returned to their original state, that is to say, came to contain only 11 or 12 per cent. of water.

The density of a fragment saturated with water was 1.80 at 6 degrees, 1.812 at 6 degrees, 1.797 at 13 degrees; that of a fragment containing 0.111 of water, 1.67 at 2 degrees; and that of a dried fragment 1.53 at 5 degrees. In the two last experiments, the absorption of water, and the 'disengagement of gases were prevented, by covering the surface of the fragments with a thin coat of olive oil. Lastly, the density of this substance, when weighed dry out of the water, and under the water, after an absorption of eighteen hours, was found to be 2.215 at 13 degrees of the centigrade thermometer.

Chemical Characters.-Exposed to the heat of a lamp in a small matrass, this mineral affords water; gently heated in a platina crucible, it gives out all its water without losing its resinous lustre ; it becomes a little more translucent, with a tint of yellow opaline colour; when quickly heated, it decrepitates, splits, whitens, and becomes opaque by the intrusion of the air into the fissures which are formed.

It is infusible before the blowpipe. The thinnest splinters, when strongly heated, become transparent, and assume the vitreous lustre and hardness of hyaline quartz. It is affected, like pure silica, with all the chemical agents. Caustic potash in concentrated solution very readily attacks it at a boiling heat; it is dissolved almost instantaneously. Muriatic acid precipitates it in large white gelatinous flakes, when the liquor is concentrated; and, on the contrary, when a sufficient quantity of water is used, a precipitate is not immediately obtained, and by evaporation a transparent jelly is procured.

Analysis.-The water is not combined in this siliceous substance. I thought, at first, that it was; having been deceived by the difficulty of chasing the last portions of this fluid, whic is experienced when the heat of boiling water only is emplo but, I found, that, by a prolonged desiccation, the water al diminished, and at length was entirely expelled. The re of its analysis are the following:

JANUARY-MARCH 1827.

It contains no lime, nor have the oxides of iron or of manganeşe been detected in it. I have in vain searched for alkalies by means of carbonate of lead.

Observations.This siliceous substance differs from the quartzes and flints in many of its characters, and especially in the density, which, in these minerals, is about 2.65; but it has a great resemblance to the siliceous sinter (Quartz concretionné thermogène of Haüy). Both have the same lustre, the same hardness, the some fracture. The density differs but little; Klaproth found that of the thermogenous quartz to be 1.807. These two minerals appear to be a siliceous jelly scarcely consolidated; they are both equally soluble in potash, and they have both the property of retaining water, and the power of absorbing a new dose of it. The difference which exists between these two substances is the manner in which they appear in nature. The siliceous sinter or thermogenous quartz is almost always in stalactites or concretions in the neighbourhood of hot springs, particularly those of the Geysers in Iceland. A subspecies occurs in the island of Ischia, upon a decomposed granite, and is considered as a volcanic production*. The position of the gelatinous quartz is different.

Geognostical and geographical positions.—It occurs in the Commune of Tortezais, in the Department of the Allier, and is very abundantly diffused there, sometimes serving as a cement to sandstones, and sometimes occurring in the midst of these sandstones, in masses often of considerable size. On the route from Noyant to Cosne, between Bussière and Tortezais, one of these masses is seen intersecting the road for a length of 30 metres, and recurring on each side in the fields in detached pieces over a great extent.

It is fissured in various directions, without any appearance of regularity, The surfaces exposed to the air are always more or less altered, and pass into floatstone (nectic quartz) I have not been able to meet with it in the form of concretions. If this

* Vide Jameson's System of Mineralogy, and Manual of Mineralogy.

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substance has been deposited by hot springs, they must have been very large and very numerous, and it would be astonishing should no remains of them be still visible. None of those which I saw were either saline, or warm, or incrusting. The nearest #warm spring is that of Bourbon l'Archambault, and it does not form siliceous deposits.

The sandstones which contain this gelatinous quartz must have been deposited at the same time with itself, for they are intimately mixed. The gelatinous part always contains rounded grains of quartz, and it is rare that the sandstones have not this jelly, which serves as a cement to it, although it is only in small. quantity; and there is a transition from the one to the other by a change in the proportions of the rounded grains, and of the dissolved portion.

The variety of sandstone which abounds most in gelatinous silica, is formed of grains of hyaline and milk-white quartz, rounded, and of a small size; some grains of opal also are seen in it, but there is no felspar or kaolin. When the silica is in the nectic state, it is difficult to determine whether there be kaolin or not, from the mutual resemblance which these two white and friable substances possess. Another variety of sandstone, is, in a great measure, formed of grains of hyaline quartz; some scales of mica and spots of red oxides of iron are also perceived in it. The red spots are seen to increase in size and number; they are formed of a siliceous paste, coloured with tritoxide of iron. The red colour at length predominates, and the mass becomes entirely of that tint; a multitude of small grains of quartz and of gelatinous spots are, however, seen in it.

These sandstones are supported by conglomerates composed of blocks of quartz, granite and micaslate. These conglomerates rest immediately upon the primitive formations. Above the red-sandstones there occur strata of sandstone and bituminous slate, with impressions of ferns and junci, containing beds of black coal and iron-ore. They have the same direction and inclination as the coal-sandstones which they support. No rock of volcanic origin is found in all these formations. It is, therefore, in an intermediate deposit, which might be referred to the old red-sandstone, or the lower beds of a coal-formation, that

this gelatinous silica occurs. This position is very different from that of the thermogenous quartz of the islands of Iceland and Ischia.-Annales des Mines 1826.

Experiments to compare the specific Heat of Air, under a constant volume, with its specific Heat under a constant pressure. By Mr HENRY MEIKLE. (Communicated by the Author.)

IT has been long known, that gaseous bodies emit heat when

compressed, and absorb it when dilated, a property, by the by, which is not easily reconcileable with the creed of those who suppose heat to be mere motion. Little, however, was ascertained, for a considerable time, regarding the amount of the change of temperature accompanying a given change of density. The earliest experiments to determine this question seem to have been those of Professor Leslie. Mr Dalton and M. Gay Lussae have also engaged in the same inquiry *. As the heat evolved or absorbed by a change of density, depends on the difference between the specific heat under a constant pressure, and that under a constant volume, if we could find the ratio of these quantities, we should be enabled to determine their relation to the heat evolved or absorbed, and from this the change of temperature, and conversely. From certain experiments of MM. Delaroche and Berard, the Marquis de Laplace instituted some calculations †, which happened to come nearer the point than could have been expected; for these experiments were not at all suited to the purpose; and it is the more remarkable, that they should

According to the experiments of this last author, tinder or amadou is inflamed by the sudden compression of air into one-fifth of its bulk. 'Some have even questioned the fact, and others conjecture, that combustion commences at lower temperatures, as the air is denser. But may we not suppose, with more probability, that the pressure on the tinder, being suddenly augmented in an almost nine-fold ratio, should elicit much heat from this compressible substance itself? So that, till something else be known on the subject, we need neither doubt the fact, nor believe that a fivefold compression of air would of itself generate an inflammatory temperature. The melting of fine wires, or thin metallic leaves, would afford a surer test of the temperature in compressed air, than the kindling of soft spongy bodies.

Annales de Chimie et de Phys. iii. 238.

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have been used as the basis of such calculations, considering that, at as early a period (1812), MM. Desormes and Clement, had, with a very different view, made some better-contrived experiments, from which an approach to the true quantity could have been made with more certainty. Their method was very simple, and required no thermometer to shew the variations of temperature,—a contrivance which is said to have been first suggested by Lambert. No notice, however, seems to have been taken of these latter experiments,—probably because they were associated with a most fanciful inquiry after the absolute zero, till MM. Gay Lussac and Welter undertook a similar and more extensive series of experiments, giving nearly the same results. Of both of these and the inconsistent conclusions deduced from them by MM. Laplace and Poisson, I have had occasion to speak in the first volume of this Journal, where I have shewn that, whatever be the ratio of the specific heat of air under a constant pressure, to its specific heat under a constant volume; if that ratio only be constant, the variations of the quantity of heat in a mass of air must be uniform, while those of its volume, under a constant pressure, form a geometrical progression; and it is remarkable, that our first-rate authorities on the subject, who admit the constancy of this ratio, did not see that it was directly at variance with the commonly received theory of the airthermometer.

But, although the value of the ratio referred to have nothing to do with the true law of temperature, yet its exact determinabition would be of great moment in various researches. Consitoliderable deference is due to the experiments of the illustrious philosophers above mentioned. They were well calculated for shewing that the ratio of the specific heats is constant; because, supposing any inaccuracy to attach to them, it would be common to all the cases. But I had always some doubt whether their apparatus was the most eligible for determining th

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value of that ratio. The apparatus mostly employed principally of a glass balloon, to the neck of which wa brass cap and stop-cock. From the side of the cap, n horizontal pipe, communicating with a vertical gla minating in some light liquid to act, the part of a gauge or measure of the variations of pressure.