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The chromium is next to be separated: For this purpose the liquid is evaporated, the residue dissolved in water, and filtered, to < separate the silica that may be present. Pronitrate of mercury is then poured in, which occasions a precipitate of chromate of meroury, which, being dried and heated, gives the green oxide of u chromium. There remains for examination the first precipitate of * iridium, titanium, iron, chromium, and alumina. There is like wise a little osmium, which is removed by digesting in muriatic acid, distilling, and precipitating by zinc as before. If there remain portions undissolved, they must be triturated with nitre, as at first; and we observe that the oftener this operation is repeated : the more blue dò the muriatic acid solutions become, because they contain less and less iron and titanium, which, as more easily dissolved, are first acted upon by the acid, and leave behind them a greater proportion of iridium.

Now iridium has this property: when in that state of oxidation that it forms' red solutions in acids, it is only precipitated by salammoniac, and that in the state of a triple salt. It is therefore brought to this state by boiling its' muriatic solution with pitric acid. The liquid is neutralized by ammonia. By boiling, the iron and 1 titanium are thrown down. The iridium is then precipitated by salammoniac; and the triple salt obtained, when exposed to a red heat, leaves iridium in a state of purity.

This metal, so difficult to separate from the singular alloy which concealed it from all eyes, possesses remarkable properties. Its s colour and lustre are very similar to those of platinum. It is more difficultly fusible. It is insoluble in the simple acids, difficultly soluble in nitro-muriatic acid; but potash and nitre oxidize it, and combine with it into black powder, which gives a blue-coloured solution. With boiling nitro-muriatic acid, it forms a red solution. Its blue solutions themselves become red when boiled; but both the blue and the red solutions are discoloured by sulphate of iron, sulphureted hydrogen, iron, zinc, and tin. Oxymuriatic acid causes them to resume their colour. It is iridium which gives a red colour to the last precipitates of the triple salt of platinum, while the first precipitates, into which it does not enter, are yellow.

The properties of osmium are not so easily determined, in consequence of the case with which it is oxidated and volatilized. Its oxide is white, and very caustic. It exhales an unsupportable odour. It is flexible and fusible, like wax; and as soon as it touches an animal matter, it blackens it. Its solution in water becomes blue by nutgalls, &c.

M. Mongez, member of the Class of Ancient Literature, has read a memoir on the bronze of the ancients, in which he shows, from experiments made by M. Darcet, that it is not by immersion in cold water that bronze becomes hard, as is the case with steel;

but that it acquires its hardness by being heated red-hot, and then allowed to cool slowly in the air, M. Darcet has taken advantage of this property to make symbals, instruments hitherto made only in

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Turkey, and, it is pretended, by a single workman in Constantinople, who possesses the secret

The falls of stones from the atmosphere, since the reality of the phenomenon has been constated, are observed so often, that by and by the most astonishing thing will be the long incredulity entertained respecting them; A remarkable fall took place this year in the department of the Lot-et-Garonne. It happened on the 5th of September, and, as usual, in fine weather, with a strong explosion and a whitish cloud. The number of stones was considerable; one of them was said to weigh 18 pounds. They were scattered over a surface of about a league radius. Their external characters and composition are absolutely the same as in other stones of the same origin, only their fracture has tints a little more marbled' than common. Excellent reports by two good observers at Agen, MM, de Saint-Amans and Lamouroux, have made us fully acquainted with the details of the phenomenon,

M. le Comte Berthollet has presented to the Class, on the part of Mr. Tennant, one of the stones that fell last year in Ireland, and which resemble all the others, excepting that they contain a little more iron.

It is known, and we have had repeated occasions to mention it, that the stone called arragonite furnished the strongest objection that could be produced against the employment of crystallization in the classification of minerals; because chemists had found no difference between its composition and that of calcareous spar, though the crystalline forms be essentially distinct. This objection is now removed. M. Stromeyer, Professor of Chemistry at Gottingen, has discovered the constant presence of three per cent. of strontian in arragonite, while none exists in calcareous spar. M. Laugier, Professor in the Museum of Natural History, has repeated this analysis, and obtained the same result. It remains to be explained how the addition of so small a quantity of a constituent can change so completely the form of the primitive molecule of a mineral.

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of these mountains is unknown; but as their summits are perpetually covered with snow, we are sure that it at least exceeds 8000 feet. It runs in a northerly direction for nearly three degrees of

. latitude; then easterly, with still a northerly inclination, for six degrees of latitude; then nearly south ; afterwards south-east; and lastly, nearly east, over a space occupying nine degrees of latitude and 13 degrees of longitude. Its size is fully' as great above a thousand miles before it joins the Mississippi as at the junction, yet a great number of large rivers join it in the interval. This shows us the great evaporation to which it is subjected. It joins the Mississippi nearly in north latitude 39°, west longitude 90°, from Greenwich. The Mississippi, after this junction, flows for 10° of latitude south, a course, including the windings, certainly not so little as 2000 miles ; so that the whole course of the Missouri, from its source to the ocean, exceeds 5000 miles. This is a length, of course, that will not easily be paralleled any where else; and almost the whole of this immense river is navigable. What is still more important, a great part of its banks consist of fertile plains ; and from the observations of Lewis and Clarke, it would a coal country occupies about a thousand miles of these plains, What a country is this likely one day to become!

II. An Analogy respecting Volcanoes. The phenomena of volcanoes are some of those that have hitherto baffled the sagacity of philosophers. Hence I conceive that every analogy which has any tendency to throw light upon their origin, however insignificant in itself, ought not to be neglected. This induces me to venture to state the following one. Some time ago I got a salt from Mr. Trimmer, which is sublimed during the burning of London bricks. Mr. Trimmer informed me that the sand which is mixed with the clay in the bricks is brought from below Woolwich, and is therefore in all probability impregnated with common salt. This will account for the acid in the sal-ammoniac. I was at a loss to account for

the decomposition of the common salt; but a correspondent from Bristol (Annals of Philosophy, v. 157.

) suggests that sulphate of ammonia is generally formed during the combustion of coal. This must no doubt be the case, as pyrites is almost a constant companion of pit coal. Here, then, we have during the burning of bricks containing common salt by common pit coal the sublimation of sal-ammoniac. Now it is well known that sal-ammoniac is sublimed from Mount Vesuvius. Hence is it not likely that the fuel wbich supports Vesuvius is coal, and that sea water has access to it? This supposition has been made long ago; but the preceding analogy seems to me to strengthen it. Would it not be proper to ascertain whether sal-ammoniac be sublimed from all volcanoes ? and if not, whether there be any thing peculiar in the position and geology of those that yield this salt? It all volcaDoes yield sal-ammoniac, I should be inclined to suspect that the quantity of coal contained in the bowels of the earth is much

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greater, and that it goes much deeper, than has been hitherto suse pected.

III. Arragonite. It may be worth while to state, that Stromeyer's discovery of the presence of strontian in arragonite has been confirmed more than six months ago at Paris by the experiments of Vogel, Laugier, and Vauquelin, all of whom succeeded in obtaining crystals of nitrate of strontian from arragonite.

IV. Whether Cast-Iron expands on congealing. In answer to the queries and doubts of my Correspondent N. N. on this subject, I have to observe that Reaumur made a set of experiments on the melted metals, and found that three of them expand in the act of congealing; namely, cast-iron, bismuth, and antimony; while all the rest contract. If you throw a piece (of tolerable size) of gold or silver into these metals in fusion, it immediately sinks to the bottom; but a piece of iron will swim on the surface of the melted mass. When melted iron congeals, the surface is not concave, as it would be if it contracted, but convex. The allowance alluded to by N. N. for the shrinkage is in consequence of the diminution of bulk before congealing, partly from contraction, and partly from leakage. If the iron had congealed before the addition was made, it would be in vain to make it, as the addition would not unite with the old portion. V. On the Production of Complimentary Colours by the successive

Reflection of Polarized Light from Gold and Silver. Dr. Brewster has discovered that if a pencil of polarized light undergoes more than two reflections between two polished plates, coloured light, so jours being complimentary to each other. The either of gold or silver, it consists of two portions, A and B, of

B portion A is polarized in the plane of reflection, and B in a plane perpendicular to it. The colours of A and B

vary after every two reflections. The 2d and 3d, the 6th and 7th, the 10th and 11th, &c. reflected images are llue, or composed of the most refrangible tays; while the 4th and 5th, the Sth and 9th, the 12th and 13th, &c. are red, or composed of the least refrangible rays. The angle of incidence upon the plates sbould be between 80° and 90°, and the plane of reflection inclined 15° to the plane of the primitive polarization. In order to perceive the colours, the reflected pencil must be analyzed by a prism of Iceland spar. This singular property is not possessed by mercury, lead, steel, 'nor any other metal which has been tried; but all these metals possess another property different from that of the gold and silver. These experiments have been repeated and verified by M. Biot, of the Institute of France. VI. On the Multiplication of Images, and the Colours which

ively examined the multiplication of images, and the beautiful colours which accompany them, in some specimens of leeland spar. All these philosophers ascribe the phenowena to fissures within the crystal, and almost all of them explain the multiplication of the images by jaternal reflections. Dr. Brewster has discovered the true cause of all these appearances, and can communicate the faculty of producing them to any piece of Iceland spar, or any doublérefracting crystal, What was supposed to be an accidental fissure, bre has shown to be an interrupting stratum, either of the same or of a different substance, crystallized in a different manner from the rest of the mineral, and producing the complimentary colours by depolarizing two of the images, and he has found that this stratum is always perpendicular to the shorter diagonal of qtie of the shomboidal faces. The artificial rlombaids which have been constructed upon this principle imitate, in the most perfeet manner, -all the phenomena which appear in the real crystal. 9,13 0 20316

3.1.9. VII. Optical Properties of Iodine. In attempting to obtain a measure of the refractive power of jodine by ascertaining the angle at which it polarizes light, Dr. Brewster found that it possessed the property of polarizing the light which it reflected in two opposite planes, a property which is peculiar to metallic bodies. VIII. Queries respecting the Visibility of the Stars in the Day-time.

(To Dr. Thomson.) SIRS Having for some time past made a considerable number of observations on the celestial bodies in the day-time, the following conclusions, amongst many others, have been established, from a great number of observations and experiments, viz. that in every ınstance an increase of the magnifying power of the telescope has the prin. cipal effect in rendering a star epsily perceptible that the diminu

; tion of the aperture of the object glass produces a very slight effect

, in some cases none at all, and that when the aperture is contracted beyond a certain limit it produces a hurtful effect, and even prevents the object from being distinguished. These conclusions, I have

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, reason to believe, coincide with the deductions of Mr. Short and others who have made similar observations; so that it may be considered as a fact sufficiently established, that magpifying pouver is requisite for distinguishing a star in the day-time, and that the the magnifying power is increased, the more splendid and brilliant the star appears.

Query: What is the cause why the magnifying power of telescopes si produces this effect 2

In regard to the planets, a probable solution might be given from the consideration that the telescope, by enlarging the angle of vision, augments the apparent size of the object, tkus presenting a larger

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