CHAPTER III. 



HEAT: ITS SOURCES, EFFECTS AND MODES OF TRANS- 

 MISSION. 



HEAT and the different forms of attraction are called physical 

 agents, forces, or forms of energy. They are known to us chiefly 

 by their effects. 



Heat was once called an element ; later it was regarded as a 

 subtile form of matter contained in bodies, and manifesting itself 

 with more or less intensity. Now it is thought to be the vibra- 

 tory motion of the molecules of bodies, which may be increased 

 or diminished in various ways. When this motion is increased 

 the body becomes hot, the molecules are separated and the body 

 expands. When the motion is diminished the body grows cold, 

 the molecules approach each other and the body contracts. 

 Concerning the ultimate source of this motion which affects us as 

 heat, science as yet has no suggestions to make. 



The process by which the blacksmith sets the tire of a wheel 

 shows that heat causes iron to expand and that when the heat 

 is removed or its intensity lessened, the iron contracts. Lay a 

 rod of iron or copper, ten or twelve inches long, on a board, and 

 drive a nail at each end so that the rod just touches the nails ; 

 then place the rod in the fire and when red-hot test its length 

 by the nails. A few experiments with different solids will show 

 that heat causes them to expand and that some expand more 

 than others. Purchase from the druggist an alcohol lamp, 

 some test tubes fitted with corks, and a few pieces of nar- 

 row glass tubing, and have holes cut in some of the corks of 

 such size that the glass tubing will fit them closely. If a lamp 

 seems too costly, one can be made with little trouble. Get a 

 bottle holding from two to four ounces, with a wide mouth, fit it 

 with a cork, bore the cork for a small tube made of tin, or the 

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