The following Communications were read :—

1. On the Existence of peculiar Crystals in the Cavities of the Topaz. Part I. By Sir D. Brewster.

2. On the Use of Colourless Ink in Writing. By Sir George Mackenzie.

Many years ago, the author had attempted to separate the component parts of common ink, with the view to get rid of its inconvenience in soiling everything with which it came in contact, by committing some of the parts to paper, and some to the ing with solutions, he found that, in all his trials, the paper was, pen. Worksooner or later, discoloured more or less, so as to unfit it for the market, and he abandoned the attempt. He afterwards tried salts of iodine, but failed to fix the colour which they yielded. After another interval, the subject again recurring, he was led, by an accidental circumstance, to think he might attain the long sought-for object by using dry powders for the paper, instead of solutions. The first trial satisfied him that he was, at length, to succeed; and, after persevering a considerable time, he brought certain processes so far as to yield good paper. With an almost colourless ink, prepared with permuriate of iron, traces were instantaneously produced, dark enough for ordinary purposes. The powder introduced

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into the machine for preparing the paper, is compounded of galls, anhydrous ferro-prussiate of potassium, and carbonate of lime, so diluted with rice flour, that enough, and no more of the powder than enough, remains among the fibres of the paper. The paper is sized before being passed through the machine, and is afterwards finished in the usual manner. Specimens of different qualities of paper were laid on the table, and written on with the colourless ink by the members present.

3. On the Use of Metallic Reflectors for Sextants, and on the Determination of the Errors arising from NonParallelism in the Mirrors and Sun-Shades of Reflecting Instruments. By John Adie, Esq.

The object of this communication was to shew that, by the use of metallic reflectors for sextants, greater accuracy was obtained in the observed angles, and also that larger angles could be observed. Objects were seen reflected by metallic mirrors, which could not be seen when glass was used; and that when the alloy was formed of pure metals, it was not subject to rust or tarnish, even when exposed to action of the sea air.

The author then exhibited a sextant fitted with these mirrors, which had been employed for a season in the survey of the north coast of Scotland, under Mr Mossman, and read extracts of letters from that gentleman.

In the second part, he describes methods by which the nonparallelism in mirrors and shades may be determined with great accuracy, before they are applied to reflecting instruments.

A Ballot then took place for the following Candidates, recommended by the Council at last Meeting for filling places in the Foreign Honorary list :—

all of whom were declared to have been duly elected Foreign Honorary Members of the Society.

The following Donations were presented :

List of Specimens of Birds in the Collection of the British Museum. Parts 1. and 3. Accipitres, Gallinæ, Grallæ, and Anseres. List of the Specimens of Lepidopterous Insects in the Collection of the British Museum. Part 1.

List of the Specimens of Myriapoda in the Collection of the British

Museum.

Catalogue of the Tortoises, Crocodiles, and Amphibanians, in the Collection of the British Museum.-By the Trustees of the British Museum.

The Electrical Magazine, conducted by Mr Charles W. Walker. Vol. I. No. 7.-By the Editor.

Tijdschrift voor Naturlijke Geschiedenis en Physiologie-Uitgege

ven door I. van der Hoeven, M.D. en W. H. De Vriese, M.D. Deel XI. Stuks 3, 4.-By the Editors.

Cast of the Bust of the late Professor Playfair, which was executed by the late Sir Francis Chantrey.—By Sir George Mackenzie, Bart.

Fifteenth Report of the Scarborough Philosophical Society.-By the Society.

Monday, 3d March 1845.

SIR T. M. BRISBANE, Bart., President, in the Chair.

The following Communications were read:

1. On the Existence of peculiar Crystals in the Cavities of Topaz. Part II. By Sir D. Brewster, K.H.

The author, after alluding to his former papers on the fluids in topaz, described, in Section I., the form and position of the strata in the minerals in which the cavities occur. They generally occur in immense numbers, occupying extensive strata, and injuring the transparency of the mineral. These strata occupy every possible

position, and have every possible curvature; their shape is equally irregular; and it is probable that, in every case, some edge or angle of the stratum touches the surface. The cavities are sometimes concentrically arranged, and sometimes occur in parallel straight lines. In one specimen, they radiate from a centre. When different strata occur in the same specimen, they generally differ in the character of the cavities, one stratum containing flat, another deep cavities, &c. The whole facts lead to the conclusion, that the strata of cavities have been formed under the influence of forces propagated through a plastic mass, carrying with them gases and vapours, which came to a position of rest previous to the crystallization of the mineral. In Section II. the author describes some new observations on the two fluids formerly discovered by him. In some cases of flat cavities, the faces of which are parallel to the planes of easy cleavage, the application of heat forced the fluid between the lamina of the crystal to a distant part. In one specimen, a white ball was seen to be projected from one cavity to the edge of the specimen, as in a case formerly described. In other specimens, where the dense fluid was accompanied by a bubble of some gas, the application of heat increased the size of the bubble, which then threw off a smaller to a distant spot. On cooling, the latter disappeared, and the former recovered its original size. Apparently the gas was here absorbed by the liquid on cooling. While the bubble expanded by heat, the liquid was forced into minute tubes or slits, from which, on cooling, it returned. In Section III. he described the form and position of the crystals in the cavities of topaz. They are both fixed and moveable, and often beautifully formed. They are very numerous, and occur in several different forms, which are enumerated, a very frequent one being the cube. In Section IV. he treated of the physical properties of these crystals. Many of them melt or dissolve in the fluid in a gentle heat, others with difficulty, some not at all. Those which melt are commonly reproduced, on cooling, of their original form, with modifications. The tessular crystals have no action on polarized light; but there seem to be two substances in this form, as some tessular crystals melted, while others were found infusible in any heat used. The doubly refracting crystals would appear to be of three kinds, as some melted easily, others with difficulty, others not at all. They did not depolarize white light, or the highest order of colours. One crystal melted, and was reproduced, without any fluid being present. In another cavity, several crystals, when heated, darted across the cavity, while others rotated

rapidly round their middle point. Too strong a heat often bursts the cavities, separating the laminæ of the topaz. The volatile fluid, escaping, leaves a crystalline residue; the dense fluid disappears entirely, and is probably a condensed gas. In one specimen, the faces of the cavities formerly filled with the volatile fluid, are corroded, as if by a solvent, developing crystalline structure; an appearance analogous to that which has been observed on the external surface of topazes. In Section V. the author described crystals embedded in the mass of the topaz, some of which can only be detected by polarized light, as they do not affect the transparency of the crystal by common light. He concluded by describing cavities lined with a doubly refracting crust or shell, with optical and crystallographic axes a phenomenon which has no parallel in mineralogy. These cavities have the appearance of embedded crystals, but are detected by depolarizing a uniform tint with a variable thickness of crystal.

2. On the Extraction of pure Phosphoric Acid from Bones, and on a New and Anomalous Phosphate of Magnesia. By Dr Gregory.

The author, after explaining the methods hitherto proposed for purifying the phosphoric acid of bone earth, and pointing out their inconveniences, mentioned that the chief difficulty was the separation of the magnesia always present in bone earth. The lime, it has been for some time known, may be entirely removed by means of sulphuric acid. In repeating the process of Liebig, which did not succeed in his hands, and which requires the use of alcohol, he found that, after the separation of the lime, there is obtained, by evaporation and heating, a clear and colourless glass, containing all the magnesia that this glass dissolved completely by boiling in water; but that the solution, if again evaporated and heated to 600° for a quarter of an hour in a platinum capsule, became turbid, and deposited the whole magnesia as an insoluble salt. When water was added, so as to dissolve the phosphoric "acid, and the insoluble salt separated by the filter, the filtered liquid was found absolutely free from magnesia, and was a solution of pure phosphoric acid. The insoluble salt of magnesia above mentioned is new. It is remarkable for its insolubility in water and acids; and its composition, as ascertained by several concurring analyses, is—