and put in a rubber stopper with tube, as shown in the figure. Replace the flame, and the steam formed by boiling will collect in the flask above the surface of the water and increase the pressure. As this pressure increases it will do work by forcing water out of the tube.

In the process of evaporation the molecules, moving in every direction, strike the surface of the liquid from below, and some of them escape into the air. If the temperature is raised, the velocity of the molecules is increased, and a

5.

0 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° 110° 120° 130° 140° TEMPERATURE IN CENTIGRADE DEGREES

FIG. 260.-Pressure of Water Vapor at Different Temperatures

greater number escape. When boiling begins, at 100° under ordinary atmospheric pressure, the rise of temperature is stopped, and all the heat energy applied is used in changing water to steam. If the boiling takes place in a closed vessel, the repeated blows of the molecules of the vapor upon the surface of the liquid oppose the escaping molecules more and more as the vapor increases in quantity and pressure; and if the temperature is kept at any fixed point, as 100°,

the boiling soon stops. If the temperature is now raised, the molecular velocity increases, the internal pressure becomes greater than the vapor pressure, and the boiling re

commences.

The relation between the vapor pressure, or vapor tension, of water and the temperature that produces it is a most important one, and is applied in determining the required strength of steam boilers. Figure 260 is the curve showing this relation. Dry steam confined in a boiler at temperatures above 100° is called superheated steam. The pressure of this superheated steam is the source of its expansive power. When it passes into a steam cylinder, it expands, does the work of driving the piston, gives up some of its heat (§ 286),

FIG. 261

and if it cools to 100°, has no further expansive power at ordinary atmospheric pressure. Figure 260, however, shows the actual vapor tension at temperatures below the boiling point as well as above it. The existence of this pressure may be demonstrated as follows:

Demonstration.

Set up a barometer tube B (Fig. 261), as in the demonstration in § 170, and then introduce a few drops of water under the lower end of the tube with a fountain pen filler. When the drops are introduced, vaporization takes place, and the mercury falls slightly in the tube. In order to be sure that the space above the mercury is filled with saturated vapor, enough water should be put in to leave a little on the surface of the mercury.

If ether is introduced instead of water, the change in the mercury column is much greater, as shown at C. In the case of any liquid

il

the vapor tension is equal to the difference between the height of the mercury column below it and the height of the barometer A at the time.

312. The Steam Engine. A steam engine is a machine for transforming the pressure and expansive power of steam into mechanical energy. Since the steam is produced by the application of heat, the steam engine really transforms heat into mechanical energy. So great, however, are the losses in the burning of the coal, the expansion of the

steam, and the working of the engine, that the best modern steam engine does not utilize more than 17 per cent of the energy in the coal.

A simple form of steam engine is shown, partly in section, in Figs. 262 and 263. A is a steam pipe connecting a boiler with the steam chest B. The live steam passes from the steam chest through the port C into the left end of the cylinder, between the cylinder head D and the piston P. The pressure of this live steam forces the piston to the right, and drives the exhaust steam, on the other side of the piston, out of the port E under the slide valve V, and out of the exhaust port F, which leads either to a condensing chamber or to the open air.

When the piston has been forced over a part of its stroke, the slide valve will be moved by its rod r to the left, closing C, and the work done during the rest of the stroke will be due to the expansive power of the steam. By the time the piston has reached the end of its stroke to the right, the slide valve will be moved so far to the left that the live steam will now come into the right end of the cylinder through E, and the exhaust steam in the left end will go out of the ex

haust port F, through the port C. The piston rod R is attached to a crank arm M on the main shaft S, and in this way the to-and-fro, or reciprocating, motion of the piston is changed to the rotary motion of the shaft. On the shaft are fixed one or two heavy flywheels W, the momentum of which serves to give steadiness to the engine; and one or more belt wheels, over which run the belts by which the motion of the shaft is transmitted to machinery.

In condensing engines the exhaust steam passes into a compartment containing water for condensing the steam. The condensation greatly reduces the back pressure which opposes the motion of the piston. In noncondensing en

gines, such as the locomotive, the exhaust steam passes into the open air sometimes by way of the smokestack, in order to increase the draught through the fire box. The back pressure in noncondensing engines is the pressure of the atmosphere.

In compound engines, the steam gives up only part of its heat and expansive power in one cylinder; the exhaust steam from this escapes under pressure to a second cylinder, where it does more work. By thus using two or more cylinders in succession, a greater percentage of the energy of the steam can be utilized. A comparison of Stephenson's locomotive, the Rocket, built in 1829 (Fig. 264), with a modern type of locomotive (Fig. 265), shows the development that has taken place.

FIG. 264. The Rocket

313. The Steam Turbine is another kind of steam engine, in which expanding steam strikes directly upon curved blades in a wheel, causing it to rotate by impact or by reaction, just as the turbine water wheel does (§ 147). The principle of a simple turbine is illustrated in Fig. 266, which shows how steam is delivered to the blades of the wheel through four tubes. In each of these tubes there is a check valve which permits only the proper amount of steam to pass through. On escaping through this, the steam acquires a high velocity in expanding, and strikes upon the blades of the wheel with great force.

Another type of turbine is divided into stages; that is, there are several rotating wheels for each set of steam pipes.