44 POWER AND THE PLOW 



earlier, and in a steam boiler under considerable pressure, 

 the boiling point will be higher. We have seen that it requires 

 one B.t.u. to raise the temperature of one pound of water one 

 degree F. However, to overcome the cohesion of the water 

 particles and turn one pound of water into steam, requires 

 the application of approximately 967 B.t.u. In evaporating 

 water under pressure, not only this internal resistance, or 

 latent heat, of the water itself must be overcome, but also 

 the outside pressure upon it. With 100 pounds of pressure 

 it requires a temperature of 337 F. to convert water into steam. 

 During the evaporation of water into steam, the temperature 

 of the steam will not rise above that of the water, but if heat 

 be applied to the steam away from the water, it will rise in 

 temperature and become a superheated, invisible gas. So 

 long as it remains in contact with water, steam will carry some 

 water in suspension, hence to produce absolutely dry steam, 

 without superheating, is next to impossible under the con- 

 ditions surrounding the ordinary boiler. The amount thus 

 carried is seldom less than 2 per cent., but anything over 3 

 per cent, is regarded as objectionable. 



COMBUSTION 



Fuels used in the steam engine contain a high percentage 

 of carbon, varying with the material, and amounts of hydrogen, 

 sulphur, nitrogen, oxygen, moisture, and the mineral elements 

 which make up the ash. Air is a fairly constant mixture, 

 composed of about four parts of nitrogen, one of oxygen, and 

 traces of carbonic acid gas, (or carbon dioxide), water, nitric 

 acid and ammonia. At a given temperature, depending on 

 a number of factors, the free oxygen of the air will unite with 

 the free carbon and other combustible elements to form the 

 gases of combustion. Heat is thrown off as the union takes 

 place, and the more perfect the combustion, the greater the 

 heat produced. In an insufficient supply of air, for instance, 

 a pound of carbon burns to carbon monoxide (CO) instead of 



