THERMODYNAMICS. 359 



this impact of the planetary projectile would equal that derivable 

 from the combustion of 5600 globes of coal each equal to the earth 

 in size. This is the measure of the potential energy of the earth 

 considered as a mass separated from the sun. 



568. Chemical Affinity. We have already seen 

 that there are forces in nature compared with which the 

 force of gravity is insignificant. (Read carefully the first 

 paragraph, in this chapter.) When coal is burned, the 

 carbon and oxygen particles rush together with tremendous 

 violence, energy of position being converted into energy of 

 motion. The molecular motions produced by this clashing 

 of particles constitute heat and have a mechanical value. 



569. Heating Powers. If a pound of carbon be 

 burned, the heat of the combustion would raise about 

 8000 pounds of water 1 C. In like manner, the combus- 

 tion of a pound of hydrogen would yield about 34000 heat 

 units (pound-centigrade). 



(a.) The following table shows the heating powers of several 

 substances when burned in oxygen : 



Hydrogen 34,462 



Marsh gas (CH 4 ) 13,063 



Petroleum 12,300 



Carbon 8,080 



Alcohol (C 3 H 6 0) 6,850 



Phosphorus 5,747 



Carbon protoxide (CO) 2,403 



Sulphur 2,220 



(ft.) The calorific powers mentioned above maybe adapted to Fah- 

 renheit degrees by multiplying them respectively by |. As they 

 stand, the numbers represent the number of times its own weight 

 of water that could be warmed 1C. by burning the substance in 

 oxygen. 



57O. The Steam -Engine. The steam-engine is a 

 machine for utilizing the tension of steam. Its essential 

 parts are a boiler for the generation of steam, and a cylinder 

 for the application of the tension to a piston. 



