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results of subsequent observation, detected this fallacy; for in the perusal of the following pages it will be seen, that where different coloring matter was employed, the white and yellow maintained no such superiority in the scale of colors. The material of color therefore is not to be disregarded in this enquiry.

In the further prosecution of this investigation, five small pieces of plate glass were procured, cut from the same strip of equal size, their weights being rendered alike by the addition of small fragments from the same plate. They were next painted on one side with water-colors, as follows:

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In every color the paint was not spread quite uniformly over the surface, it being found impossible to blend the shades into one unvarying tint.

The plates were placed side by side upon a board, and exposed, in the first and second experiments, upon the roof of a house, and in the third upon an open grass plot. After each experiment, if any defect of coloring appeared, the plates were retouched. The results were as follows:

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Arranging the colors according to the quantity of dew gained,

the following orders are obtained:

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The average gain of the several colors, arranged as above, is seen in the annexed scale.

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It is impossible from these results to detect any controlling influence in color, but the fact, that the least amount of moisture was gained by the black, in two cases out of three, and that it holds the lowest rank in the table of averages, is not in accordance with the experiments of Dr. Stark.

Six white pasteboard cards were next taken, belonging to the same pack, and presenting the same extent of surface. They were equalized in weight in the manner before mentioned, and at first painted on one side as follows:

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Thus prepared they were exposed on the grass plot, during five nights, upon the same board employed in the last experiments; and after each trial the cards were carefully weighed and equalized. At the close of the third trial, they were each painted on the remaining side, the colors being interchanged in the following manner.-B, red; A, yellow; F, green; E, black; C, blue; and D, being turned over afforded a white ground.

In the fourth and fifth trials, these second surfaces were exposed. The table below presents the results of this investigation.

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Colors arranged according to the amount of dew gained:

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It is seen in the experiments just detailed, that among the different colors the blue and red alone follow an invariable order; the former always occupying the highest and the latter the lowest rank; and that this is true notwithstanding the cards were changed after the third trial: a fact which shows that this law follows the pigment and not the card. But does the law attach to the color or the coloring matter. The latter appears to be the case. In Prof. Bache's essay on the influence of color upon radiation, a list is given of twenty-five substances, arranged in the order of their radiating powers. Prussian blue ranks the second, indigo the eighteenth, and vermilion the fifteenth. Prussian blue therefore, radiating more than vermilion, should contract more dew as accords with the fact. But it is evident that this effect is produced by the material of the pigment and not by the color, inasmuch as indigo has a less radiating power than vermilion, and consequently it would give but comparatively little dew. A fact which is shown in Table A, where the blue ranks very low in the scale of colors.

The results developed in these investigations afford no support to the assertions of Dr. Stark, that dark colors are more favorable than light to the deposition of dew; but, to their full extent, lead to the inference that color has no controlling power in this particular.

It is with much satisfaction that I have perceived the results of my humble researches, to accord with those, which Prof. Bache deduced from his elaborate investigations; wherever a comparison could be made. Thus with him I find, that color is not a determining quality in the radiation of non-luminous heat, that vermilion possesses a low radiating power, and that while Prussian blue possesses this property in a very high degree, indigo ranks amid the lowest on the scale. In Prof. Bache's experiments the rate of cooling was the test of the radiating power, in those just detailed, the amount of dew is the criterion. Now, without asserting that the non-influence of color upon radiation is fully established, it is certainly a circumstance worthy of consideration, that two independent modes of analysis have led to the same conclusions.

ART. XV.-A new Method of extracting Pure Gold from Alloys and from Ores; by C. T. JACKSON, U.S.G.S.

THE following method of obtaining pure metallic gold in the form of a spongy mass has been practised by me for several years, and no account of the process has, to my knowledge, heretofore been published. It is very useful to the chemist and to the manufacturer, and is more economical than any other method that I am acquainted with.

After separating the gold from silver by means of a mixture of nitric and chlorohydric acids as is usually done, the solution containing gold and copper is to be evaporated to small bulk and the excess of nitric acid is thus driven off.

A little oxalic acid is then added and then a solution of carbonate of potash sufficient to take up nearly all the gold in the state of aurite of potash is gradually added. A large quantity of crystallized oxalic acid is then added so as to be in great excess and the whole is to be quickly boiled. All the gold is immediately precipitated in the form of a beautiful yellow sponge which is absolutely pure metallic gold. All the copper is taken up by the excess of oxalic acid and may be washed out.

Boil the sponge in pure water so long as any trace of acidity remains, and the gold is then to be removed from the capsule and dried on filtering paper. It may be pressed into rolls, bars or thin sheets, by pressing it moderately in paper. I have made several useful applications of the gold sponge thus prepared, and had a tooth plugged with it in October, 1846, to which purpose it is well adapted.

By moderate pressure, the spongy gold becomes a solid mass and burnishes quite brilliantly.

The jeweller or goldsmith will find spongy gold to be quite convenient when he requires it for a solder, and it is a convenient form of the metal for making an amalgam for fine gilding. I have used it for some years in soldering platina, and prefer it to the filings or gold foil for that purpose. This method of separating fine gold from coarse, is very simple, and cheaper than the usual processes. It is applicable in the separation of gold from ores that may be treated by acids, and is vastly preferable to the method commonly used by chemists and assayers.

When making oxyd of gold for dentist's use, the chemist will find that oxalic acid added to his potassic solution, will at once recover all the gold that is dissolved in an excess of the alkaline solution.* Many other applications of this very simple method will occur to chemists and artisans.

* Much gold is lost by the usual method of preparing the oxyd.

ART. XVI.-Discovery of Tellurium in Virginia; by C. T. JACKSON, U.S.G.S.

EARLY in May last, Mr. Knowles Taylor of New York gave me two specimens of native gold, in mica slate rock, from an auriferous vein recently discovered in Whitehall, near Fredericksburg, Va. In one of the specimens I observed a considerable mass of a splendent foliated and sectile mineral, of the color of antimony, which I recognized as an ore of tellurium. The gold was imbedded in a mass of it, and it was also observed to exist disseminated through the rock in shining metallic leaves. On submitting this mineral to analysis, I discovered that it was a telluret of lead and gold or foliated tellurium ore. In the open glass tube before the blowpipe, telluric acid sublimes, and condenses in the cooler part of the tube in a yellowish white film which melts into drops. A little greyish sublimate also deposits, which is metallic tellurium. The residual matter, cupelled on mica, gave a well characterized glass of litharge, and a minute globule of pure gold. This interesting mineral has not, I believe, been heretofore discovered in the United States, and it is extremely rare in Europe. It had been mistaken for sulphuret of molybdenum, and was considered to be of no value. That error should be corrected, for it is not only valuable as an extremely rare mineral, but since, as I am informed, it occurs in abundance in the Virginia mine, it should be saved and wrought for gold, in the same manner as is practiced in the tellurium and gold mines of Transylvania. It is very easy to expel the tellurium by heat, and then the gold may be obtained by the usual processes of amalgamation by mercury, and discharge of the mercury by heat. Since I detected the tellurium, I have conversed with T. A. Dexter, Esq. of Boston, who has recently visited the mine, and has seen a considerable quantity of this tellurium ore in the vein. He gave me two very well characterized specimens, which he took from the vein in place; so there can be no doubt of its existence in a true auriferous vein.

I announced this discovery at the annual meeting of the American Academy of Arts and Sciences last month.

Boston, June 18th, 1848.

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