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decomposition of sulphuretted hydrogen. It is well known that alkaline and earthy sulphates are reduced to sulphurets by organic matters, with the aid of heat, or even at ordinary temperatures, in presence of water. To the decomposition of these sulphurets by water and carbonic acid, we are to ascribe not only the sulphuretted hydrogen of solfataras, which, by its oxydation under different conditions, gives rise either to free sulphur, or to sulphuric acid and to gypsum by epigenesis, but also the sulphuretted hydrogen which appears in springs and in stagnant waters, where the sulphur produced by the decomposition of the gas is often mingled with sedimentary gypsums.* (See Bischof, Lehrbuch, ii, 139-185.) This author has suggested the decomposition of chlorid of magnesium by alkaline or earthy sulphurets as a source of sulphuretted hydrogen and hydrate of magnesia, into which sulphuret of magnesium is readily resolved in the presence of water. (Chem. Geology, i, 16.) If a salt of calcium were present, this reaction could only take place in the absence of carbonic acid, for carbonate of magnesia is incompatible with chlorid of calcium. The direct reduction and decomposition of sulphate of magnesia by organic matter and carbonic acid may, however, yield sulphuretted hydrogen and carbonate of magnesia, and thus, in certain cases, give rise to magnesian sediments.

69. In the preceding sections, we have supposed the waters mingling with the solution of sulphate of magnesia to contain no other bicarbonate than that of lime, but bicarbonate of soda is often present in large proportion in natural waters, and the addition of this salt to sea-water or other solutions containing chlorids and sulphates of lime and magnesia, will, as we have seen, (81) separate the lime as bicarbonate, and give rise to liquids, which, without being concentrated brines as in the previous case, will contain sulphate of magnesia, but no lime salts. A farther portion of bicarbonate of soda will produce bicarbonate of magnesia, by the evaporation of whose solutions as before, hydrated carbonate of magnesia would be deposited, mingled with the carbonate of lime which accompanies the alkaline salt, and in the case of the waters of alkaline springs, the compounds of iron, manganese, zinc, nickel, lead, copper, arsenic, chrome, and other metals, which springs of this kind still bring to the surface. In this way the metalliferous character of many dolomites is explained, as also the frequent association of metals, such as copper, nickel, cobalt, chrome and titanium, with serpentine, steatite, diallage, olivine, and other magnesian silicates, which owe their origin to the alteration of magnesian sediments such as we have described.

* On certain modes of decomposition of the sulphates, see Jacquemin, Comptes Rendus, June 14, 1858.

70. As the separation of magnesian carbonate from saline waters by the action of bicarbonate of soda does not suppose a very great degree of concentration, we may conceive this process to go on in basins where animal life exists, and thus explain the origin of fossiliferous magnesian limestones like those of the Dudswell (§ 53,) and the Silurian rocks of the western United States, whose fossils, as I am informed by Mr. James Hall of Albany, are generally such as indicate a shallow sea. To the intervention of carbonate of soda is I conceive to be referred the origin of all those dolomites which are not accompanied by gypsums, and which make up by far the larger part of the magnesian limestones; nor will the dolomites thus derived be necessarily marine, for the same reagent with waters like those of the Danube and Arve would give rise to dolomites and magnesites in fresh-water formations, which unlike those mentioned in § 67, would not be accompanied by gypsums.

71. To the first stage of the reaction between alkaline bicarbonates and sea water I am disposed to ascribe the formation of certain deposits of carbonate of lime which although included in fossiliferous formations, are unlike most of their associated limestones, not of organic origin, but have the characters of a chemical precipitate of nearly pure carbonate of lime, in which are often imbedded silicified shells and corals.* It is not perhaps easy in all cases to distinguish between such precipitates, which may assume a concretionary structure, (see on this ques

*The large proportion of dissolved silica which many river waters contain (§ 64) appears in sedimentary deposits, not only replacing fossils and forming concretions and even beds of flint, chert and jasper, but also in a crystalline state, as is seen in the crystallized quartz often associated with these amorphous varieties, and in some beds of sandstone which are made up entirely of small crystals of quartz. Elie de Beaumont long since called attention to the crystalline nature of certain sandstones which as Daubrée has remarked, could not have been derived from the disintegration of any known rock, and Mr. J. Brainard at the meeting of the American Association for the Advancement of Science, held at Cleveland, insisted upon the crystalline character of the grains composing sandstones in Ohio, as evidence that these were chemical deposits. He however fell into the error of supposing that all sandstones and even quartzose conglomerates have had a like origin, while the latter and the greater part of the former are undoubtedly mechanical deposits from the ruins of pre-existing quartzose and granitic rocks.

These crystallized sands according to Daubrée, are met with in beds in the sandstone of the Vosges, the variegated sandstone (Triassic and Permian,) in the tertiary of the Paris basin and elsewhere. Other sands are made up of globules of calcedony, apparently like the crystallized sands a chemical deposit, and associated with oolitic iron ores in the lias, and with glauconite grains in the green-sand. (Daubrée Recherches sur le Striage des Roches, etc., Ann. des Mines 1857, 6 livr.) We may here mention the so-called gaize from the green sand of the Ardennes, which gave to Sauvage 560 p. c. of amorphous soluble silica mixed with quartz sand and glauconite. (Bischof, Lehrbuch, i, 768-811.)

Maschke has shown that under certain conditions silica is soluble in about twentyfive parts of pure water; from this solution it separates by evaporation or by the addition of concentrated saline solutions in a form insoluble in water. (Jour. für prakt. Chemie, lxviii, 233.) In these reactions we have a key to the formation of silicious deposits.

tion Bischof, Chem. Geology, i. 428,) and those deposits which like travertines have been formed from subterranean springs. In neither case however, should they be confounded with the tufaceous limestones mentioned in § 63.

72. The union of the mingled carbonates of lime and magnesia to form dolomite, is attended with contraction, which in case the sediment was already somewhat consolidated, would give rise to fissures and cavities in the mass. Should the dolomitic strata be afterwards exposed to the action of infiltrating carbonated waters, the excess of carbonate of lime and any calcareous fossils would be removed, (§ 30,) leaving the mass still more porous, with only the moulds of the fossils. Insoluble however as it appears to be at ordinary temperatures, the filling up of such cavities both in magnesian and in pure limestones, not less than its deposition in veins and druses, indicates that dolomite is under certain conditions soluble.

The lowest temperature at which hydrous magnesian sediments may be transformed into magnesite and dolomite has yet to be determined. The requisite heat has however doubtless been attained by the accumulation of overlying sediments, in virtue of that law which causes the temperature to increase as we penetrate the earth's crust. This increase we may suppose with Mr. Hopkins to have been much more rapid in former epochs than at present.-(Geol. Journal, viii, 59, also Phillip's Manual of Geology, 609.)

Conclusions.

1. The action of solutions of bicarbonate of soda upon sea water separates in the first place the whole of the lime in the form of carbonate, and then gives rise to a solution of bicarbonate of magnesia, which by evaporation deposits hydrous magnesian carbonate.

2. The addition of solutions of bicarbonate of lime to sulphate of soda or sulphate of magnesia gives rise to bicarbonates of these bases, together with sulphate of lime, which latter may be thrown down by alcohol. By the evaporation of a solution containing bicarbonate of magnesia and sulphate of lime, either with or without sea salt, gypsum and hydrous carbonate of magnesia are successively deposited.

3. When the hydrous carbonate of magnesia is heated alone under pressure it is converted into magnesite, but if carbonate of lime be present, a double salt is formed which is dolomite.

4. Solutions of bicarbonate of magnesia decompose chlorid of calcium, and when deprived of their excess of carbonic acid by evaporation, even solutions of gypsum, with separation of carbonate of lime.

5. Dolomites, magnesites and magnesian marls, have had their origin in sediments of magnesian carbonate formed by the evaporation of solutions of bicarbonate of magnesia. These solutions have been produced by the action of bicarbonate of lime upon solutions of sulphate of magnesia, in which case gypsum is a subsidiary product; or by the decomposition of solutions of sulphate or chlorid of magnesium by the waters of rivers or springs containing bicarbonate of soda. The subsequent action of heat upon such magnesian sediments, either alone or mingled with carbonate of lime, has changed them into magnesite or dolomite.

ART. XLI.-On Gallic and Gallhumic (Metagallic) acid; by Dr. F. MAHLA, Ph.D., Chicago.

It is mentioned among the reactions of gallic acid in almost every handbook of chemistry, that its solution produces a deep bluish-black color with a solution of the salts of the sesquioxyd of iron, which disappears, when the solution is heated. As I have nowhere found an explanation of this fact, I have tried to investigate it by some experiments.

When the solutions of the sesquioxyd of iron and gallic acid are used in a diluted state, the resulting mixture appears only slightly colored, but if they are concentrated, it assumes after being heated to ebullition, a dark brown tint, and then causes black spots on the skin, which can be washed away only with the greatest difficulty. Such a solution might perhaps be used advantageously as a hair dye.

If the iron-solution was not added in too large proportion, liquid ammonia no longer precipitates hydrated sesquioxyd of iron, but the proto-sesquioxyd (black oxyd). A reduction takes place therefore, the oxygen transforming some of the carbon of the gallic acid into carbonic acid, which is freely evolved during the ebullition.

To a portion of gallic acid, dissolved in water and heated to ebullition, a solution of sesquichlorid of iron was carefully added in small quantities and the mixture heated again after each addition. This treatment was continued, until a drop of the solution mixed with a little water ceased to give the characteristic bluishblack precipitate of gallic acid with sesquichlorid of iron. A solution of carbonate of soda was then added in slight excess and the black precipitate separated by filtration. A portion of the filtered dark-brown liquor, after being exactly saturated with hydrochloric acid, deposited a voluminous black precipitate, which if dried, formed a black shining mass but when freshly

precipitated, was easily redissolved by free muriatic acid. Such a solution, containing but little free muriatic acid, produced black insoluble precipitates with limewater, with the different salts of lime and baryta, with sulphate of zinc and sulphate of copper. Another portion of the filtered liquor super-saturated with acetic acid, caused precipitates of a black color in solutions of acetate of lead and nitrate of silver. From the silver precipitate, metallic silver was soon separated.

The

The lead precipitate was carefully washed with distilled water, and after being dried in an air-bath at a temperature not exceeding 200° F. (94° C.) for ten hours, it was heated over a spirit lamp, until the organic matter was perfectly destroyed. residue, consisting of a mixture of oxyd of lead and metallic lead, was treated with acetic acid, and from it the whole quantity of oxyd of lead was calculated.

1052 gram. gave 0.662 of the mixture of PbO+Pb, which left after being treated with acetic acid 0.037 metallic lead, a quantity corresponding to 0.041 oxyd of lead. The acetic acid extracted 0.625 oxyd of lead, which quantity added to the above found 0.041 gives 0-666. This is equal to 63.30 per cent.

Gallhumic (metagallic) acid, which was detected by Pelouze in the residue of distillation, when gallic acid was suddenly heated to 480° F. (249° C.) shows the same reactions, and its lead salt, 2PbO, C12H303, contains 63:04 per cent of the oxyd of lead. No doubt can therefore exist about the identity of Pelouze's acid and my product. Two equivalents of gallic acid are divided exactly into one equiv. of gallhumic, two equiv. of carbonic acid, and three equiv. of water:

CH01=

C12H303
C2 04
H303
C14H6O10

This origin of gallhumic acid forms another and interesting argument, that pyro-acids can be obtained otherwise than by the

action of heat.

If some powdered "red precipitate" is added to a solution of gallic acid and heated over a spirit lamp, it is immediately reduced; gallic acid precipitates suboxyd of copper (red oxyd) in a solution of sulphate of copper; this reaction appears with the greatest facility if the solutions are heated together. It also reduces a cold solution of neutral chromate of potassa, producing the green sesquioxyd. The gallic acid is in each of these cases transformed into gallhumic acid. The action of these substances on gallic acid and the formation of the new product, is explained by assuming gallhumic acid to be only an intermediate product, the final result being carbonic acid and water.

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