Solid bodies conduct caloric in all directions, upwards, downwards, and laterally.
Exp. Hold a rod of iron obliquely over a lamp or furnace and it will be heated both above and below the fire.
26. Some bodies conduct caloric much more rapidly than others.
Exp. Coat with wax the ends of two rods of the same size, and length, one of glass and the other of iron, or any other metal; then place the uncoated ends in a vessel of boiling water. The wax on the metallic rod will be melted, while that on the glass remains cold.
27. Liquids, and elastic fluids, do not conduct calorics but convey it by changing the place of their particles.
Illus. That portion of the fluid which is nearest the source . of heat, is expanded, and rises up, carrying with it a quantity of caloric, while that portion which is colder descends, and in its turn, is heated and rises, while another portion descends, and thus the process goes on as long as the fluid is capable of imbibing heat.
Exp. 1. Take a tall glass jar, or vial, and nearly fill it with cold water, and put into the water some particles of amber, or any other substance, about the specific gravity of the water ; then immerse the jar in a vessel of boiling water. The motion of the particles of amber will show two currents in the ja?, one upwards, along the sides where the heat is greatest, and one downward, in the centre.
2. Take a glass tube, 10 or 12 inches (long, closed at one end, and put into it an inch or two of coloured water, and then fill the tube by carefully pouring in common water by means of a dropping, so as to keep the two strata quite distinct. If now the upper end of the tube be heated, the water may be made to boil at this part while the two strata at the bottom remain undisturbed. But if the tube be afterwards heated at the bottom, the coloured fluid will rise up and tinge that above it.
OF COMBINED, OR LATENT CALORIC.
28. When a solid is converted into a fluid, or a fluid into the aeriform state, a quantity of caloric is absorbed, which is not appreciable by the thermometer, nor sensible to the touch. This is called combined or latent Caloric. .
Obs. The quantity of caloric absorbed by melting ice, is much more, than can be accounted for by the corresponding difference between the temperatures'of the ice and the water, as indicated by thelthermometer, or by sensation. A quantity of caloric, then, is absorbed by the melting ice; combines with the water, and forms a part of it. Thus water is a compound of ice and caloric. But the caloric is so confined by its intimate union with the water, as not to be given out.during its liquid state, and consequently it cannot excite sensation. The following experiment will illustrate this fact.
Exp. Expose a pound of water at 32° and a pound of ice at 32° in a room the temperature of which is 60°, and uniformly the same during the experiment. Place the bulb of one thermometer in the vessel of water, and another in that containing the ice. The thermometer in the vessel of water will very soon begin to rise and will gradually reach the temperature of 60° , while that placed in the ice will steadily continue at 32°, for several hours, or until the ice is entirely converted to water; it will then begin to rise ; and from this time, it will take just the same number of minutes, for the water in this vessel to reach the temperature of the room, as it did in that at the beginning of the experiment.
Obs. Although during this experiment the ice remained at 32°, yet it must the whole time have been absorbing caloric, because a colder body cannot be in contact with a warmer one without receiving caloric from it. The caloric therefore which turned the ice into a liquid, or caused its fluidity remains fixed, or latent in the water.
Corol. The fluidity of water is owing to a quantity of caloric which it contains in a latent state.
Commentaires