receiver R. When the piston begins to descend, the valve v” closes and the valve in piston p" opens, letting the air escape into the space above the piston. The air can thus be considerably rarefied by a few strokes, but it is impossible to produce a perfect vacuum, owing to the difficulty of securing perfectly fitting joints for the apparatus.

A more perfect vacuum is attained by the aid of the Sprengel mercury pump. This consists simply of a vertical tube of narrow bore, something over a yard in height, a few inches from the top of which a lateral tube is made to connect perfectly air-tight. If now mercury be poured into the vertical tube by the aid of a funnel, in falling it draws the air from the vessel connected with the lateral tube until a complete vacuum has been established therein, when a column of mercury thirty inches in height will stand in the vertical tube. These pumps are used exclusively in producing the vacuum needed for incandescent electric-light globes.

Condensing Pumps.—By a different arrangement of the valves in the cylinder and side-tube, instead of exhausting a given space, additional air may be forced in and so brought under considerable pressure. A condensing pump is also useful in condensing other

P

W

FIG. 16.

Lifting pump.

gases than air, as in forcing carbon dioxide under pressure into solutions.

Lifting and Suction Pumps.-An important application of the principle of atmospheric pressure is seen in the pumps devised for the lifting of water from wells and cisterns. One of the common forms is illustrated in Fig. 16. The action here exactly corresponds to that in the cylinder of the air-pump. As the piston P is drawn up, the valve v', known as a clack-valve, opens upward, and air with water following it is drawn up into the cylinder. As the piston descends, the valve v' closes and v opens. After a few strokes the water is lifted by this action from the depth w to the cylinder and there remains, filling the whole length of

the tube. It is obvious that the action of the piston first exhausts the cylinder of air, and that the water rises into the vacuum thus

formed, where it remains unless the leakage of air into the cylinder causes the water to fall again to the level of that below. Other forms of pumps combine the principle of exhaustion and pressure, and thus can deliver a continuous stream of water.

The Siphon.-This is simply a tube bent at an acute angle, open at both ends, and with legs of unequal length. If the siphon be filled with liquid and the longer end closed temporarily, on dipping the shorter leg into the liquid contained in an open vessel a flow begins when the longer leg of tube is unstopped, and the liquid will drain from the vessel until the level falls below the end of the short leg. The explanation of the action of the siphon is simple. When it is in action, the pressure on the surface of the liquid in the vessel is the atmospheric pressure minus the vertical distance from this surface to the top of the shorter leg of the siphon, while the pressure on the liquid where it is flowing out is the atmospheric pressure minus the vertical distance from the point of escape to the bend at the top of the longer leg. This latter pressure is less than the former, hence the liquid continues to flow out as long as this inequality continues.

CHAPTER III.

RADIANT ENERGY.-I. Heat.

I. THE NATURE OF HEAT.

HEAT is a form of energy due to molecular vibration. This vibration seems to be taking place in greater or less degree in all bodies, and, when communicated to the ether which fills all space around the vibrating body, is transmitted to the nerves of sensation, and so is felt as heat. The presence of air is not essential for this transmission of heat vibrations, as they are transmitted equally in vacuo as in air. When these vibrations become more rapid the heated body may become luminous, and this manifestation of radiant energy is called light. Electrical energy seems to be due to vibrations of the same nature, but of still greater rapidity of movement. Such is the undulatory or vibratory theory of heat and light. The fact that one of these related forms of energy can be changed readily into the others, and that all of them can be produced from mechanical energy, renders this theory a very probable explanation of the observed phenomena.

An older theory that heat as well as light was due to the emission of material particles from heated bodies, or sources of heat, is now practically abandoned. Similar statements with regard to the existence of a supposed material substance called caloric are now considered as devoid of any foundation of truth.

II. SOURCES OF HEAT.

1. Physical Sources.-By far the most important source of heat known is the sun's radiation. What the source of the sun's heat may be is not known, nor can any exact estimates be made as to the temperature existing upon the sun's surface. The quantity of heat received upon the whole surface of the earth in any given unit of time, as in a day, is, however, enormous. Not only is it the greatest of the present sources of heat, but by its past activity it has been the means of accumulating for us the immense stores of coal, petroleum, and other valuable heat-producing substances used at present as fuel.

The earth possesses also a heat of its own, readily noted as we descend to any considerable depth below its surface, and made evident to us in hot springs and volcanoes. The explanation most generally accepted for this is that the earth has cooled from a much more highly heated state, probably that of an incandescent gas, and that, while a hard crust has formed upon the surface, the interior of the globe is yet in a molten and liquid state.

2. Chemical Sources.-Most forms of chemical combination, as we will see later, are accompanied by the development of heat in definite amounts, or are exothermic. Hence every case of combustion going on about us in nature contributes to the development of heat. This includes the rapid combustion of all forms of fuel, and the slow combustion or decay of organic matter. It includes also the respiration of all kinds of animals, and the processes of assimilation of food equally due to chemical and heat-producing changes. Those forms of fuel which are richest in the elements carbon and hydrogen possess the greatest value as fuel, as by the oxidation of these elements the maximum of heat can be developed. Hence the value for heating purposes of the several varieties of coal, of petroleum, of hydrocarbon gases, and of so-called "water-gas."

3. Mechanical Sources.-Friction and percussion are among the commonest of the methods by which heat is developed. The old device of obtaining sparks from a piece of flint and a steel, and the still older one of the savage of rubbing together two dry sticks to kindle a fire, are illustrations of the development of heat by friction. The "hot-box" on a railway car, where the heat developed by the friction of the car-axle in its box often suffices to ignite the oil-soaked waste, is also an illustration. The striking of the blacksmith's hammer upon the anvil readily illustrates the heat developed by percussion. In this case the energy of the mass of the hammer in descending is changed when it strikes into the molecular energy of the particles known and recognizable by the senses as heat.

Mechanical Equivalent of Heat.-The fact that visible mechanical or mass motion may be changed into molecular motion, or heat, suggests that a mechanical equivalent may be established for a given development of heat. This idea, first brought forward by Mayer, was experimentally established by Joule. He determined the number of foot-pounds (see p. 20) of energy equivalent to a unit quantity of heat. He used a copper vessel filled with water and provided with brass paddle-wheels, which

were set in motion by the falling of weights. The weights being known and the space through which they fell, as well as the difference in temperature of the water at the beginning and at the end of the fall, the amount of fall corresponding to an increase of one degree was readily calculated. The result expressed in English units is that to raise one pound of water one degree Fahrenheit requires 772 foot-pounds, or to develop one degree Centigrade requires 1390 foot-pounds.

III. EFFECTS Of Heat.

If we refer for a moment to the explanation given as to the nature of heat, we can conjecture what the effects of the application of heat to a body would necessarily be. In the first place, the vibrations already existing between the molecules or particles of the body will be increased in rapidity, and the body, to use the common expression, becomes hotter. This is made evident by rise of temperature, appreciable by the senses or by more accurate recording instruments. In the second place, these vibrations will increase not only in rapidity but in amplitude, and the body expands, the force of cohesion having been weakened while that of repulsion tends more strongly to drive the molecules apart. In the third place, this loss of cohesive power may go so far that change of physical condition results. A solid body under the influence of heat may fuse or liquefy, and a liquid when heated may boil and be entirely volatilized. We shall take up these several effects of heat in succession for more detailed study.

1. Rise of Temperature. This is the first and most general effect of heat. Indeed, it might be called the invariable effect except for the case which will be referred to later, when the heat applied is consumed in doing the work of changing the physical condition of the body, or, as it has been termed, is rendered latent. In this case it does not show in the raising of the temperature of the body.

Rise of temperature is in some cases appreciable by the senses, but more delicate means of distinguishing are clearly obtainable in the use of what are termed thermometers, or heat-measurers. These usually depend upon the expansion and contraction under the influence of heat and cold of a liquid or gas, but the expansion and contraction of metals are also availed of at times.

The ordinary thermometer is that in which mercury is used. It consists of a glass bulb of spherical or cylindrical shape connected