426 SMITH'S INTERMEDIATE CHEMISTRY 



Source^/ ihe^World^ Energy. The energy that does the 

 world's work comes mainly from two sources, namely, water power 

 and the combustion of wood, or of coal (which is fossil wood). 

 The water comes from vapor, generated by the sun's heat, con- 

 densed as rain, and collected in lakes or reservoirs. The source 

 of the energy of coal or wood is a little less obvious. When wood 

 (which is largely cellulose) burns, it gives carbon dioxide, water, 

 and heat. In fact, its combustion is represented by the equa- 

 tion given on p. 397, when the equation is read backwards. Thus 

 the sunlight, working through the machinery of the plant, takes 

 the carbon dioxide and water, furnishes the energy (as light), 

 and gives us wood and oxygen. And the wood and oxygen, when 

 burned, give us back the original substances, and the equivalent 

 of the original energy in the form of heat. Hence, our other 

 main source of energy turns out to be the same as the first the 

 sun's rays although the route by which the energy comes to us 

 is a little less direct. 



If, instead of burning the starch of the plant, we consume it as 

 food, it goes through a series of changes instead of only one. But 

 the end products are the same, namely, carbon dioxide and mois- 

 ture issuing from our lungs, and heat and other forms of energy 

 such as are developed in living organisms. Thus, whether we 

 use our muscles, a steam engine, or a waterwheel to do work, 

 sunlight is in each case the ultimate source of energy employed. 



Exercises. 1. In fermentation, why does not carbon dioxide 

 appear in bubbles at once? 



2. How do we ascertain that acetic acid in aqueous solution is 

 only slightly ionized? Give as many methods as possible. 



3. (a) Why are charcoal and coke smokeless fuels? (b) Ex- 

 plain why bituminous coal burns with flames while anthracite 

 does not. 



4. Point out the analogies between the processes used in making 

 coke and charcoal, and between their properties and uses. 



