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The latent heat of air, and of all elastic fluids, may be forced out by sudden compression, like squeezing water out of a sponge. The quantity of heat brought into action in this way is very well illustrated in the experiment of igniting a piece of tinder by the sudden compression of air by a piston thrust into a cylinder closed at one end: the developement of heat on a stupendous scale is exhibited in lightning, which is produced by the violent compression of the atmosphere during the passage of the electric fluid. Prodigious quantities of heat are constantly becoming latent, or are disengaged by the changes of condition to which substances are liable in passing from the solid to the liquid, and from the liquid to the gaseous form, or the contrary, occasioning endless vicissitudes of temperature over the globe.
The application of heat to the various branches of the mechanical and chemical arts has, within a few years, effected a greater change in the condition of man than had been accomplished in any equal period of his existence. Armed by the expansion and condensation of fluids with a power equal to that of the lightning itself, conquering time and space, he flies over plains, and travels on paths cut by human industry even through mountains, with a velocity and smoothness more like planetary than terrestrial motion; he crosses.
the deep in opposition to wind and tide; by releasing the strain on the cable, he rides at anchor fearless of the storm; he makes the elements of air and water the carriers of warmth, not only to banish winter from his home, but to adorn it even during the snow-storm with the blossoms of spring; and like a magician, he raises from the gloomy and deep abyss of the mine, the spirit of light to dispel the midnight darkness.
It has been observed that heat, like light and sound, probably consists in the undulations of an elastic medium. All the principal phenomena of heat may actually be illustrated by a comparison with those of sound. The excitation of heat and sound are not only similar, but often identical, as in friction and percussion; they are both communicated by contact and radiation; and Dr. Young observes, that the effect of radiant heat in raising the temperature of a body upon which it falls resembles the sympathetic agitation of a string, when the sound of another string, which is in unison with it, is transmitted to it through the air. Light, heat, sound, and the waves of fluids, are all subject to the same laws of reflection, and, indeed, their undulatory theories are perfectly similar. If, therefore, we may judge from analogy, the undulations of some of the heat-producing rays must be less frequent than those of the extreme red
of the solar spectrum; but if the analogy were perfect, the interference of two hot rays ought to produce cold, since darkness results from the interference of two undulations of light, silence ensues from the interference of two undulations of sound; and still water, or no tide, is the consequence of the interference of two tides. The propagation of sound, however, requires a much denser medium than that either of light or heat, its intensity diminishes as the rarity of the air increases; so that at a very small height above the surface of the earth, the noise of the tempest ceases, and the thunder is heard no more in those boundless regions where the heavenly bodies accomplish their periods in eternal and sublime silence.
A consciousness of the fallacy of our judgment is one of the most important consequences of the study of nature. This study teaches us that no object is seen by us in its true place, owing to aberration; that the colours of substances are solely the effects of the action of matter upon light, and that light itself, as well as heat and sound, are not real beings, but mere modes of action communicated to our perceptions by the nerves. The human frame may therefore be regarded as an elastic system, the different parts of which are capable of receiving the tremors of elastic media, and of vibrating in unison with
any number of superposed undulations, all of which have their perfect and independent effect. Here our knowledge ends; the mysterious influence of matter on mind will in all probability be for ever hid from man.
THE sun and some of the planets appear to be surrounded with atmospheres of considerable density. According to the observations of Schröeter, the atmosphere of Ceres is more than 668 miles high, and that of Pallas has an elevation of 465 miles. It is remarkable that not a trace of atmosphere can be perceived in Vesta, and that Jupiter, Saturn, and Mars, have very little. The attraction of the earth has probably deprived the moon of hers, for the refractive power of the air at the surface of the earth is at least a thousand times as great as the refraction at the surface of the moon. The lunar atmosphere, therefore, must be of a greater degree of rarity than can be produced by our best air-pumps; consequently no terrestrial animal could exist in it.
What the body of the sun may be, it is impossible to conjecture; but he seems to be surrounded by a mottled ocean of flame, through which his dark nucleus appears like black spots, often of
enormous size. These spots are almost always comprised within a zone of the sun's surface, whose breadth, measured on a solar meridian, does not extend beyond 30° on each side of his equator, though they have been seen at the distance of 391. From their extensive and rapid changes, there is every reason to suppose that the exterior and incandescent part of the sun is gaseous. solar rays probably arising from chemical processes that continually take place at his surface are transmitted through space in all directions; but notwithstanding the sun's magnitude, and the inconceivable heat that must exist at his surface, as the intensity both of his light and heat diminishes as the square of the distance increases, his kindly influence can hardly be felt at the boundaries of our system. The power of the solar rays depends much upon the manner in which they fall, as we readily perceive from the different climates on our globe. In winter the earth is nearer the sun by about a thirtieth than in summer, but the rays strike the northern hemisphere more obliquely in winter than in the other half of the year. In Uranus the sun must be seen like a small but brilliant star, not above the hundred and fiftieth part so bright as he appears to us; but that is 2000 times brighter than our moon to us, so that he really is a sun to