HEAT 



2748 



HEAT 



(2) Hot springs and volcanoes supply heat, 

 and are proofs of the belief that the interior 

 of the earth has a very high temperature. 



(3) Electric currents are another source of 

 heat. Whenever an electric current passes 

 through a substance, its temperature is raised. 

 The hottest furnaces in the world are the elec- 

 trical furnaces at Niagara Falls; these have a 

 temperature of about 7,000, or more than 

 twice the heat of boiling steel. Flames as hot 

 as these cannot be produced in a blast furnace 

 because they consume iron containers and tools 

 as though they were paper and reduce fire 

 brick to the consistency of putty. 



(4) Such mechanical actions as friction and 

 compression produce heat. An English scien- 

 tist, Sir Humphry Davy, in 1810, demon- 

 strated that two pieces of ice could be melted 

 simply by rubbing them together. It was then 

 that the idea first began to prevail that heat 

 could be produced by mechanical work. Water 

 can be heated by friction until it boils. A 

 piece of wood may be set on fire by friction. 

 Many a hunter or trapper has resorted to the 

 device savages employ of obtaining a flame 

 by twirling a pointed stick in the hollow of a 

 dry piece of wood. A piece of metal may be 

 heated by striking it or by rubbing it against 

 another surface. Every child has probably 

 heated a penny by rubbing it on a carpet or has 

 burned his skin sliding down a rope. 



The other mechanical means of producing 

 heat is by compression, and this is a well-known 

 heat effect. If a gas is compressed its tempera- 

 ture is raised. But here, just as in the heat- 

 effects produced by friction, work is required 

 to produce heat, and in each case the amount 

 of heat produced depends on the amount of 

 work done. 



(5) Many chemical actions, such as the com- 

 bustion (burning) of substances like carbon 

 and oxygen, produce heat. An ordinary flame 

 is the result of the combustion of these two 

 elements. It is the combustion of carbon and 

 oxygen that produces the heat of our bodies. 

 Carbon is one of the waste products of the 

 body, the result of the .breaking down of cells 

 which occurs with every movement. When 

 this carbon comes in contact with the oxygen 

 supplied by the lungs, heat is produced. This 

 is the reason that, if you work very hard and 

 fast, or run rapidly, you get warm. 



(6) Changes in the arrangement of the mole- 

 cules of which matter is composed will produce 

 heat. When water freezes, heat is given off. 

 A tub of water is sometimes placed in a con- 



servatory on a night when frost is expected, 

 because, as the water freezes, it gives up heat 

 energy to the surrounding room, or "takes the 

 frost out of the air," as the household doubt- 

 less expresses it. 



How Heat Travels. Energy or heat is trans- 

 ferred from the molecules of one body to those 

 of another in three ways: by convection, by 

 conduction and by radiation. 



Convection. In this process of heat trans- 

 ference the molecules of the body whose tem- 

 perature is raised move away to other places 

 in the body and thus tend to raise the tempera- 

 ture of the entire body. The molecules of 

 solids cannot 



move about from 

 place to place, so 

 convection is 

 limited to fluids. 

 If a vessel of 

 water, or any 

 other liquid, is 

 placed over a 

 fire, the portions 

 of the liquid near 

 the bottom be- 

 come hot and so 

 expand and be- 

 come lighter, or 

 less dense, than 

 the rest of the 

 liquid. This 

 causes them to 

 move upward to- 

 ward the top of 

 the liquid, and 

 thus the heat is 

 transferred. The 



BY CONVECTION 



When the water in the coils 

 within the stove is heated it 

 becomes less dense that is, 

 lighter ; this causes a current 

 within the tank, and eventu- 

 process of convec- ally all the water in it be- 



tion is of great comes heated ' 

 importance in the drafts in chimneys, ventila- 

 tion, in hot-air, hot-water and steam-heating 

 plants, and in the great phenomena of nature, 

 such as winds and ocean currents. When a fire 

 is started in a fireplace, the air in the chimney 

 becomes heated and begins to rise and the cold 

 air pushes in from below, forming the so-called 

 draft along the floor and up the chimney. You 

 can see why the draft is not very great when 

 the fire is being started and why it increases 

 in strength as the fire gets hotter. 



Conduction. In conduction the separate por- 

 tions of matter do not move away bodily, but 

 the vibrating molecules hitting against each 

 other pass the energy from one portion to 

 another. If a poker has one end in the fire, that 



\ 



