122 MODERN SCIENCE READER 



producer so it may be learned not always to regulate 

 combustion at the chimney damper, but to keep this open 

 sufficiently to pull in all the air we need as a maximum 

 above the fire, and to regulate the combustion by combined 

 movements of the door grids and ashpit dampers. 



Safety valves are locked up from tampering; why not 

 also lock the chimney damper ? It should be locked, for it 

 is not fit to be used as a regulator of the combustion of 

 bituminous coal, for this is a double process, the coal burn- 

 ing partially as solid fuel on the grate and partly as gas 

 above the fire, and each operation requires separate and 

 yet conjoint air regulation. 



Ordinary coal has a calorific capacity of about 14,000 

 B.t.u. per pound. The volatile matters distilled from it 

 have a capacity of 18,000 to 24,000 B.t.u. The extra 4,000 

 to 10,000 heat units they now possess are borrowed from 

 the heat of combustion of the solid fuel on the grate, and 

 when the green gas is wasted unburned it is carrying with 

 it the latent heat of distillation. Assuming 20,000 as its 

 average heat value and assuming one third of the coal to be 

 volatile, the green gases carry off nearly half the heat value 

 of the coal. 



Though the molecular structure of coal may not be dis- 

 coverable, there can be no doubt as to the results of the 

 systems of combination ordinarily adopted. If fired on the 

 coking system, the gas is driven off more or less steadily and 

 continuously, and places less of a tax on the surface at any 

 one moment in respect of maximum air supply above the 

 fuel to burn the gas than is levied when fresh coal is spread 

 heavily over a fire at more or less wide intervals of time. 



In solid fuel the carbon has not changed its state, but 

 any hydrogen has been somehow rendered solid by its 

 combination with carbon. 



The gaseous hydrocarbons become liquids when their 

 molecular weight gets up to about seventy to eighty, and 

 solids begin to appear when the molecular weight reaches 

 128 or 136. 



