CHAPTER XXI 

 STEAM AND GAS ENGINES 



138. Steam is water vapor or water in gas form which 

 has a temperature of 100 C. at standard pressure. When 

 water changes to steam it increases in volume about 

 i ,600 times, that is, one gallon of water will make 1,600 

 gallons of steam and fill a space of 214 cubic feet at 

 standard pressure. The molecules of steam are very 

 active and move about with high speed. The speed of 

 the molecules varies with the temperature. To change 

 water at boiling point to steam requires 536 calories for 

 each gram of water, and the steam has the same tempera- 

 ture as the boiling water. These 536 calories of heat are 

 used to make the molecules move faster. The faster 

 they move the more space a given number of them re- 

 quires and the harder they strike against the walls of 

 the vessel enclosing them. Since the molecules of steam 

 move at such a high speed they will rush with great 

 force through any valve or opening that is made in the 

 containing vessel. The energy of the steam is derived 

 from the wood or coal which is oxidized to make heat. 



139. The Steam Engine. More than 150 years ago 

 James Watt, an instrument maker living in England, 

 studied the crude engines in use in his day and invented 

 the double-action steam engine, which was the same in 

 principle as the steam engines of the present time. Most 

 of the improvements that have been made on Watt's in- 

 vention have been on the machinery to which the engine 



