of heat. Bodies which radiate heat best have the power of absorbing it in the same proportion, and the least power of reflecting it; hence, in leading steam through a room, it would be absurd to use black pipes, because, in that case, much of the heat would escape by radiation before the steam would be carried to the place where it was to be used. If the steam is used to heat the apartment, black pipes are the best. Hence the cylinder of a steam engine ought to be polished, but the condenser should not. Vessels intended to receive heat should be black. The comparative quantities of heat existing in different bodies may be ascertained by marking the time which equal quantities of them require to cool a certain number of degrees, reckoning their capacities for heat to be as these times estimated by the volume; or, if divided by the specific gravity of the substance, by the weight. It is necessary here to distinguish carefully between what is called the specific heat of a body, and its capacity for heat, these two terms being often confounded. If we take two bodies at the same temperature, and expose them to the action of a greater heat, it will be found that one body will have absorbed a greater quantity of heat than the other, by the time that they have acquired an equal tem perature; and the amount of this additional heat, referred to some standard, is denominated the specific heat of the body. Thus if it be found that it requires 1 degree of heat to raise water from one temperature, T, to another temperature, t, and if to produce the same change of temperature in steam it requires 0.847 degrees, then is 0.847 the specific heat of steam, water, as the standard, being 1.000. The capacity of one body for heat compared to another is not the relative quantities of heat required to raise them a certain number of degrees, but the absolute quantities contained in them at the same temperature. CAPACITIES OF BODIES FOR HEAT. Solution of muriate of soda, 1 in 10 of water,... •9360 •9250 •9050 Olive oil,...... 7100 Nitric acid, specific gravity 1.29895,. 6613 Sulphuric acid, with an equal weight of water, 6050 Nitrous acid, specific gravity 1·354,.. •5760 Linseed oil,.... ⚫5280 Quicklime, with water, in the proportion of 16 to 9, 4391 Soft bar iron, specific gravity 7·724,.......... •1190 Brass, specific gravity 8.356, 1160 Copper, specific gravity 8.785, •1140 Large quantities of heat must enter into bodies, and be concealed, to enable them to pass from the solid to the fluid state, or from the fluid state to that of vapour. Thus the quantity of heat necessary to convert any given weight of ice into water, would raise the same weight of water 140 degrees of Fahrenheit. This quantity of heat is not sensi ble, but is, as it were, kept hid or latent; nor can it be de tected by the touch, or by application of the thermometer. Every addition of heat applied to water in a fluid state, raises the temperature until it arrives at the boiling point; but however violently the fluid may boil, it does not become hotter, nor does the steam that arises from it indicate a greater degree of heat than the water; hence, a large proportion of the heat must enter into the steam and become latent. The quantity of heat that becomes latent in steam, was found by Dr. Black to be 810 degrees of Fahrenheit. Under the common pressure of the atmosphere at the surface of the earth, (15 lbs. on the square inch,) water cannot be raised above a temperature of 212 Fahr.; but when exposed to greater pressure, by being confined in a vessel, the water may be raised to a much higher degree of heat, and if, in this state of confinement, the heat applied be insufficient to cause the water to boil: if the vessel should be open, steam will rush out, and the water which remains will fall in temperature to 212. On the contrary, water boils at very low temperatures when the pressure is diminished; as in an exhausted receiver, or at the tops of mountains. When the temperature of steam is reduced, it assumes again the fluid form, and the quantity of latent heat set free by steam in passing to the state of water, has been found, by Mr. Watt, to be 945 degrees. He also found that a cubic inch of water may be converted into a cubic foot of steam; and that when this steam is condensed, by injecting cold water, the latent heat which the steam gives out in passing to the fluid state, would be sufficient to heat 6 cubic ches of water to the temperature of 212, or the boiling point. It is generally considered that steam raised from boiling water occupies 18 hundred times as much space as the water did from which it was raised, and instead of making the latent heat of steam 810, as Dr. Black found it, more correct experiments show it to be 1000, at the ccm mon pressures of the atmosphere; but the latent heat of steam is inversely proportional to the degree of pressure under which it is produced; that is, the latent heat is greatest where the pressure is least, and least where the pressure is greatest. It has lately been discovered that the sensible heat and latent heat of steam at any one temperature added together, give a sum which is constant; that is to say, which is the sum of the sensible and latent heat of any other temperature, or under any other pressure. Now, the sensible heat of steam at the ordinary pressure of the atmosphere is 212-32 = 180; and the latent heat has been found to be 1000, their sum is 1180, which is the constant sum of the latent and sensible heats of steam under any other pressure. Thus, at the temperature of 248, where the elastic force of the steam is equal to two atmospheres, or a pressure of 30 lbs. on the square inch, the sensible heat will be 248-32 216, wherefore the latent heat is 1180 — 216 = 964, and so of the other temperatures. It has also been found that while the elasticity of steam increases in geometrical progression, with a ratio of 2, the latent heat diminishes with a ratio of 1·0306, differing not very materially from a unit. Many experiments have been made to ascertain the elastic force of steam of various temperatures. The most valuable of them are those recently made by the French academicians, the results of which are given below in a tabular form; and the practical man will duly estimate the value of this gift of science. The following simple rule is easily remembered and applied, and comes near enough to the truth for all practical then 2.3 × 2·3 × 2·3 × 2·3 × 2·3 × 2·3 = 148.0359, this divided by 30, gives the atmospheres, In constructing this table the results were derived from experiments up to 24 atmospheres, after which the formula which follows was employed. E=(1+T+0-7153) 5 Where E represents the elasticity, and T the temperature, by the centigrade thermometer, regarding 100° as unity, and T the excess of temperature above 100° It may be observed that this formula is more accurate in very high temperatures than for low. |