Images de page
PDF
ePub
[ocr errors]

Silica.

. 40:3 Alumina

40.5 Lithina

4.3 Oxide of iron

4.85 Oxide of manganese.

1.5 Boracic acid

1.1 Volatile matters

3.6

96.15 7. Foliated Pyrope, from Greenland. This mineral has a deep blood colour. Its lustre is not adamantine, like that of the

pyrope, but common. It is composed of scaly distinct concretions. It is softer than pyrope. Its specific gravity is 3.634.

. According to the analysis of Pfaff, its constituents are as follows: Silica.

41.82 Oxide of iron.

32:42 Alumina.

17.82 Magnesia .

4.90
Oxide of manganese

3:12
0.80

.

[ocr errors]

Lime ....

100.88

This approaches to Klaproth’s analysis of the pyrope. (Schweigger's Journal, xxi. 236.)

8. Rutilite, from Arendahl.- This mineral has a dark hair brown colour, passing into blackish brown.

It is always crystallized ; but so confusedly that Professor Pfaff was unable to make out the form, though he thinks that it approaches most to a four-sided prism.

The external surface is dull or slightly glimmering. The lustre of the longitudinal fracture is glistening, that of the cross fracture shining, and the kind of lustre approaches that of the diamond.

The principal fracture is foliated with a two-fold cleavage meeting under angles of 74o and 106o. The cross fracture is small conchoidal.

It is composed of thick scaly distinct concretions.
The fragments are quadrangular and sharp edged.
It is translucent at the edges.
It scratches glass, and even garnet.
It is brittle and easily frangible.
The colour of the powder is light brown.
Specific gravity 3.879.

Its constituents, according to the analysis of Professor Pfaff, are as follows :

[ocr errors][merged small][merged small][merged small]

1

[ocr errors]

а.

Professor Pfaff has observed that there is striking resemblance between zirconia and the oxide of titanium. To prove this he has drawn up the following table :

(1.) Zirconia and oxide of titanium are both insoluble in caustic alkalies.

(2.) Both are somewhat soluble in carbonates of potash and soda.

(3.) The solution of zirconia in muriatic acid, when heated to a certain temperature, becomes milk white, and runs in some measure into a jelly, especially if it has been concentrated to a certain point by evaporation. The muriatic solution of oxide of titanium exhibits the same appearances.

(4.) From the muriatic solution of zirconia, oxalic acid throws down a white precipitate, which is again re-dissolved by an excess of the acid. This is the case also with the solution of oxide of titanium.

(5.) Zirconia and oxide of titanium are precipitated from their acid solutions by the neutral succinates and benzoates in copious white bulky flocks, which are again readily dissolved by the addition of succinic acid.

(6.) Tartaric acid, or tartrate of potash, occasions a precipitate when dropt into the solution either of zirconia 'or oxide of titanium.

(7.) Malic acid produces, in both solutions, a copious white precipitate.

(8.) Prussiate of potash throws down a green precipitate in the common solution of oxide of titanium ; which, by a certain increase in the oxidation of the titanium, becomes almost quite blue. From a moderately neutral muriatic solution of zirconia prussiate of potash throws down a greenish blue precipitate, which, on the addition of muriatic acid, becomes more blue; but, after a certain interval of time, changes into celadon green. The liquid above both precipitates remains of the same green colour.

(9.) Hydrosulphuret of ammonia produces, in the muriatic acid solution of oxide of titanium, a dark olive or blackish green precipitate in very loose flocks. This precipitate may be washed without any loss of colour ; but when exposed to sunshine it becomes quite white. The same phenomena take place when

a

hydrosulphuret of ammonia is dropped into a solution of zirconia, and the precipitate undergoes the same change of colour when exposed to the solar rays.

(10.) The only re-agent which acts in a strikingly different manner upon solutions of oxide of titanium and of zirconia is the tincture of nutgalls.' In the common solution of oxide of titanium it throws down a reddish brown precipitate, whereas in the solution of zirconia it occasions a deposition of yellow flocks. The addition of ammonia renders the colour more inclining to brownish red, and makes the precipitate more abundant.

(11.) Both the solution of oxide of titantium, and of zirconia, have an astringent taste.

It is obvious from this detail of particulars, that if zirconia and oxide of titanium be two distinct substances, as is believed at present, we are still ignorant of a method of separating them from each other.-(Schweigger's Journal, xxi. 240.)

This historical sketch has extended already to so great a length, that I must pass over the notice of the new analyses of various minerals which have been inserted in the twelfth and present volumes of the Annals of Philosophy. I refer the reader to

. Annals of Philosophy, xii. 388, 465, 768; and xiii. 65, 141, 144, 232, 310.

III. CRYSTALLINE FORM OF CINNABAR. This mineral, which is almost the only one of mercury, occurs in great abundance, but seldom in crystals. Hence its crystalline form had not yet been determined with accuracy. Haüy, when he published his Mineralogy, had seen only two crystals, and he was led from them to suspect that the primitive form was a regular six-sided prism. M. le Chevalier de Parga has lately sent him a set of very complete crystals of this mineral from the mine of Almaden, in Spain, which has enabled him to determine the primitive form, and the laws of crystallization of this mineral, with all the requisite precision. He has accordingly published a memoir on the subject which will be duly appreciated by mineralogists. As it is scarcely possible to make his deductions intelligible, without the assistance of figures, I think it will be better to insert the memoir entire in a future number of the Annals. It may be sufficient to observe in this place, that the primitive form of the crystals of cinnabar, ac

. cording to Haüy, is an acute rhomboid, the smallest incidences of the faces of which are 71° 48', and the greatest 108° 12'. The ratio between the demidiagonals of each rhomb is 3 to 18. (Ann. de Chim. et Phys. viii. 64.) IV. ON THE CAUSES OF THE DIFFERENT CRYSTALLINE

FORMS OF MINERALS. The great variety of forms which the same mineral species is known to assume, has drawn much of the attention, and occa

[ocr errors]

sioned the most laborious part of the investigations of mineralogists. The known forms of calcareous spar exceed 600; and perhaps those of iron pyrites and of some other species, if they were fully examined, would not be found much fewer. Leblanc was the first of the modern chemists that attempted to account for this diversity ; but the progress which he made was inconsiderable. The subject has been lately taken up by M. Beudant, who has published a most interesting and elaborate paper on the subject. I regret that I am prevented, by want of room, from laying the substance of his researches before the reader. I can do no more than merely state the general results which he obtained.

1. The state of the atmosphere, the greater or less rapidity of evaporation, the form of the vessel, its nature, the quantity of liquid, the state of its concentration, seem to have no effect whatever upon the crystalline forms which salts assume ; they merely influence their beauty and size.

2. When the atmosphere is moist, the salts have a tendency to form crystalline vegetations on the edges of the vessel.

3. Very dilute solutions, excluded from the air and prevented from evaporating, may yield crystals after a longer or shorter interval of time. But this is particularly the case with those salts which have but little solubility.

4. The nature of the vessels, by exercising different attractions on the salts, occasions the crystals to deposit themselves more or less quickly, and to accumulate in different ways in different parts of the solution. If the vessels are covered with a coat of grease, the crystallization takes place only at the surface.

5. The position in which the crystals are deposited in the midst of a liquid mass, has no other influence than that of producing more or less extension of the crystal in one direction, rather than another. The bounding faces are always of the usual number, and in the usual position.

6. The temperature and electrical state seem to have no influence on the forms of crystals ; excepting that at high temperatures crystallization is very irregular, and the saline masses produced are very fragile.

7. Substances in suspension, almost permanent in a saline solution, have no effect in varying the crystalline form. These substances are often deposited in the crystal in concentric layers.

8. The crystallization of a salt cannot take place in the midst of a deposit of foreign matters in very fine and incoherent particles, unless the deposit be covered to a certain height by the liquid. Crystals, formed in these circumstances, always contain a portion of the foreign matters which are found disseminated more or less regularly in their mass, and never deposited in concentric layers. When the solution is not much con

7

a

centrated, the crystals are always of a simpler form and more regular than when they are crystallized in a pure liquid. When the solution is very concentrated, isolated crystals are formed in it, whose faces are crossed like the hopper of a mill.

9. The crystallization of a salt may take place in the midst of a gelatinous mass without the necessity of any supernatant liquid. In that case the crystals contain none of the foreign matter, and undergo no change of form ; but they are almost always isolated and remarkably regular and complete in all their parts.

10. When several salts are in solution in the same liquid, it would appear that they are capable of mutually affecting one another's crystallization, even when they are not susceptible of uniting or of acting chemically upon each other. Thus common salt takes the form of a cubo-octahedron when it crystallizes in the midst of a solution of borax, or still better of boracic acid.

11. The forms which the same salt is capable of assuming, vary according to the nature of the liquid from which it is precipitated. Thus alum assumes the cubo-octahedral form when it crystallizes in nitric acid, and the cubo-icosahedral form when it crystallizes in muriatic acid.

12. Whenever several salts are capable of mixing chemically, that is to say, of uniting without entering into a definite combination, that salt, whose system of crystallization predominates, always assumes particular forms which differ from those which it adopts when it is pure. The different salts present likewise, in general, different forms in the same system of crystallization, according as they contain more or less of acid ; and the double salts according as one or other of the component salts exist in more or less quantity.

13. The chemical action which tends to determine a particular form, by altering the composition of a salt, produces different effects according to its energy, and often gives occasion at once to several varieties of crystals. Thus the action of an insoluble carbonate upon alum determines in the same solution octahedral crystals, cubo-octahedral crystals, cubic crystals, and an incrystallizable matter which contains still less acid than the preceding

14. When simple crystals of different forms belonging to the same salt are dissolved together in the same liquid, two different things may happen. If the crystallization takes place slowly, the crystals are deposited in succession and separately; but if the crystallization be rapid, a single mixed compound is formed, exhibiting crystals partaking at once of all the different simple forms. Thus octahedral and cubic crystals of alum may unite and constitute cubo-octahedral crystals. 15. Crystals of complex form may be sometimes decomposed

« PrécédentContinuer »