The degree of moistness of air is expressed by the phrase hygrometric state. The hygro metric state does not express the density of the water vapor present, but, instead, expresses the quotient obtained by dividing the density of the vapor present by the density of the vapor re quired to saturate the air. If pressures were employed instead of densities in getting the quo tient, substantially the same result would be ob tained. Still another common way of defining hygrometric state is to take the quotient obtained by dividing the pressure of the vapor corre sponding to the dew-point by the pressure of vapor saturated at the temperature of the air, a method closely agreeing with the former ones.
The Critical State.— When the temperature rises, the saturated vapor in contact with its liquid becomes denser, while the liquid it self expands and becomes less dense. If the heating of the liquid and vapor takes place in a strong closed vessel containing not too much or too little of the liquid, after a while a tempera ture is reached at which the saturated vapor be comes as dense as the liquid. At this point they become identical in their physical properties; the line of demarcation of liquid and vapor fades away, and the two fluids begin to mix. The tem perature at which this phenomenon occurs is called the critical temperature; the correspond ing pressure and density are called the critical pressure and critical density, and the liquid is said to be at the critical state. Above the criti cal temperature it is impossible to distinguish between a liquid and its vapor. No .matter how great the pressure, a gas or vapor cannot be forced into the state of a liquid that is obviously distinct from the vapor unless the vapor be cooled below the critical temperature.
Radiation.—We have described two methods by which heat energy may be transferred•from one place to another — by conduction and by convection. A third method remains to be studied. How does the heat of the sun reach us? By means of waves in the luminiferous ether. Go to a quiet pond in which a piece of wood may be floating. Standing on the shore, vibrate your hand up and down in the water. Waves run from your hand over the surface of the water to the wood and cause it to vi brate up and down. Energy from the hand has been transferred to the wood by means of waves. These waves consist of the successive vibration of successive particles of water, each particle receiving energy from behind and pass ing it on to the front. It is much the same with heat waves. The ether, which fills all space, is capable of being set into vibration by vibrating molecules and of handing this vibra tion on step by step in the form of waves. Molecules acted upon by these waves are them selves set into vibration. The vibrating mole cules of the sun generate ether waves, and the ether waves generate vibration of the mole cules of bodies on the earth. These ether waves are called radiant heat. We now have a very wide range of ether waves under ex perimental control. From the large waves generated by electrical oscillations used in wire less telegraphy and sometimes several miles long we may pass by insensible gradations, with only two breaks, to the extremely minute waves which constitute Rontgen's X-rays and the Gamma rays of radium. Dark heat waves, or infra-red rays, ordinary light and ultra violet light belong to the middle of the series. The shortest electrical waves thus far tested are about one-tenth of an inch long, while the length of the longest heat waves is about 1/300 inch. The shortest ultra-violet waves are about 1/300,000 inch long. All ether waves have the same velocity as light, namely, 186,300 miles a second. All may be reflected, ref Tamed, polar ized, diffracted, and made to interfere. They may be absorbed by transmission to a degree depending upon the substance used for trans mission and the particular wave-length of the rays.
Thermodynamics.—The most cogent reason for discarding the caloric theory of heat is that heat may be generated from that which is not in any sense substance —heat may be derived from mechanical energy. Heat is generated when a brass button is rubbed on the carpet, when a bullet is struck with a hammer, and when two pieces of ice are rubbed together, a process resulting in their melting. The rela tion between mechanical energy and the heat energy generated by its consumption was first carefully investigated by J. P. Joule before
1850. One pound calorie of heat energy is ob tained from 1,399 foot-pounds of mechanical energy. That is to say, the energy due to the fall of 1,399 pounds through the distance of a foot is sufficient if transformed into heat to raise the temperature of a pound of water through one degree centigrade. This number of foot-pounds is called the mechanical equiva lent of heat, for it has been found that the process is reversible. When by means of an air-engine or a steam-engine one pound calorie of heat is consumed in generating mechanical energy, 1,399 foot-pounds of the latter are ob tained. The first law of thermodynamics states that when mechanical energy is converted into heat, or when heat is converted into mechan ical energy, the quantity of mechanical energy is equivalent to the quantity of heat energy. The second law of thermodynamics states that it is impossible for a machine without the con sumption of external energy to make heat pass from a body at a low temperature to one at a high temperature. However, when mechanical energy is supplied, such a transfer of energy from a cold body to a hot body becomes possi ble through the use of a °reversible engine.* A heat engine is *perfectly reversible' when a reversal of its cycle of operation is attended by a reversal of all its energy changes, in kind and in amount. Thus, during a direct cycle in a hot air engine, an amount of heat energy, H, is taken from a high temperature *source' and partly converted into an amount of me chanical energy, W, and partly transferred as an amount of heat energy, H', to the low tem perature *escape.* Here W + H', assum ing these quantities to be measured in the same equivalent units, such as foot-pounds. Now, when the hot air engine is reversed in motion by the external application of the same amount of mechanical energy, W, it will, if perfectly reversible, take, during one complete reversed cycle, the amount, H', of heat energy from the low temperature escape and, combining with it the mechanical energy, W, deliver to the high temperature source a quantity of heat energy equal to H. Of course, the perfect reversibil ity here supposed is a theoretical ideal to which practical engines only approximate. The ratio of the mechanical energy generated in a per fectly reversible engine to the heat energy drawn from the high temperature source, i.e., the ratio of W to H, is dependent upon the temperature of the source and the temperature of the escape. This ratio, sometimes called the ther modynamic efficiency of the engine, must, in the case of perfect reversibility, be absolutely inde pendent of the construction of the engine and of the nature of the working substance, be it air, steam, ether vapor, liquid or solid. For, following Carnot, if we suppose that there could be two perfectly reversible engines work ing between tht same hot body as a source and the same cool body as an escape but with ,different thermodynamic efficiencies, we can imagine that these two engines are coupled together in such a way that the more efficient engine, working directly, will drive the less efficient engine, working reversely. The re sult of this combination would be that more heat energy would pass from the cool escape up to the hot source than would pass from the hot source down to the cool escape. In other words, with a self-contained device not employing external energy we would thus suc ceed in making heat pass from a body at a low temperature to a body at a high temperature, which is contrary to the second law of thermo dynamics. The thermodynamic efficiency of a perfectly reversible engine is, then, dependent only upon the two extreme temperatures be tween which it works. In good practical con densing steam engines, which, of course, are only imperfectly reversible, the thermodynamic efficiency may occasionally exceed 20 per cent. This means that 20 per cent of the heat energy supplied to the engine from the boiler is con verted into mechanical energy, the remaining 80 per cent escaping partly, as it should, at the condenser and partly, as it should not, by conduction, radiation, etc. This thermodynamic efficiency may be as much, in a good practical steam-engine, as 70 per cent of the thermody namic efficiency of an ideal engine working between the same boiler temperature and the same condenser temperature.