is no loss of energy, only a change of form. A body hurled to a certain height possesses at its highest point no energy of motion (kinetic energy), but it possesses energy of position (potential energy) just equal to the energy of motion that sent it up. When it falls down it reaches the ground with the same speed as it started its upward motion with, and during its fall it could furnish the same amount of work as was expended in raising it.

54. Heat is a Form of Energy.-Heat is a form of energy due to some invisible motion of the molecules of a body among themselves, and not of the body as a whole. When a falling body strikes the ground its visible kinetic energy disappears, but it has not been lost. It has been converted into the energy of heat and the energy of sound, and the sum of these different forms of energy is equal to the whole visible energy of motion possessed by the stone when it strikes the ground. The energy of sound consists of moving particles of air, but the energy of heat consists in increased motion of the molecules of the body manifested as increase of heat. Since this invisible molecular motion called heat is able to perform work, we call heat a form of kinetic energy.

As examples of the transformation of the visible energy of motion into heat, the following experiments may be performed:

Experiment 27.-Hammer a piece of lead smartly several times upon an anvil, and test the temperature of the lead and the hammer before the experiment and after. An increase of temperature can be detected by means of a thermopile and a galvanometer. An iron nail may be hammered until its rise of temperature can easily be felt.

Experiment 28.-Place a small piece of German tinder at the end of a stout glass fire-syringe (Fig. 49). Now introduce the tight-fitting solid piston and force it down quickly. The suddenly compressed air becomes so hot that the tinder is ignited. Mechanical motion is converted into heat.

Many other instances of the production of heat through rubbing, pressure, filing, grinding, etc., might FIG. 49.— be given. The visible energy of motion can there- ringe.

fore be transformed into heat.

A fire-sy

It should also be noted that the energy of heat is often being changed into the visible energy of motion. Every

locomotive engine supplies an example of this transformation. As another example of the disappearance of heat when motion is produced, note this experiment:

Experiment 29.-Let air or other gas that has been compressed in a cylinder and allowed to come to the ordinary temperature escape against the bulb of a sensitive thermometer, and a fall of temperature will be

noticeable.

55. Electrical Energy.-Various bodies, such as sealingwax, vulcanite, and glass can be brought by rubbing (i.e. by doing work upon them) into a condition such that they attract other bodies near them. This condition is called electrification, and the bodies are said to be electrified, or to be charged with electricity. Since electrified bodies can produce motion and heat, they are said to be endowed with electrical energy.

Experiment 30.-Rub a piece of sealing-wax or vulcanite with a piece of dry flannel, and then bring it over some bits of light paper, straw, or pith. These will be attracted even at a short

distance. The electrified wax does work in raising the bits, and must therefore be endowed with energy.

Experiment 31.-Suspend a pith ball by a silk thread to a support, and bring it between an insulated metal plate joined to the knob of an electrical machine and a metal plate having

FIG. 50.-Particles attracted by a rubbed rod of glass.

FIG. 51. (From Joubert's "Electricity and Magnetism.")

metallic connection with the earth. The pith ball moves backward and forward between the two metal plates.

In a charged Leyden jar we have two coatings with opposite kinds of electrification, and on bringing the separate states into

connection, the electrical energy is transformed into heat and light as shown by the spark that passes across the connection.

n

In the electric current produced in a voltaic cell or battery of cells we have electricity in motion. The electrical energy of such a current is partly transformed into heat when it is sent through a thin platinum wire, for the wire offers so much resistance to the current that it becomes red-hot. The energy of the electric current is able to produce motion in a magnetic needle, for if the wire conveying a current be placed over a magnetic needle pointing north and south, the needle is at once moved so as to set itself at right angles to the current.

Not only does the energy of the electric current often get transformed

S

Cu

Z n

FIG. 52.-Deflection of compass-needle by an electric current.

into heat, but heat gives rise to electric currents.

If we

solder together a piece of antimony and bismuth at one end, and have the free ends united by a thin copper wire, we can detect an electric current passing along the wire on heating the junction. This current would, as just stated, move a small magnetic needle. An instrument applying these facts, and called the thermopile, can be made which will measure very small changes of temperature.

56. The Energy of Chemical Action.-The carbon of coal and the oxygen of the air have a certain affinity or attraction for each other, and this chemical attraction is a store of energy, the potential energy of chemical separation. When the coal is made to burn, that is, unite with the oxygen of the air, the molecules of the two substances move together, and as the carbon and oxygen unite, the energy becomes kinetic or active, and the potential energy stored in the fuel soon becomes transformed into light and heat. When gunpowder explodes and sends out a 50-lb. ball from a cannon, the energy of chemical

action resulting from the union of the ingredients of the powder is converted into the mechanical energy of visible motion. In a later chapter we shall have many more examples of the energy of chemical action.

57. Radiant Energy.-The molecules of a heated body are in a state of rapid vibration, and are constantly communicating some of their energy to the surrounding and all-pervading ether. This energy is carried along the ether by means of vibrations or waves, and these ether-waves may either be again transformed into heat, or cause the sensation of light, or give rise to certain chemical actions. Energy transmitted as vibrations of the ether is called radiant energy, and the process of transmission is often called radiation. What is spoken of as radiant heat and light are forms of radiant energy (see pars. 72 and 80).

57a. Transformations of Energy. It is evident, from what has been said, that one form of energy is often being transferred or converted into another, and it is not difficult to put together a series of interesting transformations. The potential energy of cold fuel is changed as it unites with oxygen during burning in a furnace into the energy of invisible molecular motion called heat; the heat energy of the burning fuel passes into water and changes it into steam; the energy of the steam sets in visible mechanical motion a piston and its connected machinery; the moving machinery may drive a dynamo and produce electric currents; and the electric currents may make white-hot the carbon filament of an incandescent lamp, drive a tram-car, ring a bell, or cook a dinner.

As the student progresses in his studies, those and other transformations will be better and better understood. But it is important to notice even now that the energy which is being dealt with passes into more than one of the other forms, and that the sum of these is equal to the energy applied. There is, in fact, no loss of energy, for energy like matter is indestructible. Yet in any change of energy from one form to another, some of it always takes the form of heat, and we cannot convert heat entirely into any other form, but only partially, for some of it is always lost to further use by conduction and radiation.

CHAPTER VIII.

HEAT.

58. Distinction between Heat and Temperature. If we add a pint of hot water to a pint of cold water, we notice that the mixture is cooler than the hot water was and warmer than the cold, or, in other words, that the temperature of the mixture is different from the temperature of the two masses before mixing. The quantity of heat in the mixture, however, must be the same as the sum of the quantities in the two masses mixed, for no heat has been added or abstracted. There is clearly, therefore, a difference between heat and temperature. Heat is, in fact, a form of energy that consists of vibratory motions in the molecules of a substance, while temperature is a quality of a body dependent on the quickness of this molecular motion. The quantity of heat in a body will therefore depend upon the rapidity of molecular vibration and the total number of molecules which are in vibratory motion; the temperature will depend upon the speed of vibration merely. When heat is transferred to a body its molecules are set in more rapid vibration, and its temperature thus rises.

Temperature being a quality of bodies, we speak of degrees of temperature; heat being a form of energy, we speak of the quantities of heat possessed by bodies. Bodies may have the same temperature and yet contain different quantities of heat; thus, a glass of water taken from a large boiler will have the same temperature as that left in the boiler, but the quantity of heat in the glass of water, which is the sum of the energy of its molecules, must be much less than the quantity of heat. contained by the water left in the boiler.

The temperature of a body is measured, as every one