its course is parallel with the others, and terminates amongst them; in another part of its course it intersects these same concretions, and then it ends. It is evident, that if this concretion had continued parallel with the others throughout its whole extent, we would not hesitate to consider it as of cotemporaneous formation with them. If this inference be admitted, and we do not see on what ground it can be refused, it is evident, that, if the intersecting portion of the concretion is a continuation of that parallel with the other, both must be considered as having crystallized at the same time, and as a simultaneous formation with the whole rock in which it is contained. But the tabular concretions intersected by the one part of the concretion, are equivalent to veins or dikes, because they are tabular masses intersected by another tabular mass; and as all these concretions are of simultaneous formation, it follows, that the crossing concretion and that which is crossed, which are equivalent to two veins, of which the one crosses the other, have been formed at the same time. EDINBURGH, April 1819. ART. XXVIII.—Notice respecting a Singular Optical Property of Tabasheer. By DAVID BREWSTER, LL.D. F.R.S. Lond. and Edin. Communicated by the Author. THE substance called Tabasheer*, has been long known in eastern countries, and formed an important article in the Materia Medica of the Arabian Physicians. In the Gentoo language it is called Vedro-Paloo, or Bamboo milk; in the Malabar, Mungel Upoo, or Salt of Bamboo; and in the Warriar, Vedroo Carpooram, or Bamboo Camphor. It is found in the joints of the female bamboo, sometimes in a fluid state like milk, sometimes with the consistency of honey, but generally in the form of a hard concretion. Some specimens of it are transparent, and resemble very much small fragments of the artificial pastes made: in imitation of opal; others are exactly like chalk; while a third kind is of an intermediate character, and has a slight degree of translucency. Pliny clearly describes Tabasheer under the name of Sugar. The word is derived from the Persian Scher, or the Sanscrit Kschiram, signifying milk. See Humboldt on the Natural Family of the Grasses, The first person that examined the properties of this substance was Mr Macie* (now Mr Smithson), who analysed a portion of the Tabasheer from Hyderabad, which Dr Russell + had the preceding year presented to the Royal Society. "From its indestructibility by fire;-its total resistance to acids ;—its uniting by fusion with alkalis in certain proportions into a white opaque mass, in others into a transparent permanent glass, and its being again separable from these compounds entirely unchanged by acids," he considers it "as perfectly identical with common siliceous earth.” In the year 1804, Messrs Humboldt and Bonpland brought with them from America some specimens of Tabasheer, called Guaduas butter by the Creoles, taken from the bamboos which grow to the west of Pinchincha in the Cordilleras of the Andes. These specimens were analysed in 1805, by Messrs Fourcroy and Vauquelin §, who found them to be different from the Tabasheers of Asia. Instead of being wh: lly composed of silex, they contained only 70 per cent. of this earth, and 30 per cent. of potash, lime and water. The Tabasheer which I employed in my experiments, was sent from Nagpore by Dr Moore to Dr Alexander Kennedy, who was so kind as to favour me with a considerable portion of it. It had the same general chemical characters as the Tabasheer from Hyderabad, which was used by Mr Smithson, the same specific gravity nearly, and the same external appearance; so that I have no hesitation in considering it as also composed principally of silex. When the semi-transparent specimens of this substance are immersed in water, they imbibe it with great rapidity, emitting numerous bubbles of air. The transparency increases whenever the air has been discharged, and after a few minutes the water pervades, and renders transparent the whole mass. If a small portion of water, on the contrary, is laid upon the Tabasheer when dry, instead of adding to its transparency as might have been expected, it actually renders it as opaque and white as chalk; and, from the same cause, the Tabasheer which has been saturated with water becomes opaque, as the water eva * See Philosophical Transactions, 1791, p. 368. Humboldt's Personal Narrative, vol. i. Introd. p. xiii. Note porates; reaches its maximum degree of opacity; and recovers its semi-transparency when perfectly dry. The increase of transparency from the absorption of water, is an effect easily explained, and is one with which mineralogists have been long familiar in the phenomena of hydrophanous opal; but the production of opacity, by the absorption of a smaller portion of the same fluid which produces transparency, is a fact entirely new and not easily explicable upon known principles. After having determined that the white opacity was not the result of any chemical change, and must, therefore, have resulted from optical causes, I attempted to frame some hypothetical explanation of the phenomenon. In tracing the progress of a ray of light through a porous body, having a small quantity of water in its pores, and through another which had these pores filled with water, I saw that opacity could be produced in the first case only upon the supposition that the Tabasheer had a refractive power considerably lower than water. Improbable as this supposition was, I immediately formed one of the semi-transparent specimens into a prism, and found, to my great surprise, that the refractive power of Tabasheer was not only lower than water, but so much dower, as to be almost intermediate between water and the gases. I repeated this experiment with various specimens from Nagpore, and also upon one from Hyderabad, with which I was favoured by Dr Hope, and which, as it formed part of the parcel of which Dr Russell had presented a portion to the Royal Society of London, was the same as that which was analysed by Mr Smithson*. The following were the results. Air,... Index of Refraction. ..1.0000+ Tabasheer from Hyderabad, yellowish by reflected light, 1.1115 We trust that some of the Members of the Institute of France will be induced to measure the refractive power of the Tabasheer brought from Quito by M. Hum The physical properties of Tabasheer are not less singular than its optical qualities, and indicate a structure of a very remarkable kind. A detailed account of my experiments on this subject, has been transmitted to the Royal Society of London, and will probably appear in the Second Part of the Philosophical Transac tions for 1819. EDINBURGH, May 1. 1819. ART. XXIX.-Account of the Expedition to Baffin's Bay, under Captain Ross and Lieutenant Parry. Drawn up from Captain Ross's account of the Voyage, and other sources of information. IN the year 1815, and the two succeeding years, numerous masses of ice were seen floating in the Atlantic; and in 1817, it was reported by vessels from the arctic regions, that a very considerable extent of ice had disappeared between Greenland and Spitzbergen. These unusual appearances directed the attention of the learned to the almost forgotten subject of a passage across the Pole. An ingenious and intelligent writer published several dissertations in the Quarterly Review, for the purpose of demonstrating the practicability of a passage across the polar seas. The breaking up of the ice on the east coast of Greenland held out to him the prospect of arriving directly at the Pole through an open sea; while the want of precision in Baffin's narration of his voyage, combined with physical and hydrographical considerations, induced him to expunge the Bay of Baffin from our maps; and to predict the existence of a passage to Behring's Straits by the northern extremity of the American continent. Captain Scoresby, who had distinguished himself in no fewer than sixteen voyages to the Arctic Regions, had maintained, boldt, if any of it is still in existence. It will be interesting to know if the 30 per cent, of potash and lime produces any perceptible effect upon the refractive power and other properties of the Tabasheer. I have sent a quantity of the Nagpore Tabasheer to M. Berzelius, with the hope that he may have leisure to submit it to an accurate examination. As this distinguished chemist is now in Paris, he would do a service to science by comparing directly the Asiatic and American Tabasheers, ар that the Pole, guarded by a frozen barrier, could only be proached by the alternate use of boats, and of sledges drawn by dogs; while his learned opponent, on the authority of every iceberg that travelled to the south, insisted that a change of temperature had effected an opening through the frozen ridge; and that, while we accomplished the great object of a passage across the Pole, we might execute, also, the more romantic enterprize of releasing the lost colony of Eastern Greenland, whom the accumulated ice was supposed to have for ever separated from the rest of the world. The public took a deep interest in speculations like these, where the dry details of hydrography were enlivened by discussions and schemes almost bordering upon romance; and though they were assailed by poetical theories of climate, and the usual allowance or malevolent predictions, yet the general expectation of advancing the interests of natural science, and of practical navigation, would not permit itself to be damped; and there were a few, more sanguine than the rest, who expected that the British flag would be fixed upon the Pole of the world, whether it was deposited from a sledge and four, or more formally transplanted from the quarter-deck of a British vessel. With the greatest liberality and love of science, the British Government equipped four vessels, viz. the Isabella of 385 tons, and the Alexander of 2521 tons, under the command of Captain Ross and Lieutenant Parry, for the purpose of exploring the passage through Baffin's Bay; and the Dorothea of 382 tons, and the Trent of 249, under the command of Captain Buchan and Lieutenant Franklin, with the view of penetrating directly into the Polar regions, by the way of Spitzbergen. These vessels were adapted in the most scientific manner for the perils which they had to encounter, and every precaution was taken for insuring the health and comfort of their respective crews, and for accomplishing in the most satisfactory manner the general object of the expedition. The expedition under Captain Ross left Deptford on the 18th April 1818. It reached Lerwick in Shetland on the 30th, and on the 1st of June it entered Davis' Straits, after encountering an iceberg about 40 feet high and 1000 feet long. On the 14th of June it reached Whale Islands, in latitude 63° 54', |