The heat which man is enabled to measure beneath the earth’s surface, appears to be alone due to the conducting powers of the rocks themselves; it has been observed that the line of equal temperature follows, as nearly as possible, the elevations and depressions which prevail upon the surface, and the diminishing rate of increase beyond this line, certainly is such as would arise, was all the heat so measured, the result of the passage of the heat by conduction through the crust of rocks.
Whether or not the subterranean bands of equal heat have any strict relation, upon a large scale, to the isothermic lines which have been traced around most portions of our globe, is a point which has not yet been so satisfactorily determined as to admit of any general deductions.
The Oriental story-teller makes the inner world a place of rare beauty—a cavern temple, bestudded with self-luminous gems, in which reside the spiritual beings to whom the direction of the inorganic world is confided.
The Philosopher, in the height of his knowledge, has had dreams as absurd as this; and amid the romances of science, there are not to be found any more strange visions than those which relate to the centre of our globe. At the same time it must be admitted, that many of the peculiar phenomena which modern geological researches have brought to light, are best explained on the hypothesis of a cooling sphere, which necessarily involves the existence of a very high temperature towards the centre.
We have already noticed some remarkable differences between solar and terrestrial heat; but a class of observations by Delaroche[63] still requires our attention. Solar heat passes freely through colourless glass, whereas the radiations from a bright fire or a mass of incandescent metal are entirely obstructed by this medium. If we place a lamp or a ball of glowing hot metal before a metallic reflector, the focus of accumulated heat is soon discovered; but if a glass mirror be used, the light is reflected, but not the heat; whereas, with the solar rays, but little difference is detected, whether vitreous or metallic reflectors are employed. It is well known that glass lenses refract both the light and heat of the sun, and they are commonly known as burning-glasses: the heat accumulated at their focal point being of the highest intensity. If, instead of the solar beam, we employ, in our experiments, an intense heat produced by artificial means, the passage of it is obstructed, and the most delicate thermometers remain undisturbed in the focus of the lens. Glass exposed in front of a fire becomes warm, and by conduction the heat passes through it, and a secondary radiation takes place from the opposite side.[64] It has been found that glass is transcalescent, or diathermic, to some rays of terrestrial heat, and adiathemic, or opaque for heat, to others[65]—that the capability of permeating glass increases with the temperature of the ignited body—and that rays which have passed one screen traverse a second more readily. It would, however, appear that something more than a mere elevation of temperature is necessary to give terrestrial heat-radiations the power of passing through glass screens, or, in other words, to acquire the properties of solar heat.
To give an example. The heat of the oxy-hydrogen flame is most intense, yet glass obstructs it, although it may be assisted by a parabolic reflector. If this flame is made to play upon a ball of lime, by which a most intense light is produced, the heat, which has not been actually increased, acquires the power of being refracted by a glass lens, and combustible bodies may be ignited in its focus.
It certainly appears from these results, that the undulatory hypothesis holds true, so far as the motion of the calorific power is concerned. At a certain rate the vibrations are thrown back or stopped by the opposing body, while in a state of higher excitation, moving with increased rapidity, they permeate the screen.[66] This does not, indeed, interfere with the refined theory of Prévost,[67] which supposes a mutual and equal interchange of caloric between all bodies.
The most general effect of heat is the expansion of matter; solids, liquids, and airs, all expand under its influence. If a bar of metal is exposed to calorific action, it increases in size, owing to its particles being separated farther from each other: by continuing this influence, after a certain time the cohesion of the mass is so reduced that it melts, or becomes liquid, and, under the force of a still higher temperature, this molten metal may be dissipated in vapour. It would appear as if, under the agency of the heat applied to a body, its atoms expanded, until at last, owing to the tenuity of the outer layer or envelope of each atom, they were enabled to move freely over each other, or to interpenetrate without difficulty. That heat does really occasion a considerable disturbance in the corpuscular arrangement of bodies, may be proved by a very interesting experiment. A bar of heated metal is placed to cool, with one end supported upon a wedge or a ring of a different metal the other resting on the ground. In cooling, a distinct musical sound is given out, owing to the vibratory action set up among the particles of matter moving as the temperature declines.[68]
Heat is diffused through all bodies in nature, and, as we shall presently see, may be developed in many different ways. We may, therefore, infer, that in converting a sphere of ice into water, and that again into steam, we have done nothing more than interpenetrate the mass with a larger quantity of heat, by which its atoms are more widely separated, and that thus its molecules become free to move about each other. Hence, from a solid state, the water becomes fluid; and then, if the expansive force is continued, an invisible vapour. If these limits are passed by the powers of any greatly increased thermic action, the natural consequence, it must be seen, will be the separation of the atoms from each other, to such an extent that the molecule is destroyed, and chemical decomposition takes place.