possess a more definite character, in conscquence of the thermo-lumeniferous axis being symmetrical. When a linear aperture is used, the heat will be propagated from both sides, and the central ray will probably be in that state which naturally arises out of the transmission of a solar ray at that distance from its calorific base. If, again, the aperture be circular, the same thing will hold; but the attainment of the cold point will be in this case more difficult, and probably not in the plane of the perforated lamina, but behind it. The lumeniferous rays, then, in being constituted in these apertures, must have not only a motion in the course of the ray, necessary and proper to the propagation of light in that direction, but a transverse motion radiating from the centre of the aperture, or from the axis of the ray, which will require that the light, in being transmitted, shall be modified, so as to produce colours symmetrically related to each other. The smaller the aperture, the more intense the transverse excitement of the ray. By admitting sunbeams thus, through small slits and apertures, and by the use of dense atomic bodies, the natural symmetry of the sunbeam, and the state of its polarity, are changed together. The radiant medium, in fact, at the dense body, is as it were changed from a tessular crystallization to one possessing free polarity; consequently, when the illuminated structure is viewed with a telescope, as by Fraunhofer, or received on a white surface, (as Mr Herschell did with polarized laminæ, when detecting the lemniscoid developed by the confluence of two axes,) spectra and chromatic axes are displayed, as when other polarized media are viewed in a similar manner. The beauty of the coloured rings, spectra, and fringes, which are thus developed, surpasses that of the equatorial lamina of other crystalline media, in the proportion that the radiant medium surpasses all others in symmetry and transparency, and fitness for the development of chromatic phenomena. Those which Fraunhofer saw, by admitting a sunbeam through four small apertures, must have been truly magnificent; and we have only to inspect the form which was developed, by admitting beams through four small apertures placed in the angles of a square, to see more of the structure of the radiant medium than could have been expected in an accidental experiment. Drs Brewster and Seebeck have shewn how sensitive the polarization of a mass destitute of molecular individuality, such as a bit of glass, is to changes of temperature; and a thermometer has even been suggested founded on this principle. But I hasten to attempt the structure of a variety of chemical and natural bodies, according to the principles that have already been laid down, and, after the perusal of the following pages, the preceding may, perhaps, be found more interesting. N BOOK III. OF NATURAL AND CHEMICAL ON ATOMS, PARTICLES, AND MOLECULES. THE Radiant Medium, from its universal diffusion, from the minuteness, the solidity, the pungency, and other features of its atoms, gives rise to phenomena in the economy of nature, not a little different from those of other aëriform media. These phenomena, however, are only its natural and specific properties, depending on its physical constitution, and all other bodies possess analogous properties, in as far as their physical constitutions are analogous. It has been treated of apart from other chemical bodies, not because it is essentially different from them, but because it is the element to which they are related in such a manner, that their particles are symmetrical structures constituted of radiant atoms. These particles, commonly called the Atoms of Bodies, possess, as is always received in chemistry, the same form, magnitude, and structure, in the same species of substance. But in dissimilar bodies they differ in these particulars, and this constitutes the specific difference between one body and another; the solid impenetrable matter of which they consist being at the zero of heat, or in a state of rest, universally the same in nature and properties. But as two bodies may be in very different states of calorific excitement, while their tem perature, that is, their disposition to part with their heat, so as to warm or affect by heat a third body, may be the same, specific heat must be regarded as an element modifying the condition of the ultimate atoms of bodies. In acquiring a certain specific heat, then, when it enters into the composition of any specific body, the substance of an atom of common matter acquires a specific modification, and this necessarily affects it in all those qualities by which only it can be known to us. But in a state of absolute coldness, or when the calorific excitement of any number of bodies is every way the same, the impenetrable matter of which they consist is the same in nature and properties universally. This doctrine, though little attended to by many chemists, has generally found advocates in those great philosophers, whose general knowledge of physics enables them to form expansive views of the structure of the universe. The reason why it is but little entertained by some chemists is, because they think that, if all undecomposed substances really consisted of the same kind of matter, doubtless they ought to be able, in some experiment or other, to reduce them to the same. But this is to entertain far too high a conceit of chemical power. requires the highest energy of analysis to separate oxygen from silicon, though each of these substances possesses a highly symmetrical form and specific individuality of its own, and they be merely retained in union by a well-balanced chemical affinity. Until the vigour of our decomposing apparatus be greatly encreased, there is no reason to suppose that, though dissimilar masses were constituted of radiant matter, or the matter of light, they ought to have returned to this state during the manipulations of the chemist. In many experiments, a few millions, perhaps, of the external particles over which the voltaic repulsion or other destructive energy is diffused, are dissipated into atoms, as, doubtless, in others, such as sulphur, phosphorus, and carbon, they are resolved into hydrogen. But how many millions must be destroyed before a loss of weight would be indicated? Even though the surface of the mass exposed for decomposition were half a square inch, and the weight of a particle as great as that of gold, which is believed to be more than 100 times as heavy as one of hydrogen, it is certain that a complete stratum of the superficial particles might be dissipated, without diminishing the weight of the remainder 10th of a grain; for a single grain of gold may, even mechanically, be made to cover completely 600 cubic inches. 1000 If we could operate with our voltaic and oxy-hydrogen foci upon a few particles of a substance, perhaps even with our present apparatus we might be able to resolve any body into radiant matter. But we must operate upon sensible masses, whose quantity constantly exercises a powerful energy to reproduce the substance as fast as it is destroyed, and whose dissipation, though a particle were destroyed every second, would require the experiment to be sustained for many ages. The convertibility of one undecompounded substance into another is, however, everywhere apparent, by the aid of those instruments of analysis which constitute the organization of plants and animals, and, not unfrequently, even by the aid of the more rude apparatus of the laboratory. Thus, it appears that more carbon is ejected from the lungs than can possibly be absorbed from the food consumed, though some be also added to the system. Herbivorous quadrupeds, which are not known to consume any phosphorus or nitrogen, generate it out of grass or water in many ounces every day. During the hatching of eggs earthy matter is developed. Plants, fed upon pure water, supply themselves with carbon, oxygen, hydrogen, potassium, silicon, iron, &c. &c.; the introduction of which, from without, in such quantities as are found in their substance, has, in many experiments, been rendered impossible. All the phenomena of assimilation are easily accounted for, on the supposition that the unchangeable element of matter is more minute than any of our chemical substances, and by such a supposition, a reason is at the same time given why the assimilating apparatus, which has such an office to perform, is of a structure so perfect and so complicated. But, on the hypothesis that plants and animals receive from without all the undecompounded substances which enter into the |