other metals, have failed; and it is, therefore, considered a simple substance. It is grey, with a shade of (red,'having scarcely any taste or 'smell. It is soft, brittle, and yet said-to be a little malleable when made red hot. As a metal it has 1 been put to no use; but an oxide of it is employed, as we have al"readystated, to tinge glass blue. ''Near Exeter, in Devonshire, a substance is found in great abundance, which is called the black ^tiUfcte 6f'manganese, and under this '•naftwig'probably known.-to our ■Wade*s. It is frequently used in ^-Viirtoufrferts, and has already been 'sfetteral times mentioned in our "pages.- For a long time chemists we're1 puzzled what to-makc of this 'rnitiefraH some of'them called ft an ''ofee'ofiiieta, and some a particular 1 species of earth:-at length l&erg'man* showed that it was a metallic 'tr^kfe; and, wotilong afterwards

Dr. Gahn succeeded in reducing it to a metallic state. He lined a orucible with charcoal powder moistened with water; put into it some of the mineral, formed into a ball by means of oil, then filled up the crucible with charcoal powder, luted another crucible over it, and exposed the whole for about an hour to a very intense heat. On opening the crucible a metallic button, or rather a number of metallic globules, was found, equal in weight to one-third of the mineral employed. This metal is manganese. It is of a greyish white colour, like cast iron, and has a good deal of brilliancy. It is softer than cast iron, and may be filed; at the same time it is brittle, and can neither be hammered into plates nor drawn into wire. When exposed to the air it soon loses its lustre and becomes an oxide. It combines readily with other metals, particularly iron, from which it is hardly ever quite free. It has been put to no use, and is only procured in small quantities by the chemical process just described.

Of cerium, though placed by chemical writers among the metals, absolutely nothing is known.---' There is a heavy mineral called cerites, which has hitherto been found only in the mine of Kiddarhytta, iii Westmannland, in Sweden, and is of course very rare. If < is found mixed with ores of copper and some other metals, and is. of a rose red or flesh red colour, tinged occasionally with olive brown. It ■ is supposed that the. basis of this mineral is a peculiar metal, to which the name of cerium Iras been given; but all attempts to obtain it in a separate and metallic state have failed. My the experiments of some very celeb rated chemists,. carburets and oxides have been made; but the metal, the supposed simple, uudecompoundabie base, has never yet been detected. >• .i i. i»i ..

Uranium, though almost as little known as cerium, and put to no use, has . an undoubted existence. In the George Wagsfort ruiae, in Johan?Geoirgenstadti ia Saxopy,

there is found a peculiar mineral, from which the celebrated chemist, Klaproth, obtained a peculiar metal, which he called uranium. It has since been detected in other substances, but is not profusely scattered throughout nature. It is of an iron grey colour, and yields to the file; it combines with oxygen; and this is all which is known of the metal uranium.

Zinc has been much longer known than either of the metals we have just mentioned, and is, consequently, a great deal bettor known. The ancients were acquainted with a mineral, which they called cadmia, which,, when melted with copper, formed brass; but there is no proof that they were acquainted wjth, ziec in its metallic state. This metal is first mentioned in the writings of Albertus Magnus, a great ttfehymist, who died in the thirteenth century, under the name of marcasite of gold. The word zinc occurs first in the writings of Paracelsus. In this country ores of, zinc are found in the state of sulphurcts, or combined with sulphur; and in the neighbourhood of Bristol..and, Swansea there are works,Jbr, extracting ,t!ie metal. The pre i$ roasted, reduced to powder, and*- exposed,4o a strong beat in largo closed clay pots. The meta), is gradually reduced, drops down, and ilows out through an iron,.tube, into vessels containing watfir. Jfis afterwards cast into ingots and used for making brass, or scnt.abroad. In England; among the .workmen it is known by the name of spelter. It is of a brilliant white <©»)oui", .having a shade of blu?,-:a#d ,whe«: rubbed by the fingers they become of a black colour, acquire a peculiar taste, and emit a peculiar smell.. His not so malleable/as copper, lead, or tin; but neither is it so brittle as some mctalS, Wmcn'wnThereaftor fee de

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addjfcjpn of tin. To taake brass, the calamine is previously roasted; it is then mixed with charcoal and granulated copper, exposed in crucibles to a heat that will not at first melt the copper, but after a time the heat is increased, the copper is fused, and the compound is run into ingots. Brass is perhaps one of the most useful of all the metallic alloys. Unlike copper, it does not tarnish on being exposed to the air; is more readily melted, and is more malleable when cold. It is of great use as wire, and of it sieves of extraordinary fineness are wove, which could not be made of any other metal. It is said that the.Dutch or German brass, (for the principal manufactory of this article is on the Harz mountains in the kingdom of Hanover,)possesses superior ductility to that made in England, and is on that account preferred, by watchmakers and other artists. This is said to be owing to the Dutch brass containing more copper than the English. By that process called cementation, which was described under the head of " Iron," copper plates and calamine being the materials emr ployed,.the Germans make a superior, Icind of brass, which is hammered out in thin leaves, and sold in dn's.qoun$ry under the name of Dutch gold. It is about five times as.jth^pk.as gold leaf, or about one sixty-thousandth of an inch thick. Having ourselves all the materials for,making this substance, we do not^gee.why some of our maDufacturerSjpr artists may not reach or excel the Germans; and having stated the fact iu which their superior)^ is, supposed to be founded, we ,mus^,j leave, the: proving of it, and its application, to our readers.

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