Cooling by solution of a solid occurs because of the work which has to be done in pulling apart the molecules of the solid crystals. Molecular velocity is lost; that is, heat disappears in doing this work.

The freezing point of salt water is lower than that of fresh water because the attraction of the salt molecules for the water molecules tends to keep the latter from arranging themselves into crystals of pure ice. At a lower temperature (smaller energy of agitation) the cohesive forces get the mastery and form the pure ice.

A mixture of salt and ice becomes cold because, on account of the low freezing point of salt water (temperature of equilibrium between salt water and ice) the ice begins to melt when brought in contact with the salt and the 80 calories per gram necessary for this melting is taken from the mixture.

QUESTIONS AND PROBLEMS

1. After the water in an open vessel first begins to boil, a long time elapses before it is all boiled away. Explain.

2. What is the meaning of the statement that the heat of vaporization of liquid ammonia at 5° C. is 314 calories per gram?

3. After water has been brought to a boil, will eggs become hard any sooner when the flame is high than when it is low?

4. The hot water which leaves a steam radiator may be as hot as the steam which entered it. How, then, has the room been warmed?

5. Why are burns caused by steam so much more severe than burns caused by hot water of the same temperature?

6. In a certain radiator 2 kg. of steam at 100° C. condensed to water in 1 hr., and the water left the radiator at 90° C. How many calories were given to the room during the hour?

7. How many calories are required to convert 10 g. of ice at 0° C. into steam at 100° C.?

8. How many grams of steam at 100° C. must be condensed in 1 kg. of snow at 0° C. to convert the snow into water at 0° C.?

9. How may we obtain pure drinking water from sea water?

10. Explain why salt is thrown on icy sidewalks on cold winter days.

11. If pieces of ice are dropped into water maintained at 0° C., will the ice melt? Why? If a saturated brine solution has a temperature of 0° C. and ice is dropped into it, what will happen to the ice? to the temperature of the brine solution?

12. Give two reasons why an ocean freezes less easily than a lake.

13. When the salt in an ice-cream freezer unites with the ice to form brine, about how many calories of heat are used for each gram of ice melted? Where does it come from? If the freezing point of the salt solution were the same as that of the cream, would the cream freeze?

14. How many kilograms of ice can be formed from water at 0° C. by taking from the water enough heat to vaporize 10 kg. of ammonia? (Consider the heat of vaporization of liquid ammonia to be 314 calories per gram.)

STEAM ENGINES

234. The modern steam engine. Thus far in our study of the transformations of energy we have considered only cases in which mechanical energy was transformed into heat energy. In all heat engines we have examples of exactly the reverse operation, namely, the transformation of heat energy back into mechanical energy. How this is done may best be understood from a study of various modern forms of heat engines. The invention of the form of the steam engine which is now in use is due to James Watt (see opposite page 133), who, at the time of the invention (1768), was an instrument maker in the University of Glasgow.

The operation of such a machine can best be understood from the ideal diagram shown in Fig. 176. Steam generated in the boiler by the fire passes through the pipe S into the steam chest V, and thence through the passage N into the cylinder C, where its pressure forces the piston P to the left. It will be seen from the figure that as the driving rod R moves toward the left the so-called eccentric rod-R', which controls the valve V, moves toward the right. Hence, when the piston has reached the left end of its stroke, the passage N will have

been closed, while the passage M will begin to admit steam, thus throwing the pressure from the right to the left side of the piston, and at the same time putting the right end of the cylinder, which is full of spent steam, into connection with the exhaust pipe E. This operation goes on continually, the rod R' opening and closing the passages M and N at just the proper moments to keep the piston moving back and forth

throughout the length of the cylinder. In the actual engine the valve V is so constructed and adjusted that the supply of steam from the boiler is cut off after the piston has made a part of its full stroke, the rest of the stroke being completed under the diminishing expansive pressure of the steam within the cylinder. The shaft carries a heavy flywheel W, the great inertia of which insures constancy in speed. The motion of the shaft is communicated to any desired machinery by means of a belt which passes over the wheel W. Within the boiler the steam is at high pressure and high temperature (§ 225). The steam falls in temperature within the cylinder while doing the work of pushing the piston. A steam engine

THE ROTOR OF A STEAM TURBINE

The great rotor of a 60,000-kilowatt steam turbine,- the latest and most efficient form of steam engine. The figure shows the enormous number of movable blades against which the steam impinges after being directed upon them at the most suitable angle by the stationary blades (half being shown below the rotor), which are arranged in rows alternating with the movable ones. The turbine can take in steam at a somewhat higher temperature and pressure than most steam engines and reject it at a somewhat lower temperature. This, in accordance with Carnot's principle, is what determines the efficiency. Experiments are now being made with turbines, using steam at 1200 pounds' pressure. Steam-turbine rotors revolve with a speed of from 200 to 30,000 R.P.M. The peripheral speed of the large wheel shown above is 12 miles per minute, or over 1000 feet per second. This turbine consumes 9650 pounds of steam per minute. (Courtesy of the General Electric Company)