STEAM AND STEAM BOILERS.

HEAT.

It

is, in fact, a

656. Heat is a form of energy. motion of the molecules composing matter.

It was stated

in Art. 434 that all matter is composed of molecules; now, these molecules are not in a state of rest, but are moving, or vibrating back and forth, with a greater or less velocity, and it is this movement of the molecules that causes the sensations of warmth or cold. If the motion is slow, the body appears cold to the touch; when the vibrations are rapid, the body becomes warm or hot.

It was shown in Art. 545, rule 100, that a body in motion has kinetic energy, the amount of which is measured in foot-pounds, and is found by multiplying the weight of the body by the square of its velocity and dividing by 64.32. Since the molecules composing matter are in motion, they must possess kinetic energy, and we are justified, therefore, in saying that heat, this motion of the molecules, is a form of energy.

657. Temperature is a term used to indicate how hot or cold a body is; i. e., to indicate the rate of vibration of the molecules of a body. A hot body has a high temperature; a cold body, a low temperature. When a body, as, for example, an iron bar, receives heat from any source, its temperature rises; on the other hand, when a body loses heat, its temperature falls.

The temperature is not a measure of the quantity of heat a body possesses. Temperature may be considered to be a measure of the velocity of the molecules of a body as they vibrate to and fro, while the quantity of heat may be considered to be the kinetic energy of the molecules composing the body. A small iron rod may be heated to whiteness and yet possess a very small quantity of heat. For notice of the copyright, see page immediately following the title page.

Its temperature is very high, which simply indicates that the molecules of the rod are vibrating with an extremely high velocity.

Temperature is measured by an instrument called the thermometer, which is so familiar as to scarcely need description. It consists of a thin glass tube, at one end of which is a bulb filled with mercury. Upon being heated the mercury expands in proportion to its temperature. Thermometers are graduated in different ways. In the Fahrenheit thermometer, which is generally used in this country, the point where the mercury stands when the instrument is placed in melting ice is marked 32°. The point indicated by the mercury when the thermometer is placed in water boiling in the open air at the level of the sea is marked 212°. The tube between these two points is divided into 180 equal parts called degrees.

658. Effects of Heat.-Suppose we take a vessel filled with water. Let the vessel be a cylinder fitted with a piston, as shown in Fig. 142. The water is say at the freezing point, and the millions of molecules composing the water are moving to and fro with a comparatively small velocity. Place the vessel in a fire or fur

FIG. 142.

B

nace.

Heat is communicated to the molecules of water, and they begin to move faster and faster and faster. That is, their kinetic energy increases, and, if a thermometer were inserted in the vessel, it would be found that the temperature of the water rises. Consequently, one effect of heat is to raise the temperature of the body to which it is applied. But, after reaching a certain temperature, the molecules of the water not only move faster, but they move further from each other and their paths are longer. It is plain that if the molecules are further apart than they were originally, the whole body of them must take up more space. In other

words, after reaching a certain temperature, the water expands as heat is added. Hence, another effect of heat is to expand bodies to which it is applied. Common examples of the expansion of bodies by heat are seen in the setting of tires, the expansion of the rails of a railway in summer, etc.

659. The heat supplied to the vessel of water has so far done three things: 1. Raised the temperature of the water and thus increased the kinetic energy of the molecules. (Let the amount of heat expended for this purpose be denoted by S.) 2. A certain quantity of heat has been used in expanding the water; that is, in pushing its molecules further apart against the force of cohesion. (Denote the amount of heat so expended by I.) 3. Since the water expands, it must raise the piston Pagainst the pressure of the atmosphere, and, consequently, more heat must be used to expand the water than would be required if there were no pressure on the upper side of the piston. (Call this extra quantity of heat, W.)

If we denote by Q the total heat given up to the vessel of water, we have

Q=S+I+ IV.

660. Ordinarily, the greater part of the heat given to a body is spent in raising its temperature, and but little is used in expanding the body. That is, the quantity S is nearly equal to the quantity Q, while the quantities I and W are extremely small.

water.

Suppose that the piston is removed from the cylinder of Fig. 142 and a thermometer inserted. As the vessel becomes more and more heated, the temperature indicated by the thermometer will rise until it reaches 212°. So far most of the heat has been used to raise the temperature of the But now, no matter how much heat is added to the water, the thermometer stands at 212° and can not be made to rise higher. This is the reason: When the temperature reaches 212° the molecules of water have been set into such rapid motion that the force of cohesion is no longer able to hold them and they tend to separate. In other words,

the water is changing to a gas (steam), and all of the heat is being used to effect this change. The temperature of the steam will remain at 212° until all the water is changed to steam; then, if more heat is applied, the temperature of the steam will begin to rise.

Suppose we take a block of ice at a temperature of say 14° and heat it. If a thermometer is placed in contact with the ice, its temperature will rise until it reaches 32° and will then remain stationary. As soon as this temperature is reached the ice begins to melt or change to water, and the heat, instead of raising the temperature further, is all used to effect this change of state. Here, then, is another effect produced by heat. It will change a solid to a liquid or a liquid to a gas.

661. Latent Heat.-The heat which is expended in changing a body from the solid to the liquid state or from the liquid to the gaseous state is called latent heat. The portion of the heat applied which raises the temperature, and which, therefore, affects the thermometer, is sometimes called sensible heat.

662. Measurement of Heat.-Since heat is not a substance, it can not be measured directly in pounds or quarts; but, like force, it may be measured by the effects it produces. Suppose a certain quantity of heat raises the temperature of a pound of water from 52° to 53°. It will take the same quantity of heat to raise the temperature of a pound from 53° to 54°, and it will take double the quantity to raise the temperature of the pound of water from 52° to 54° that it took to raise the temperature from 52° to 53°. The unit quantity of heat is the quantity required to raise the temperature of a pound of water from 62° to 63°. This unit is called the British thermal unit, or B. T. U.

663. Relation Between Heat and Work.-Suppose that in the experiment shown in Fig. 142, the piston had been allowed to remain in the cylinder while the water was being changed to steam. Steam at 212° occupies nearly 1,700 times the space that the water originally occupied.

Hence, the piston would be lifted in the cylinder to make room for the steam which was being formed. But to raise the piston, work must be done. Here, then, is an example of work being performed by heat. On the other hand, work will produce heat. If two blocks of wood are rubbed briskly together, they will become warm and may even ignite. The work of friction causes the journals and bearings of fast-running machines to heat. A small iron rod may be raised to a very considerable heat by pounding it on an anvil.

664. Since work may be changed into heat, and heat into work, it seems probable that there is some fixed ratio between the unit of heat (B. T. U.) and the unit of work, the foot-pound. By a careful series of experiments Dr. Joule, of England, discovered this ratio.

He found that one B. T. U. was equal to 772 foot-pounds; later and more careful experiments show that 778 footpounds is more nearly correct. This number, 778 foot-pounds, is called the mechanical equivalent of one B. T. U.

665. We have, then, the following important law: Heat may be changed to work, or work to heat; 778 foot-pounds of work are required to produce one B. T. U., and, conversely, the expenditure of one B. T. U. produces 778 foot-pounds of work.

EXAMPLE 1.-The burning of a pound of coal gives out sufficient heat to raise 14,000 pounds of water from 62° to 63°. If all this heat is utilized, how high will it lift a weight of 700 pounds?

SOLUTION. Since one B. T. U. raises a pound of water from 62° to 63°, it requires 14,000 B. T. U. to raise 14,000 lb. of water from 62° to 63°. Hence, the burning of the pound of coal gives out 14,000 B. T. U. One B. T. U. is equivalent to 778 foot-pounds; hence, 14,000 B. T. U. are equivalent to 14,000 × 778 = 10,892,000 foot-pounds. Then, the height to which the weight can be raised is 10,892,000 ÷ 700 = 15,560 feet. Ans.

EXAMPLE 2.-A cannon ball weighing 60 pounds moves with a velocity of 1,300 ft. per sec. Suppose the ball were suddenly stopped and all its kinetic energy changed into heat. How many B. T. U. would be developed? If all this heat were applied to 100 lb. of water at a temperature of 60°, to what temperature would the water be raised?