ment of the animal matter has evidently been made chiefly by carbonate of lime, of which the matrix consists; and if we suppose that a little less than one-half of the original animal matter has given place to calcareous and a small quantity of siliceous matter, whilst the larger proportion has been dissipated without substitution, the scale in its original state would have a constitution very analogous to the others; the apparently larger proportion of phosphate of lime being in this way accounted for. To what extent the analogy may have gone in other respects between the Megalichthys of the carboniferous strata of Burdiehouse, and the fish of the far more recent beds of Tilgate, is a point which must be left to the decision of those better qualified for the task, upon a due consideration of all the accompanying fossils.

There seems every reason to believe that the occurrence of true scales of saurian animals, in a fossil state, is very rare, compared to that of fish scales,—a fact which the chemical nature of the former fully prepares us to anticipate; and I have never had any opportunity of examining, or indeed of even seeing, an undoubted saurian fossil scale. In the Ossemens Fossiles of Cuvier, I observe only two instances of such remains, notwithstanding the numerous fossil animals of this kind described in that work. One of these occurs in a fine calcareous slab containing vertebræ and ribs of the fossil Gavial of Caen, in Normandy, in which several rows of scales occur in the precise relative position which they must have occupied in life *. and as to which, therefore, not a doubt can exist that they are of true saurian origin. The account given of them by Cuvier is, that they differ from those of living crocodiles more than any other part of the skeleton, and this crocodile of Caen was undoubtedly the best mailed of the whole genus. The scales are very thick, rectangular, thinner towards the edge, and their whole external surface is hollowed into little hemispherical cavities, of the size of a lentil or pea, and pressed against one another." If the original chemical composition of these scales was the same as those of living reptiles, which there is every reason to anticipate, they ought to contain very little phosphate of lime, and will in all likelihood be found now to consist prin

"

* Oss. Fos. tom. v. part 2, p. 139. Pl. vii. fig. 14.

cipally of calcareous matter, the matrix being of that nature, although siliceous matter may also possibly enter, to a greater or less extent, into their composition. It would be a matter of interest if the friends of science in France would have this matter ascertained.

The other instance mentioned by Cuvier,* refers to fossil scales supposed to belong to crocodiles found in the neighbour. hood of Argenton, in the Paris basin. They have a considerable resemblance to those of Caen; and it is remarkable that they are described as having a very prominent bony crest, "une crete osseuse tres saillante,”—a character apparently having some connection with the circumstances formerly stated as to carinated scales. These fossil scales would also be well deserving of an examination.

I could have wished to have made my examination of such relics more extensive, but in this place no other materials were to be met with.

EDINBURGH, 3d July 1835.

Notice of an arrangement for the measurement of the Angles of very Minute Crystals. By the Rev. EDWARD Craig, A.M., Oxon., F. R. S. E, Councillor of the Society of Arts.†

In the winter of 1833 I was led to investigate the phenomena of chemical action under the microscope, and submitted to the consideration of the Royal Society of Edinburgh, during that session, a short paper on the subject. The method which I adopted was, to place very minute portions of chemical agents upon thin plates of flat glass, and to bring them together; then, when one plate was pressed down upon the other with a uni form, or nearly uniform, pressure, the substances under examination were reduced to a very thin film, capable of observation, by transmitted light, and of being easily brought under the focus of a highly magnifying lens.

very

distinct

In this way an endless variety of operations were observed, especially those connected with the application of galvanism to

* Oss. Fos. tom. v. part 2. p. 168.

+ Received 8th April, and read and exhibited to the Society of Arts for land 20th May 1835.

chemical agents. My attention, however, was much directed to the beautifully minute crystallizations which took place in that apparently limited space; sometimes above a hundred exquisitely symmetrical and perfect crystals lying together, and capable of distinct observation, in the space of the 5000dth of a square inch.

A difficulty, however, arose as to the nature and composition of these crystals. The substances brought together in this way necessarily meet each other in unmeasured and, to us, indefinite quantities. The whole quantity used was too small for weight; and, even if each substance could have been weighed, when spread with extreme tenuity over a comparatively large surface of glass, it was impossible to ascertain the proportion, or the proportional strength in which one substance would meet others at any given point of the whole surface covered. When changes took place, and new combinations and crystallizations appeared, a degree of doubt therefore arose as to the precise nature of the substance. At all events, the nature of the substance could not be predicated so decidedly as in a case where the whole measured quantity of two or three known agents, known also in their relations, assumed in combination one marked and familiar character. And if this doubt would arise with respect to minute crystallizations originating in the combination of known. substances, it would be still more the case in the attempt to analyze by this mode of microscopic investigation some small portion of a substance whose composition was not known; and in which the varied results under the tentative process must be greatly, at least for a time, matter of conjecture.

I have no doubt, after lengthened observation, that familiarity with microscopic crystalline forms, and the accurate recording and classifying of phenomena, would go far to remove much of this uncertainty; but besides this, it appeared to me that, as in the case of many substances, the angles of crystallization are accurately known, the range of doubt might be considerably limited, if the angles of crystals so formed could be submitted to accurate measurement. And, with this view, I adopted the fol. lowing arrangement for the application of a micrometrical gonio

meter to the microscope:

The eye-piece of my microscope is a tube about an inch and

a half long, having one lens at each extremity; and the tube is smoothly and accurately fitted, not to screw, but to slip down into the tube of the object-glasses, Fig. 1.

The lower lens O' receives the image from the object-glasses, and this image is examined by means of the upper lens O. In the focus of the upper lens O, and lying directly on the plane side of the lower lens O', which is plano-convex, I placed two fine silk threads, crossing each other at right angles. The tube of the eye-piece is slipped down, not screwed down, to its right position in the tube of the object-glasses. It can be turned, therefore, either way freely, without affecting the focal arrangement of the microscope, which is regulated by other means. And, by turning the eye-piece round, these fine threads, unmagnified, may be applied to the side of any angle of a highly magnified crystal lying on the field below.

A brass circle C, about two inches and a half in diameter, with a divided scale on its outer edge, was made to screw fast on to the object-tube T, before the insertion of the eye-piece. And to the edge of the eye-piece a small index I was fixed, so as to lie close on the circle C, and point to the graduated division. This index is not a simple point, but a vernier, for indicating minutes.

To measure the angle of a crystal F, it is necessary to bring the point of the angle to the centre of the field of view, where the silk diameters cross each other; then, by turning the eyepiece round, to bring one of these silk lines to coincide with one

le of the angle to be measured, and notice the point of the

scale at which the index stands. The eye-piece must be again. turned till that thread has passed over the whole angle, and coincides with the other side of it. The difference of position of the index read off will give the measurement of the angle.

The operation requires some neatness of manipulation, but this is acquired by practice; and if the operation be repeated and a mean taken of the errors which occur each time, it will be easy to come very near the truth.

In this way the angles of crystals not more than the four or five millionth of an inch in size may be accurately ascertained. And the introduction of measurement in such cases has surely a great advantage in this respect, that such crystals are more perfect and sharp than almost any that can be obtained of a large size; and that frequently a whole family of crystals, in ́all their correlative forms and varieties, will be found lying together; such as, in any size adequate for manipulation, the diligent search of a whole lifetime could not have collected.

EDINBURGH, 8th April 1835.

Description of a New Spring for Shutting a Door, which goes both ways. By ALEXANDER BEATTIE, Foreman to Mr ROBERT RITCHIE, Ironmonger to the King, 241. High Street, Edinburgh.* With a Plate.

THE spring is contained in an iron-box, 11 inches by 7, and 14 inches deep, and consists of two horizontal wheels, marked on the section AA, Plate V., lying close above, and parallel to, one another, moving on the same axis with the door, to which the chains BB are attached, and moves with the wheels; the other ends of these chains are fixed to the levers CC, which move on the pivots DD, close to which the triple springs, EE, are made to press upon these levers.

When the lever or door is pressed or opened to the right hand, the under wheel is forced round, drawing back the lever C by means of the chain B, which lever the spring E presses

• Read to the Society of Arts for Scotland 11th December 1833.