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But, however remarkable this identity of relations among the simple forms may be, the analogy between the crystals of the two substances will still allow of more points of coincidence. Both species exhibit two faces of cleavage, of different perfection; of which the one, parallel to M, is very easily obtained, while the other, parallel to T', is less apparent. There are also in both traces of cleavage, parallel to P. The crystals of both substances are commonly elongated in the direction of the short diagonal of their fundamental form, and implanted in the same direction.

In prismatic Glauber-salt we may obtain very long prisms of that description from a solution of sulphate of soda, if allowed to cool rapidly, while the crystals produced by a slow evaporation are in general shorter, and present a greater number of wellpronounced additional faces than the former. The facility of obtaining excellent crystals of this substance, render it very valuable for the study of the hemiprismatic forms. These crystals are perfectly limpid, and often of a considerable size. If exposed to the dry atmosphere, they are soon decomposed, and fall into powder; but I have preserved them for many months in their natural state, by keeping them in a cool place, wrapt up in paper, which was occasionally moistened. The decomposition, however, is not so rapid as to be prejudicial to the employment of the reflective goniometer, particularly if care be taken to work at low temperatures. It generally begins at certain points, and from these it spreads, creeping, as it were, over the surface, and through the whole mass of the substance, and thus rendering some parts of the faces of crystallisation perfectly dull, while other parts of the same faces retain their original splendour. The dry or anhydrous sulphate of soda, which is produced by the decomposition of prismatic Glauber-salt in a state of powder, can also be obtained crystallised. It is a well known fact, that the capacity of water for dissolving sulphate of soda arrives at its maximum when the temperature is considerably below the point of ebullition (33° centigr. or 1061° Fahr. according to Gay-Lussac) *; above this temperature crystals are formed in the fluid, which consist of anhydrous sulphate of soda. Their form is that of a scalene four-sided pyramid, Fig, 11., whose

Annales de Chimie, tom. xi. p. 313.

angles have not yet been ascertained, sometimes variously modified, and cleavable with great facility parallel to AC'XC, a plane passing through the axis and the short diagonal of the base. The specific gravity of this salt is = 2.462; its hardness

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2.5, between gypsum and calcareous spar; in both these characters superior to the common hydrous Glauber-salt, whose specific gravity is = 1.481, and its hardness 1.5...2.0, somewhat lower than gypsum. The forms of the two salts are in the same relation to each other as those of hydrous and anhydrous sulphate of lime (prismatoidal and prismatic gypsumhaloide); and from the circumstance, that the forms of the dry sulphate of soda are prismatic, like those of the dry sulphate of lime, it appears that Glauberite, the combination of both, on account of its hemiprismatic forms, cannot be considered as a mere mechanical mixture of these two substances, as has been supposed by some mineralogists.

The analogy between the forms of Glauber-salt and of Epidote is so striking, that after having compared the crystals of the two species with each other, it would be superfluous to add any farther remarks on the subject. But it will be necessary to say a few words on the Inclination of the Axis in hemiprismatic forms, which we have found in one of them to be 0° 33', and in the other 14° 40', and on the consequences of this import ant observation.

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It is impossible to derive the secondary forms of any of the two preceding species according to any regular process, from a scalene four-sided pyramid, in which, like Fig. 11., the three lines AM, BM, CM, or a, b and c, are perpendicular to each other. But the derivation will be effected with the greatest facility, if we suppose the axis AX, to be inclined to a perpendi cular AM upon the base CBC'B', Fig. 10., at a certain angle, variable according to the species to which the forms refer. This inclination of the axis is implied in some of Haüy's earlier determinations, as, for instance, in those of Epidote, of Gypsum, of Felspar, &c.; but he never established it as a general principle, and his later labours proved that he preferred considering hemiprismatic forms, under such dimensions as would render it possible that equally inclined faces appear on both sides of the axis. Professor Weiss has endeavoured, by slight corrections

course.

in the angles, to transfer into this new shape the relations of the crystalline forms in those species, which Haüy had not yet considered under the same point of view. Supposing the angles given by Haüy not to be exact, he corrected them, although not guided by observation, till the species could be reduced to the anticipated form. Professor Mohs followed an opposite Accurate observations, first on Glauber-salt, then on Blue Carbonate of Copper, and on several other species, had shewn that there really were angles which could not be obtained when the axis in the fundamental form was supposed perpendicular upon its base. He introduced, therefore, another mode of considering these hemiprismatic forms, supposing that they are derived from a pyramid like Fig. 10. All the hemiprismatic species, which since that time have been the subject of more accurate inquiry, have yielded results agreeing with the general principle, which supposes that in every hemiprismatic form, the axis will deviate from the perpendicular direction upon the base, for a certain angle, however small this angle may be found. The limits of its range, as obtained from experience, are 0° 29′ in axotomous Lead-baryte, and 22° 49′ in prismatic Brithynesalt (Glauberite). In many species, which have not been recently examined, the inclination of the axis must be supposed to be0: but it is highly probable, that this perpendicular direction of the axis upon the base will not stand the test of obser vation.

The most important consequence of this fact, when ascertain ed to a greater generality, will be, that the Systems of Crystallisation, viz. the tessular, the rhombohedral, the pyramidal, and the prismatic system, must be increased by another, which contains the hemiprismatic forms. Then the tetarto-prismatic forms will likewise require to be included within a particular system, in which the fundamental form, Fig. 12., is a scalene four-sided pyramid, whose axis is inclined to the base, not in a plane which is perpendicular to this base, and passes through one of its diagonals, as in hemiprismatic forms, but in a plane which passes through neither of the diagonals of the base, if it is supposed to intersect this base at right angles. The exact direction of the latter has not yet been established in any species by direct ob

servation.

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ART, XIX.-Observations on the Specific Gravity and Tem perature of Sea-Water, made during a Voyage from Ceylon to England, in 1819 and 1820. By JOHN Davy, M. D. F. R. S., &c. &c.

THE following series of observations has been taken from the

MS. Journal of Dr Davy, who has kindly entrusted it to us for this purpose. They embrace a very large portion of the earth's surface, and, from the accuracy and sagacity of the able chemist by whom they were made, we have no doubt that they will be regarded by philosophers as forming a valuable addition to our hydrographical knowledge.

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The series of experiments commences at Columbo in Ceylon, on the 4th December 1819: and between the 1st of February and the end of June 1820, they were made on board the Eclipse.

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In both these experiments, the water was taken from the

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Feb. 4. 1820. On board the Eclipse, N. Lat. 5° 19, East

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These temperatures were taken in sight of land.

VOL. X. NO. 20. APRIL 1824.

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Feb. 5. 1820. N. Lat. 4° 10', East Long. 80° 15'.

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By comparing the chronometer and dead reckoning, it appeared that the ship had been in a current flowing towards the west the whole day.

Feb. 6. 1820. N. Lat. 2° 19′, East Long. 81° 14'.

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The ship seemed, during this day, to have been in a current from the north.

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There was much thunder and lightning during the night, and

the weather was squally with heavy rain.

Feb. 8. 1820. S. Lat. 0° 5', East Long. 81o 37.

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