124 ELEMENTARY GENERAL SCIENCE CHAP. 



provide him with proofs of the statement that heat and work are 

 convertible. When a brake is applied to the wheels of a train, 

 as it stops at a station, it is a common thing to see sparks ily. 

 The resistance of friction which overcomes the motion of the 

 train causes a sufficient amount of heat to be developed to raise 

 the particles of steel, which get rubbed off, to a red heat. By 

 continually hammering a piece of iron on an anvil it can be made 

 too hot to hold in the hand. 



The following experiments show that heat appears when 

 motion is destroyed : 



EXPT. 115. Procure a piece of lead in the form of a sphere 

 (about the size of a marble) with a neck or hook upon which a 

 piece of string can be fastened. Tie a piece of string firmly 

 to the neck, and while holding the string strike the sphere 

 several times smartly on an iron plate. Test the temperature 

 of the ball before and after the experiment by a thermopile 

 and galvanometer. 1 



EXPT. 116. Hammer a piece of lead, or saw wood, and test 

 the temperature of the lead or saw before and after the experi- 

 ment. 



EXPT. 117. Rub a brass nail or button on a wooden seat, and 

 notice its increase of temperature. 



When we rub a lucifer match along a rough surface the heat 

 into which the work is converted is enough to ignite the match. 

 In all these cases mechanical work is converted into heat. The 

 converse is true also, heat is convertible into work. In the steam- 

 engine the heat of the furnace changes the water in the boiler 

 into steam. The steam forces the piston along the cylinder, and 

 this movement of the piston in a straight line is converted into 

 the circular motion of a fly-wheel ; or is used, through the inter- 

 vention of suitable mechanism, in pumping water or performing 

 some other kind of work. The steam which enters the cylinder 

 is hotter than that which leaves it for the condenser. Thus, 

 we see, part of the heat of the steam has been converted into 

 useful work and parts of it have been lost to the condenser, the 

 air, etc. 



We can show by a simple experiment that heat often dis- 

 appears when motion is produced. 



1 At this stage the thermopile and galvanometer must simply be regarded us i\ 

 delicate means of measuring temperature, 





