ductors of caloric and electricity. Hence it is that when water is poured on a perfectly dry clean silk dress, it runs off, or collects in large globules, while it is rapidly absorbed by linen, which is a much better conductor of caloric. For the same reason, a lock of wool, cotton, down, or fur, will float on water a long time without becoming wet. Or if one end of a perfectly dry skein of silk be suspended in a vessel of cold water, it remains for hours without attracting the water through its fibres; whereas a skein of linen or hemp becomes very soon wetted throughout. But if the water be made hot, it rises through the silk also. In accordance with the above facts, it has been found that mercury, and other melted metals, instead of rising through capillary glass tubes, are somewhat depressed, because they have a stronger attraction for their own particles than for those of the glass. That they have also a stronger attraction for caloric, is evident from their greater conducting power. Hence, there can be no transition of caloric from liquid metals to the glass, for the same reason that there is none from cold water to silks, resins, &c. therefore no attraction. But if plates of gold, silver, and tin, be inserted into mercury, the latter is attracted by them, rises above its level, and is incorporated with them by virtue of the same power that causes all other attractions between liquids and solids. When mercury is poured on a marble or wooden table, it collects in large globules, for the same reason that water aggregates into large drops when poured upon dry silk, a duck's feathers, and other non-conducting bodies or for the same reason that when plates of glass are smeared with tallow they do not attract water unless previously wetted. On the Connexion between Gravity and the It has been said that the attraction of atoms is not like that of gravitation, inversely as the squares of the distance. This assertion is not only refuted by all analogy, but by the well established fact that the elevation and force with which liquids rise in capillary tubes, cæteris paribus, is inversely as their diameters. The principal difference between the attraction of atoms and that of masses is, that the former acts at exceedingly small distances, corresponding with the minuteness of atoms; while the power of masses extends to comparatively great distances. The aggregate force of cohesion with which a mass of granite is held together, other things being equal, is proportional to the number of its ultimate particles. The force of gravity with which it presses upon the earth is in the same ratio. But if caloric be the physical cause of cohesion, capillary attraction, and chemical solution, as I have demonstrated; and if gravitation result from the aggregate action of atoms, as maintained by Newton, Laplace, and many other philosophers, the annual and diurnal movements of the heavenly bodies must be owing to a modified action of the same cause.† We are so accustomed to the great powers and movements which mark the course of nature, that we are scarcely aware of their existence until aroused by some extraordinary phenomenon. What can be more obvious and familiar than the power of heat in modifying the surface of our planet? a power absolutely incommensurable, though for the most part unobserved. Were it possible to compute the aggregate forces of capillary attraction in the circulation of the + According to all the best established canons of philosophizing, that is the most important principle in physics, to which the greatest number of phenomena may be traced. But if we admit that the accelerated motion of falling bodies, the aggregation of planets, with their annual and diurnal revolutions, are resolvable into the Newtonian law of gravity, it is certain that many other equally important phenomena of nature cannot be referred to the principle of gravitation, such as those of heat, light, and electricity, together with the innumerable operations of Chemistry, Geology, and Meteorology, all of which are immediately connected with, and may be traced to that law of caloric by which it produces the opposite effects of contraction and expansion, and without which, we cannot explain the most simple modifications of molecular attraction. blood and sap of all the animals and plants that inhabit the earth, we should be amazed at the result. Yet they are all produced by the subtile agency of caloric, a definite amount of which is indispensable to all vital action, from that of the insignificant moss or animalcule, to the most perfect developements of organized existence. On beholding for the first time so grand a spectacle as the falls of Niagara, the mind is bewildered by an impression of irresistible power. But if we compare this thundering exhibition of might, with the vast but silent power of solar caloric in evaporation, or of subterraneous caloric in upheaving mountains, the ocean cataract dwindles into a fractional item; for it is demonstrable, as before observed, that about 140 cubit miles of water are daily converted into steam, and carried into the atmosphere by the expansive energy of solar heat, and that all the lakes, rivers, and springs of the earth are supplied by its precipitation. We are sometimes aroused to a perception of the wonderful powers that are in nature, by the sudden and awful coruscations of the electric fluid, when darting through the heavens like arrows of Omnipotence, rending rocks, trees, and dwellings. But few are aware that it is only a concentrated exhibition of the same agent which causes evaporation, solution, crystallization, and the growth of vegetation. It is generally conceded by philosophers, that all the operations of nature are referable to attraction and repulsion, which I have proved in the preceding chapters, are resolvable into the agency of caloric alone. But if the stupendous forces of chemistry, geology, and meteorology, are determined by the agency of solar radiation, why should it not be adequate to produce the annual and diurnal revolutions of planets? That rays of subtile matter, capable of producing heat and light, are continually proceeding from the centre of our system, is self-evident from experience and observation. It would, therefore, be contrary to all analogy, and the indications of common sense, to refer the planetary motions to some unknown hypothetical influence exerted through a vacuum, while there is a known cause sufficient to explain the phenomena. It is impossible to conceive how the sun could exert any agency whatever upon the earth, independent of his potent beams. Besides, what can be more simple and natural than the inference, that the same power which aggregates and holds together the particles of planets, guides their movements round the heavens? It was laid down by Newton himself, as a fundamental axiom, that no more causes of natural things ought to be admitted, than such as are both true and sufficient to explain the phenomena. (Principia, Book iii.) |