of a particle of each, and such also seems to be the structure of semi-opal *.

Besides these, there are many other minerals constituted in a great measure of silica; but alumina, lime, oxide of iron, soda, and potash, also enter into their composition; and when the resulting form is symmetrical, there is every reason to believe that the distribution of these substances is symmetrical, and that the molecules are large. Most of the conspicuous minerals of the system, however, seem to possess a very simple structure, and some of them will be noticed when treating of alumina, which most generally enters into union with silica. From a few of them alumina seems to be almost wholly excluded. Thus Okenite, a species of zeolite, consists almost entirely of silica, lime, and water, apparently a particle of each, and two of water +. There is another very well known

It is easy to be conceived, that, by superficial loss or otherwise, the water in these minerals might be somewhat under rated. The analyses give the following numbers:

mineral, named Table Spar, which contains little else than silica and lime. It seems extremely probable that lime should dissolve the silica, and that a united molecule should result, in which one particle of lime should be surrounded on its quinate parts by five particles of nascent silica, and this body unite with another particle of lime. But as no two chemists agree in the results of the analysis of such minerals, except when they are guided by certain hypothetical views, to which their analyses naturally conform themselves, we need not be too curious as to the structure of the molecules of rare compound minerals, when we have no better guide than the residuary matter obtained by their destruction. Like carbon and hydrogen, the affinities of the earths for each other are such that a multitude of ratios may be established, all of which are natural; and it is only in the more abundant minerals that we need expect a simple structure. There are other minerals, such as the crysolite, olivine, and serpentine, which are almost wholly composed of silica and magnesia. The crysolite, analyzed by Vauquelin, seems to have consisted of a molecule of silex, with one of magnesia, on each of the four arms, two molecules being united by a particle of the oxides of iron t.

FLUORINE.

In the Mineralogical System of Mohs, there is a genus named Fluor-haloide, including two species of very remarkable characters. One of these is rhomboidal fluor-haloide or apatite, which is a phosphate of lime; the other is octaedral fluor-haloide, and this is known to consist also of lime as a base united to a certain substance, of most extraordinary and terrific properties. As it occurs in nature, the mineral is beautiful and innocent, and fluor-spar is one of the prettiest things which the young collector finds in his cabinet. It usually occurs crystallized in tessular forms, which are sometimes very highly polygonal. Mr Phillips possessed seventy different sorts, and one, had all its faces been perfect, should have been included under 322 planes. The colours of the spar are also very fine, and have gained for it the names of False Sapphire, False Emerald, False Amethyst, and False Topaz. It is very abundant in the tin mines of Cornwall, and the lead mines of the north of England. In other countries also, such as Saxony, it is common, but in Scotland and many other places fluor is a rare mineral. The granular and imperfectly crystalline varieties are cut and polished into vases and such like pieces of furniture, and certainly Derbyshire spar is a very beautiful thing to look upon. Neither would one suppose that it contained any more formidable ingredient than marble, rock-crystal or a gem. But, on pouring oil of vitriol upon fragments of fluor contained in a glass retort, certain fumes are disengaged, which speedily dim the glass, and eat their way through it, and if the least quantity chances to touch the skin, the consequences are such as are thus described by MM. Gay-Lussac and Thenard: "Of all the properties of fluoric acid, the most extraordinary is its action upon the skin; scarcely has it touched it when it is disorganized. A strong pain is soon felt; the parts contiguous to the point of contact are not long in becoming white and

sore, and a little after a blister is formed, whose walls are white, very thick, and which some time after contain pus. However small the quantity of acid, even when it is scarcely visible, these phenomena equally take place. Their development, however, takes place slowly. It is sometimes seven or eight hours after the contact when they are observed, and notwithstanding, the scald is still so severe as to occasion acute pain, take away sleep, and give rise to fever. Too many precautions, then, cannot be taken to preserve one's self against this acid, whether when wishing to repair the lutes through which it may be escaping, or when, having raised too much fire, the tube is melted, and one wishes to take it away; and, still more, when after the operation one wishes to transfer it. Oftener than once we have been severely hurt, and we have seen several young chemists hurt themselves still more grievously, although forewarned of all the dangers. There were particularly three whose forefinger and thumb could not be cured in less than a month, and yet these organs had only been a few seconds in contact with the acid in a state of vapour. It is more than probable that they had lost their two fingers, or at least had preserved them with difficulty, if a few drops had fallen on them. That which places this beyond doubt, is what was shewn by a little dog *" .........!

Now, it is very interesting to observe, that, in combining with silica, to which, in the early experiments, this substance would, of course, be exposed, it loses much of its virulence, and thus, in destroying the chemist's apparatus, it saves himself.

All the phenomena which it presents, seem to indicate that it belongs to the same series of forms as silicon, while, at the same time, it gives signs of a relationship to phosphorus. Although many minerals, when heated, phosphoresce, in none is this property more remarkable than in fluor-spar, and its natural connection with the phosphate of lime has been already stated. Phosphorus also acts upon glass, and the wound occasioned by its combustion is not a little analogous to that occa

Recherches Physico-Chemiques, t. ii. p. 11.

sioned by fluoric acid. There are three forms to which the mind is led from such considerations, which are externally isamorphous, and consist of a particle of phosphorus or sulphur, united to one of silicon on its pole, (Fig. 85). Their atomic weights are severally 14, 15, and 16, nor do the experiments which have been made on this formidable substance place it beyond doubt which is the true one. In the absence of satisfactory evidence, we may assume that whose weight is 15. The atomic weight of fluorine assigned by Berzelius is 15-006 2. This form seems the most probable, and consists of a particle of silicon with three of hydrogen, as in sulphur, phosphorus, boron, &c. &c., and a single atom on the other pole, as in phosphorus, (Fig. 53). When the oil of vitriol is poured upon the fragments of fluor in the alembic, and heat applied, the decomposition commences, sulphate of lime is formed, the fluoric principle comes over, and if the receiver be kept very cold, and the spar employed contain no silica, it condenses in the receiver (which must of course contain no silica), in the form of a limpid liquid. MM. GayLussac and Thenard dropt a bit of potassium into this liquid, and there was instantly a violent detonation and a vivid combustion. The laboratory was filled with vapours, and nothing was left to examine. Having repeated the experiment in a more cautious manner, these chemists found that a remarkable quantity of hydrogen was generated, and a liquid remained, which fumed and became crystalline when exposed to the air. Distilled in a glass alembic, this crystalline mass yielded fluate of potassa, water, and fluo-silicic acid. It seems very probable that part of the fluorine is resolved into hydrogen and silicon, and that the liquid mass operated on consisted of quaternate molecules of fluorine, united in pairs by a particle of water. This water would afford oxygen to the potassium, and would disengage some hydrogen, but of this substance three particles would also result from each of fluorine destroyed. There seems nothing against the development of a hydro-fluoric acid analogous to sulphuretted hydrogen, &c. but perhaps it is not notably generated during the disengagement of the fluoric principle from fluor-spar.