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The liquid fluoric acid dropt in water, like anhydrous sulphuric, or a red-hot iron, hisses, and no doubt improves its symmetry, by entering in single particles, instead of quaternate, into the concave poles of a particle of water (Fig. 86). This hydro-2-molecule weighs 42, and when hydrated by six particles around the equator, and probably one on each pole, is the molecule which is diffused in the solution in water. In the retort from which the fluoric matter has been disengaged, sulphate of lime is found, and experiments of this sort by Sir H. Davy and Berzelius assign the former 15-44, and the latter 15-2, as the quantity of fluoric matter combined with 40 of lime. Now, 40 being the atomic weight of lime, these analyses afford a very strong confirmation of the view which assumes 15 to be the atomic weight of fluorine or anhydrous fluoric acid.

Cryolite. The fluoric principle, like phosphorus, is produced in the animal system, and, in union with lime, was detected by Berzelius as one of the constituents of bone. When, indeed, we consider its action upon the living fibre, we are tempted to conclude, that the virus of certain diseases which is developed in the system, by the original introduction of a few particles, thus disposing to their development in greater quantities, from nearly analogous forms previously existing, or natural to our organization, consists of forms perhaps intimately connected with phosphorus. But fluoric acid also occurs in the mineral kingdom, united in notable quantities to soda and alumina in the cryolite, and to cerium and yttria. These minerals are, however, extremely rare, and have hitherto been found only in Greenland and Sweden. The cryolite is from the former country, and is stated by Vauquelin to be composed in 100 parts of

True Ratio.

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It has attained the name of Cryolite from its icy appear

ance.

Topaz.This mineral is rendered interesting, not only on account of its value as a gem, but because it enters into the composition of certain rocks. From its analysis, it seems to consist of one particle of fluoric acid, one of silica, and five of alumina, or it is cyanite (as indicated by the analysis of Laugier) with a particle of fluoric acid added to each molecule. The topaz of Schneckenstein yielded

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Fluoric acid also occurs, in minute quantities, in mica, lepidolite, hornblende, apophyllite, wavellite, and others. But whether it always survives the rude treatment of analysis, or is discovered where it exists, may perhaps be questioned. In the laboratory, it forms two very interesting compounds with silica and boracic acid.

Fluo-silicic Acid Gas.

When sand or glass, or other siliceous mass, is mingled with fluor, and vitriol infused, a gas comes over, possessing very different properties from fluoric fumes. It does not corrode glass vessels, provided they be dry, nor is its action on the animal system so violent, though, as might be expected, it is still very destructive to animal life.

It has been already stated, that two particles of silex are requisite to form a symmetrical molecule, and that in mineral bodies it seemed very often to exist in binate molecules. There can be little doubt but it exists as such in those siliceous materials used to prepare fluo-silicic acid gas. Now, if such double molecule received a particle of fluorine on each of its six siliciferous regions, it is evident that a very symmetrical form would result. But on entering the gascous state, it seems

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not improbable that this form should, sooner or later, break up into two, as in the case of phosphuretted hydrogen, and the gaseous volume be constituted of equal measures, or an equal number of acid particles, the one containing twice as much fluorine as the other, and both symmetrical. If we suppose no water present, one of the particles of silex will have a particle of fluorine on each pole, the other particle of silex, one on each of its four equatorial arms. Now, as to the specific gravity of this mixed fluo-silicic gas, it does not admit of a doubt, that the density of the particles will be only half as intense as in hydrogen, nitrogen, &c. It is therefore analogous to ammonia and the hydracids. The specific gravity of its two parts, then, will be ascertained in a rude way by dividing half their atomic weight by that of common air. Now the atomic weight of the heaviest, or that which contains four particles of fluorine, is 140, composed of 80 silex + 60, (4 × 15) fluorine. The atomic weight of the lighter gas, again, is 110, composed of 80 silex and 30 fluorine. Half the mean of these divided by 20.4, which is the atomic number for common air, gives a specific gravity of 3.06. But every pair of particles is subject to the incidence of six times three accidental atoms, for every particle of fluorine, like one of phosphorus or sulphur, has three concave regions exposed to the access of radiant matter, and, if fully charged, a quantity must be added to the specific gravity found by dividing the half of eighteen by the atomic

number for common air, and Without a knowledge of its extent it really does become

this gives an addition of .441. exclusive power, and to what charged, we cannot condescend more minutely; but on these data, the specific gravity of the dry gas fully charged with atoms, disregarding its exclusive power, might be 3.5. Judging from the low refractive power of silica, and the analogy of this gas with sulphureous acid, we should not be disposed to allow any thing for exclusive power; but in such ponderous unnatural gases, it seems to happen that the density is not always exactly a multiple of that of other gases; and be

sides, the presence of a little water would alter its specific gravity. Dr Davy found the specific gravity 3.5735, and Dr Thomson found it 3.6. So far, then, there is nothing unfavourable to this view of the structure of fluo-silicic acid gas. When it meets with aqueous vapour, those white fumes appear which seem always to be presented when a particle of vapour in the air perches upon a triædral pole. The acid has also such affinity for water, that between three and four hundred times its volume are absorbed by that liquid.

There can be little doubt, that immediately on coming in contact with water, the fluorine is converted into hydro-molecules, such as are met with in cryolite, and were supposed to exist when the free acid is dissolved in water. There will be a moment, then, in which the silex will partly, at least, be abandoned by the fluorine. Now, it is most natural to suppose that, during this evolution, a compound will be instituted, consisting of two particles of hydro-fluoric acid (Fig. 46.), one on each pole of a particle of silica, a regularly constituted bihydro-fluate of silica, analogous to the binoxalate of potassa. If we suppose all the fluoric acid to be engaged in this way, a fourth part of the silex of the gas must be set free, and being insoluble, must precipitate. There are only six particles of fluorine to two of silex, or 12 to 4. But 12 give rise to 6 hydro-2-molecules, which can only charge three of silex, so that the fourth must go down. 100 inches of the gas are found to weigh about 109 grains; therefore 44 will weigh about 48 grains. Neglecting every thing but the essential constituents of the gas according to the view here given, it is composed of silica and fluorine, in the ratio of 80 of the former and 45 of the latter; and, therefore, contains about 30.7 grains of silica, and 17.3 of fluorine. During absorption by water, then, 307 7.67 grains ought to be deposited, but the collection of the whole must be extremely difficult. Now, Dr Davy found, that when 44 inches of fluo-silicic acid gas were absorbed by water, 7.33 grains of silica were deposited, which is as near to this theoretical deduction as could be expected.

Berzelius mentions, however, that a third part of the silex originally in the gas is let go. In a form of so delicate a structure, it is only to be expected that different results should be obtained in different hands, more especially when we consider that a highly symmetrical molecule may result, composed of a double molecule of silex, with a particle of hydro-fluoric acid on each of its six poles, into which, were the whole solution to be resolved, one half of the silica in the gas must be let go.

But when the gas is exposed to absorption by water of ammonia, the whole of the silica is disengaged. In these circumstances, then, 44 inches, according to the structure here advanced, ought to discharge 30.7 grains of silica, and Dr Davy (with whose experiments those of Dr Thomson agree very closely) found that 44 grains absorbed by ammonia, let go 29.9.

This liquid substance is powerfully acid, reddens vegetable colours, and emits fumes when exposed to the air. It goes also, in some form or other, into union with bases. Its atomic weight, exclusive of water, which, however, is evidently essential to its constitution, is 140; water included, it is 164.

Fluo-boric Acid Gas.

Though boracic acid, a substance already noticed, in the molecular state, be highly fixed in the fire, yet, in individual particles, as often happens in other cases, it is of a volatile nature; and if we could conceive fluorine nascent along with it, it seems very highly probable that a particle of boracic acid should receive three of fluorine into its three concaves on the equator, and the compound particle thus generated ascend. It is a quaternate molecule of two nearly allied bases; and there are three particles of oxygen present, so that it will evidently be a very powerful acid. Like other analogous forms, the gas will be half the common density, and every particle will be subject to the incidence of nine accidental atoms. Its exclusive power will probably be very small, and its specific gravity

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