Fire and Heat 59 



and the trackless air. They are the means by which the pres- 

 ent world of art and achievement has been created. 



29. Sources of heat. The directly available heat energy 

 of the sun is far from being sufficient for all the needs and 

 purposes of man. In cold and damp weather men must for 

 the sake of health and comfort supplement the sun's heat with 

 that produced from fuel. Hence they have had to find fuel. 



Exercise. What are the chief fuels in common use ? What is the 

 relative abundance of each? Its source of supply? What are the 

 advantages, such as cleanliness, convenience, economy, etc., attend- 

 ing its use? What does the cost to the consumer indicate as to its 

 abundance? How does the cost of transportation affect the selling 

 price of fuel? What other reasons can you find for the use of any 

 particular fuel, such as artificial gas, wood, oil, gasoline, alcohol, coal, 

 and the like? 



The fuels with which you are familiar may be altogether unknown 

 to some of the earth's inhabitants. Explorers report leaving coal 

 with entire safety among certain tribes of the Eskimos, who call it 

 " black stone " and do not know how to burn it. You may find in 

 books of travel accounts of other people who are unfamiliar with 

 various kinds of fuel. You may never have seen the kind of fuel used 

 in Ireland, and found in certain places in our country. It is called 

 peat (page 455). The inexperienced would be puzzled to make a 

 fire with it. What kind of fuel do the Chinese make use of in the 

 "hand warmers"? What other unusual fuels are used in foreign 

 lands ? 



Other sources of heat. Heat may be produced in varying 

 amounts by other means than by burning fuel. If you take a 

 small amount of borax in your hand and wet it with water, 

 you will feel the heat released by the action of the borax and 

 water upon each other. Quicklime when slaked with water 

 yields considerable heat (see page 418). If sulfuric acid is 

 mixed with water, heat is developed in a similar way. There 

 are other changes of the same sort by which heat is produced. 



All of the ways mentioned, together with the burning of fuel, 

 are described by scientists as chemical changes, for they are 

 brought about by combining two or more different substances 



