permanent for a great length of time, if kept cold, and relieved by aggregation or union, from its strongly electro-positive state. But, if recently generated, and left without pressure or union, it probably opens up in the course of time, and gives rise to another substance, to be treated of afterwards. Whether the hydrogen procured from the decomposition of water, could be in any measure destroyed, we have scarcely the means of discovering from the experiments which have been made. Its destruction could only be indicated by a diminution of volume, for when it mingled as radiant matter. with the radiant medium existing along with it, the chemist has done with it. The term Hydrogen is applied in chemistry only to a gas or body which, when disunited from the other bodies, is conceived to be capable of existing at all known temperatures only in the aëriform state. It is evident, however, that its particles may group so as to constitute a molecule, whose electrical state shall be in the condition of a much more perfect equilibrium than that of a single particle. When once elevated into the gaseous state, it does not appear, indeed, that it can be easily made to abandon it. Its extreme specific heat must produce a repulsion on every contact, except, perhaps, when the number requisite to constitute a molecule comes on at once in their proper positions. In these circumstances the disposition to symmetry and electrical equilibrium might be expected to overcome the mechanical rejection from union, arising from a vibratory movement on all the angles, and the extensive sphere of repulsive fluid surrounding each particle. When six particles of hydrogen unite by their equators, there results a senate molecule of most admirable symmetry and electrical constitution, (Fig. 33). Each of its six parts. has a solid axis, and is electro-positive, while the whole, considered as a single form, has no axis at all, and is very highly electro-negative. This senate molecule of hydrogen is a particle of water. That water consists entirely of hydrogen, may be shewn by decomposing it in a very highly electronegative medium, adverse to the development of an electro negative form. In other circumstances, where it is not restrained by such an arrangement, as soon as one particle of hydrogen is drawn out of the circle of six, the remaining five, by the proximity to the electro-positive part, tend to assume an electro-positive form. Now, this they may do by a simple movement; and thus the senate molecule of hydrogen, so singularly fitted for accommodating both the positive and negative electricities when decomposed, parts them between two forms, one highly electro-positive, the other highly electro-negative. In other cases, as will be afterwards shewn, other forms besides must result from the decomposition of an atom of water. But it is not to be denied, that, by decomposing water at a negative voltaic pole, the positive being removed to a distance, and the effects of induction as much as possible prevented, this liquid is wholly resolved into hydrogen. No other substance is either given out or absorbed; and, to conceive that particles are transferred through the connecting body to the positive pole, as, for instance, through the living body of man, without occasioning inconvenience, is more extravagant than the case demands. Whether hydrogen is ever resolved into water during chemical experiments, is well worthy of inquiry. Water is always found in hydrogen gas, even after transmission through dry chloride of calcium. Its development might certainly be expected, when hydrogen is nascent in great quantities. When escaping through the pores of a bladder, perhaps a part of it emerges from the exterior side, as aqueous vapour; and if as much hydrogen as possible were condensed in charcoal, perhaps a part of it might be extracted as water, especially if a little water were introduced into the charcoal along with the hydrogen, to determine to the evolution of more. The admirable Boerhaave concludes, that water is a permanent body, because a quantity, hermetically sealed up in a bolthead by Clavius the mathematician, remained without contracting in volume, or otherwise changing, for eighty years, during which it hung in Kircher's laboratory. It were much to be wished that modern chemists would imitate the older ones, in performing their experiments on a scale sufficiently ample. It would be very interesting to know what would become of a volume of hydrogen, cooled and compressed as much as possible, and exposed to the sunbeam, or kept in the dark, for as many days as the water was years. It is not to be forgotten, however, that 100 cubic inches. could only produce one globule of water th of an inch in bulk. 1000 WATER Is a senate molecule of hydrogen; its atomic weight is 12; its form is that of a frustular bipyramidal dodecahedron, concave on both poles, and without a solid axis, (Fig. 33). The equator is a regular hexagon, and though the matter in the centre, and on six equidistant radii, have no sensible thickness, the equator is everywhere impervious. Water, when entering into union with bodies constituted of larger molecules, which, from their unconformable shapes, cannot dispose of it in the proper quantity that they demand in individual particles, often aggregates into ternate molecules, whose atomic weight is consequently 36. Next to this there is the septenate molecule, which consists of a particle in the centre, with six around it, one on each of its edges. But that which performs the most important part in the economy of nature, is the senate molecule (Fig. 34), which results from the approach of six, and contains in the centre a hexagonal pore. Such a molecule must possess great permanency, for the subtile matter is very perfectly circulated; and there is no diameter, along which a repulsion, tending to break up the molecule, can be instituted. When fastened together by other bodies, a double molecule, in which one particle is above another, the poles not being in contact, also frequently occurs. The form of the aqueous particle enables us to anticipate those numbers which must prevail in its combinations, and the characters of the forms which will most readily combine with it. Those bodies only which have a positive axis, terminated by a trihedral angle, can find access to its poles; and the numbers in which they may be combined are one, when their particles are large compared with a particle of water, and two when they are small. Any body may unite around the equator; and those having parasitic forms, or reentrant equatorial angles, such as Figs. 6 and 7, Figs. 6 and 7, conformable to the equatorial edge of a particle of hydrogen, will be retained with the greatest force, being held in cohesion by four angles. The number in which such bodies must unite is three. The number six is possible, in as far as the form is concerned, but such a number of any other body may be expected to overcharge a single particle of water; and we may generally expect to find in hydrous molecules, that three particles of the peculiar substance are arranged around the equator of a particle of water, one on each alternate segment. The most symmetrical mode of arrangement is when one particle of the body demands one of water, and the six then group, so that in the centre is a senate molecule of water, and six particles of the hydrated substance around. A single particle of water is, doubtless, very much too small for being seen, so that we cannot obtain sensible evidence that it possesses the form which is here assigned to it. But the evidence on this subject which is afforded in nature, is almost as convincing as the actual vision of a single particle. The particles of water do not, like hydrogen, inveterately affect the aëriform state; but, on a sufficient reduction of temperature, aggregate into visible masses, and these masses, when, by being precipitated from solution, they possess crystalline individuality, are most perfect reproductions of all the prominent features of the aqueous particle. Water always exists diffused through the atmosphere, and its existence there, in single particles, is compatible only with a certain temperature, pressure and electrical state. When that state does not exist, and the temperature is too high to admit of the institution of solidity, it aggregates in little masses, which, when they are slowly formed, are probably little hollow balloons, constituted by aqueous particles, their axes di rected to the centre. This is the only symmetrical form possessing individuality, which can result from particles of water restrained from contact, and it may be regarded as a molecule of vesicular vapour, or of dew; but when the heat of the particles is less, so as to admit of a crystalline arrange. ment, very interesting phenomena must occur. The first movement of the aqueous particles must be into ternate molecules, and then into septenate, whose symmetry is as great as need be sought for; they are isomorphous with the senate, Fig. 34, the central pore being occupied with a particle of water. These we may regard as the molecules which unite. and develope crystalline forms; or it may be, that a septenate molecule becomes the nucleus, around which individual particles, or ternate molecules, are arranged. It matters not in which way it is generated; there must constantly result, upon any supposition, a form consisting of a central part, from which six radii emanate at angles of 60°. These radii may again have secondary branches, emanating at a similar angle, or even the primitive form may be reproduced, by the hexa. gon being completely filled up. The various forms which may be produced seems almost infinite, but those which can most easily be constructed, are the very forms of snow flakes, and no others. In the plates to the recent voyages to the Arctic Regions, most perfect illustration of these views will be found. The subject was long ago examined by Descartes,* who noticed many having twelve radii, or six principal ones, with their intervals bisected by inferior ones, as may be easily conceived. He also observed two flakes of snow united by a prism, as by an axle, as has been since remarked Sixsided spiculae, or prisms, which are described by Hooke as hollow, also arise very naturally, when a hexagonal base is afforded. Hence, they are frequently met with in glacial hoar frost attached to icy surfaces, as they are sometimes in the air attached to flakes of snow. Fig. 35 represents forms of snow-flakes copied from the Rev. Mr Scoresby's first work on • Princip. Meteor. p. 194. |