Fish's-eye stone consi dered as a zeo

lite,

or a feldspar.

III.

Description of the Apophyllite, Ichthyophthalmite of Dan drada and Reuss, Fischaugenstein of Werner. By Mr. HAUY*.

THE mineral, which is the subject of the present article, appears to have been anciently known, and was classed as a species of zeolite, from its property of forming a jelly with acids. It had been analysed by Rinman, who mentions it by the name of zeolite of Hellesta, in Sweden. The results of his analysis are nearly the same as have lately been obtained by my celebrated colleagues, Fourcroy and Vauquelin, and by Mr. Rose, whom Prussia has recently lost, to the great regret of every friend of science.

Mr. Dandrada's description of this stone does not appear to me to mark it by characters sufficiently precise, to allow us to decide, whether it should occupy a separate place in ́the system, or be classed with some of the known species. Mr. Brochant, after having quoted the principal features of this description, adds, that the ichthyophthalmite appears to have several of the characters of feldspar: and the name given it by Mr. Dandrada accords with this analogy, the name was for- name being equivalent to that of fish's eye, which in the merly applied: language of the old French mineralogists was applied to that variety of feldspar, which I call pearly, and which is the moonstone of our lapidaries.

to a species of

which the

but it is a dis

On examining some specimens brought hither about three tinct species. years ago by Mr. Molir, I was convinced, that the ichthy ophthalmite is clearly distinguished by its mineralogical characters, not only from feldspar, but from all other known minerals. I shall therefore proceed to detail these, which are already given for the most part in the work, which Mr. Lucas, jun., had drawn up from my public lectures, under the title of Tableau méthodique des Espèces minérales. Essential character. Divisible into a rectangular parall

Its characters.

* Journal des Mines, vol. xxiii, p. 385.

elopipedon,

opipedon, having a triple tendency to exfoliation, by fire, by acids, and by friction.

Physical char. Specific gravity 2.467.

Hardness: not scratching glass, and giving no sparks with steel scratching fluate of lime feebly, and carbonate of lime very evidently. If a fragment be rubbed sidewise on a hard substance, as if to polish it, it splits into leaves. Refraction, simple.

Electricity, easily excited by friction. It is the vitreous. Lustre. The surface of the crystals has a mean lustré between glassy and pearly, united with a transparency in general decided, without any proper colour.

Fracture, conchoidal, moderately shining.

Geometrical characters. Its primitive form, Pl. VII, fig. 3, is a quadrangular right prism with rectangular bases. The divisions parallel to M are very clear, and easily obtained. Those answering to P and T are not very evident except in a strong light f.

Chemical characters. Exposed to the flame of a candle, it splits into leaves. Before the blowpipe it fuses with difficulty into a white enamel. Immersed in cold nitric acid it divides in a few hours into small fragments, which at length become a white flocculent matter. Its powder forms in it a kind of jelly, similar to that produced under the same circumstances by the mesotype, or zeolite.

* From this character I have taken the name of apophyllite, signi. fying, "a stone that exfoliates."

†The proportions of the three dimensions C, G, B, are those of the

numbers 8, 9, √ 13.

VOL. XXVII.-Dec. 1810.

T

The

Crystalline forms.

Very uncom mon crystal,

The specimens of apophyllite I have examined exhibit a few crystalline forms, among which the most simple is that seen on a groupe in the museum of natural history. It is the primitive parallelopipedon, the eight solid angles of which are cut off by triangular facets, 0, 0, 0, fig. 4. The angle of incidence between o and M is 110° 50'.

Another variety, which I call supercompounded, is that represented fig. 5. The following are its principal angles of incidence. Between M and T, 90°: M and s 121° 57′ : M and r, 149: M and k, 118° 11': M and n, 135° 32′ : M and o, 110° 50′: M and I, 109° 32′ : M and x, 119° 1′. The specimen, from which I determined this variety, is one of the most remarkable, that has come under my notice, since I began to study crystallography. It adhered in a single point only, as I may say, to its support, from which I separated it by a slight stroke. It followed from this position, that the crystal has its terminations on every side, which is itself not very common. But a still more uncommon circumstance is the contrast presented by all the parts similarly situate, when we compare them with each other. In general, when a crystal deviates from symmetry, it is only by the absence of a small number of facets, among those that are necessary to the integrity of the whole; so that these facets, appear to have escaped the laws tending to produce them merely by accident, and the observer has little trouble to restore them in imagination. But in this crystal, which is represented exactly as it was formed fig. 6*, there is only one of the faces situate on one side, that has a corresponding face opposite to it: none of the other faces are repeated on the corresponding parts; and such is the progress of the decrements, that several of the faces which are single, as o, n, k, &c., ought to show themselves in eight different places, to leave no deficiency in the form of the crystal. It required time and study, to supply all these unexpressed circumstances of the crystallization, and reduce this sort of sketch, composed of ten faces seemingly without any connexion, to the real type of the form, which exhibits a well arranged assortment of

• The faces F', o', r', s ́, k', belong to the back part of the crystal.

forty

forty-eight faces. It will be easy to perceive the connexion of this type with the crystal that includes only its elements, from the identity of the letters marking the corresponding faces on the two polyedrons, fig. 5 and 6.

ticle.

for distin

Among the various forms of integrant particles, that are Integrant par rectangled parallelopipedons, I know no one, that does not differ perceptibly from that of the apophyllite in the ratio of its dimensions, which alone is sufficient to show, that this mineral substance ought to be considered as a distinct species. On this subject I think it may not be amiss, to repeat what I have said elsewhere: it is not simply in the Haüy's rule number and positions of the natural junctures, that the guishing spe geometrical character consists, which I employ to distin- cies. guish one species from another, but also in the comparative dimensions of the forms of the particles. Hence arises a system of crystallization, which accords only with the substance possessing this form, unless it be a limit capable of belonging to several minerals, as the cube, regular tetraedron, &c.; in which case it is necessary to add an auxiliary physical character to that derived from the form of the particles, that the species may be determined unequivocally. The analyses I have mentioned tend equally to establish an essential distinction between the apophyllite and all other minerals, and thus the results of chemistry and mineralogy with respect to this substance fully satisfy the two condi tions enunciated in my definition of a species, considering this as an assemblage of natural bodies, the integrant par-' ticles of which are similar in form, and composed of the same principles united in the same proportions. "Mine ralogy will have attained perfection, when we find throughout that conformity between the operations of two sciences, which should continually assist each other; and the agree ment of which, as they investigate nature by very different paths, must doubly confirm the truths they disclose."

Fig. 6 represents the ten faces of the elementary crystal: fig. 5 shows only the twenty four faces of the complete crystal, supposed to be seen in front; but it is easy to conceive in imagination the other twenty four, which are on the back part.

The theory of rockets not

mathemati

cians.

IV.

On the Motion of Rockets both in Nonresisting and Resisting
Mediums. By W. MOORE, Esq.; communicated by the
Author

SIR,

To Mr. NICHOLSON.

SHOULD the following Essay on the Motion &c. of Rockets be considered sufficiently interesting for your valuable and well conducted Journal, you are at liberty to make use of it.

I am, Sir,

Yours very respectfully,

W. MOORE.

Royal Military Academy, Woolwich,

November 3, 1810.

The theory of rockets is a subject, which has never yet engaged the attention of mathematicians; a circumstance investigated by which perhaps is partly to be ascribed to their not having been used until very recently as implements of warfare. The practice however of throwing them into besieged places, to cause their surrender, is now nearly universal among the English, and indeed is almost confined to them.

Their military

Ese.

The invention of the military rockets* (as they are now called) as it regards the exemption of our troops from the enemy's power of annoyance, is to be esteemed as valuable. By the help of these machines the capital of Denmark and the well fortified town of Flushing, together with much of the French navy have within a few years been taken and destroyed with scarcely the loss of a single man on which account, it is a matter of no small moment to bring the rules for discharging them and the methods of estimating their effects under various circumstances into one general

The invention of the military rocket is exclusively due to William Congreve, Esq.; a gentleman well known and esteemed by the public for his ingenuity.

and