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The planes x and y are generally striated, and afford imperfect reflections; and the crystals are frequently elongated in the direction of one of the edges of the base, so that the plane P terminates in an edge instead of a point, an irregularity of figure common to all the classes of octahedrons.

Sulphate of Potash and Magnesia.

I have not found any cleavage of these crystals, but the predominating form, and which may be regarded as the primary, is an oblique rhombic prism, modified by the planes c, e, and h, and measuring as follows:

L on M, or M'
M on M'.

P on c'

P on e, or e
P on h.




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The crystals are soft, flexible, and very fissile parallel to the plane P of the annexed figure, and there is not any distinct cleavage that I have been able to perceive in any other direction. There are, however, in some crystals, apparent natural joints parallel to the planes P of this figure; these would give an octahedron for the primary form, which, from the angles of the secondary planes, is found to have a square base. The most distinct measurements are the

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Bicarbonate of Potash.

The primary form of this substance is a right oblique-angled prism, which is not readily traced in the secondary crystals, but may be derived from cleavage, and is shown in fig. 1. There is also a cleavage parallel to a plane passing through the diagonals marked on the terminal planes.

P on M, or T........ 90° 00'
M on the diagonal plane 53 15
M on T.
103 25

The planes which appear on the crystals are represented in fig. 2; but the planes e are sometimes very disproportionately extended, so as nearly to efface T and f, giving to the crystals the character of another primary form.

The planes T do not commonly occur on the crystals, and without these they nearly resemble a secondary form of the right rhombic prism; they may, however, be distinguished by the unequal inclination of M on the two adjacent planes. On cleaving or otherwise breaking the crystal, water




Fig. 2.

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may be observed between the lamina, which probably occasions the measurements on the cleavage planes not accurately to agree.

This is also the case with many other of the factitious salts.

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Cyanuret of Mercury.

I have received for examination from Mr. Cooper, of Lambeth, some crystals obtained from oil of bitter almonds by digesting it with red oxide of mercury.

Mr. C. has also supplied me with some crystals of cyanuret of mercury, procured in the ordinary way by boiling the red oxide with prussian blue. The crystals derived from both of these sources correspond perfectly in their crystalline forms.

I have not succeeded in cleaving them, but from their measurements and modifications, a right square prism may be regarded as the primary form.

Fig. 1.

Fig. 1 represents the prism with the modifying planes which I have observed on two or three crystals, and on these only, out of a considerable number that I have examined.

Their general form is that shown in fig. 2, in which two of the planes a alternately efface all the other terminal planes at the two extremities of the prism. There are also many crystals which nearly resemble fig. 2, but in which the planes a and a" are visible, although very minute. This irregularity of form is of the same character as has been already noticed as belonging to sulphate of magnesia. The measured angles are as follows:

M on M' c on M

Con M'}

a on M


Fig. 2.


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a' on M, or M'
a' on a'i


Astronomical Observations, 1823.
By Col. Beaufoy, FRS.

Bushey Heath, near Stanmore.

Latitude 51° 37′ 44.3" North. Longitude West in time l′ 20.93”.

May 18. Emersion of 69 Leonis from the moon.


13h 41' 18" Siderial Time.


On the slow Combustion of Tallow, Fixed Oils, and Wax.
By Mr. C. J. B. Williams.

(To the Editor of the Annals of Philosophy.)

SIR, Edinburgh, May 1, 1823. ·PERMIT me to draw the attention of your chemical readers to a phenomenon (hitherto, I believe, unnoticed), illustrating the slow combustion of the inflammable gas or vapour, produced by the decomposition of oleaginous matter by heat. It may be manifested in the following manner: Extinguish the flame of a candle or lamp by blowing on it, having previously supplied the wick freely with tallow to increase the size of the flame, and to prevent any portion of the wick from remaining in a state of open combustion. If this experiment be made in a room secluded from every other source of light, a distinct phosphorescence on the surface of the wick will be perceived during several seconds, brighter in proportion to the size of the flame before being extinguished; hence it is most obvious with a long wick, provided no spark be left on it.

This combustion, which I consider analogous to that of the vapour of ether, by the aid of a platinum wire, is likewise attended with the production of a pungent acid vapour. Of the nature of this, I have had neither time nor opportunity to ascertain any thing. It is probably, as in the case just alluded to, merely a modification of the acetic acid; but since the odour differs considerably from that of the lampic acid, it might be of sufficient importance to merit an investigation.

I have ascertained that a similar phenomenon occurs when fixed oils or wax are projected in small quantities on a plate of metal heated considerably below redness; and from some experiments I am inclined to believe that this modification of combustion takes place in most cases where these substances are heated to ebullition in free contact with the air.

I have submitted the result of my observations on this subject in this imperfect state; as a long time might elapse before I could be enabled to extend them further, and in the expectation of seeing them prosecuted in abler hands.

I am, Sir, with respect, yours, &c.




Transactions of the Royal Geological Society of Cornwall, Vol. II. 1822.

(Concluded from vol. v. p. 300.)

The third order of veins described by Mr. Carne is that of true veins, which consist of the following classes, enumerated in the order of their relative ages, as determined by the principle of intersection:

1. The oldest tin lodes; 2. The more recent tin lodes ; 3. The oldest east and west copper lodes; 4. The contra copper lodes; 5. Cross courses; 6. The more recent copper lodes; 7. The cross flukans; 8. The slides.

"In describing the contemporaneous veins," be observes, 66 some were mentioned as occurring in the veinstones of other veins. There are also metallic veins which may be denominated veins within veins." These comprise black and grey silver ore, with native silver, in the copper lode of Huel Ann; wood-like and common tinstone in quartz and tinstone, the latter crossing the veinstones of the lodes; various ores of copper, in the quartz veinstones of Huel Carpenter, Huel Neptune, Huel Damsel, &c.; woodlike oxide of iron, in brown ironstone, at Botallack and Boscagel Downs; fibrous oxide of iron, in quartz, and veins of carbonate of iron at Huel Jubilee; and minute veins of native bismuth, in coarse red jasper, at Botallack.

Some remarks on the geological constitution of that part of Cornwall in which most of the veins described are found, and on the number and variety of the veins themselves, terminate the paper. After saying, "the claim of the granite which forms the chain extending from the Land's End to Brown-willy, has rarely, if ever, been seriously disputed;" and adverting to the supposition that St. Michael's Mount is transition granite, Mr. Carne


"The claims of the clayslate, however, have of late been disputed, and it has been called transition slate, and greywacke slate, by geologists whose authority certainly carries considerable weight: but by what rules are we to distinguish primitive from transition slate? In its structure, the clayslate of Cornwall appears, in general, perfectly homogeneous. It contains no impressions of any kind. Some of the oldest metals have been found in it, viz. oxide of tin, wolfram, and mispickel; these have been discovered in the slate; and they have also been found, together with sulphuret of tin, native bismuth, and oxide of uranium, in the veins which intersect it. Some of the oldest

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