(63.)

In the experiment already described, by which the latent [Pg114] heat of steam was determined, the water was supposed to be boiled under the ordinary pressure of the atmosphere. Having seen, however, that water may boil at different temperatures, under different pressures, the inquiry presents itself, whether the heat absorbed in vaporisation at different temperatures, and under different pressures, is subject to any variation? Experiments of the same nature as those already described, instituted upon water in a state of ebullition at different temperatures, as well below as above 212°, have led to the discovery of a very remarkable fact in the theory of vapour. It has been found that the heat absorbed by vaporisation is always less, the higher the temperature at which the ebullition takes place; and less, by the same amount as the temperature of ebullition is increased. Thus, if water boil at 312°, the heat absorbed in ebullition will be less by 100° than if it boiled at 212°; and again, if water be boiled under a diminished pressure, at 112°, the heat absorbed in vaporisation will be 100° more than the heat absorbed by water boiled at 212°. It follows, therefore, that the actual consumption of heat in the process of vaporisation must be the same, whatever be the temperature at which the vaporisation takes place; for whatever heat is saved in the sensible form, is consumed in the latent form, and vice versâ.

Let us suppose a given weight of water at the temperature of 32° to be exposed to any regular source by which heat may be supplied to it. If it be under the ordinary atmospheric pressure, the first 180° of heat which it receives will raise it to the boiling point, and the next 1000° will convert it into steam. Thus, in addition to the heat which it contains at 32°, the steam at 212° contains 1180° of heat. But if the same water be submitted to a pressure equal to half the atmospheric pressure, then the first 148° of heat which it receives will cause it to boil, and the next 1032° will convert it into vapour. Thus, steam at the temperature of 180° contains a quantity of heat more than the same quantity of water at 32°, by 1032° added to 148°, which gives a sum of 1180°. Steam, therefore, raised under the ordinary pressure of the atmosphere at 212°, and steam raised under half that pressure at 180°, contain the same quantity of heat,—with this difference [Pg115] only—that the one has more latent heat, and less sensible heat, than the other.

From this fact, that the sum of the latent and sensible heats of the vapour of water is constant, it follows that the same quantity of heat is necessary to convert a given weight of water into steam, at whatever temperature, or under whatever pressure, the water may be boiled. It follows, also, that, in the steam engine, equal weights of high-pressure and low-pressure steam are produced by the same consumption of fuel; and that, in general, the consumption of fuel is proportional to the quantity of water vaporised, whatever the pressure of the steam may be.[18]