128 FUELS 



and should be kept clean and free from clogging. So long as 

 the supply of air is sufficient, the flame burns with a dull blue 

 color, but when the supply falls below that needed for com- 

 plete burning of the carbon, the blue color disappears, and a 

 yellow flame takes its place, and with the yellow flame a deposit 

 of soot on cooking utensils results. 



A comparison of the heating power of the different fuels. - 

 Whether our fuel be wood, coal, oil, or gas, it costs money, and 

 it is important for us to know which fuel yields the most 

 heat for the least money. It is not difficult to measure the 

 amount of heat given out by equal quantities of the different 

 fuels. A pound of wood, for instance, is burned in the bomb 

 calorimeter (page 68), and the heat which it yields is calcu- 

 lated from the increase in the temperature of the water which 

 surrounds it. Similarly a pound of coal is burned and its heat 

 value is calculated. To test the heating power of gas, a gas 

 jet is allowed to burn in the calorimeter until a pound of gas is 

 consumed. From the results of -such experiments we reach a 

 basis of comparison. 



QUANTITIES OF HEAT OBTAINED FROM VARIOUS FUELS FOR $i 



Bituminous coal @ $6 a ton gives 1,250,000 large calories 



Wood @ $8 a cord gives 937,000 large calories 



Petroleum @ $0.15 a gallon gives 240,000 large calories 



Gasoline @ $0.20 a gallon gives 175,000 large calories 



Gas @ $i looo cu. ft. gives 170,000 large calories 



If prices alone were considered, coal, yielding as it does the 

 greatest heat, would be chosen. But we have to consider not 

 only the actual cost of the fuel, but also the labor necessary to 

 care for a fire, the length of time the fire is needed, and various 

 other conditions. There is no hard and fast rule, and the wise 

 person will vary her household fuel according to circumstances. 



