BUILDING MATERIALS 23 



of this knowledge from a course in general science, but we must 

 now review some of the elementary facts in order to answer the 

 problems which follow. 



Self-Testing Exercise 



A tree has (1), (2), (3), and green 



(4). The (5) serve to anchor the tree in the ground 



and take in (6). The stem holds to the light the 



(7), which are the (8) (9). The tree 



takes (10) materials from its environment and makes them 



into (11) and (12) matter. 



PROBLEM H. WHAT ARE THE BUILDING MATERIALS OF 

 THE WORLD AND HOW ARE THEY USED? 



Matter. Matter and energy are the fundamental things in the 

 world. Matter is anything which has weight or occupies space. 

 The tree is made of matter, as is the surrounding soil, the water, 

 and even the air which it uses. Matter is usually present in one 

 of three forms, a gas, a solid, or a liquid ; and, as we know, is capable 

 of being changed from one form to another. For example, a 

 liquid, water, may be changed into a vapor, steam, by heating, or 

 into a solid, ice, by freezing. In biology, matter is usually thought 

 of as being of two sorts, organic, or that which comes from living 

 things, and inorganic, or the material that never has been alive. 



Energy. When a tree grows, or the roots push their way 

 through the soil, or take in water, energy is being exerted. 

 Energy means the power or ability to do work. There are five 

 kinds of energy : mechanical, electrical, chemical, heat, and light 

 energy. To perform its work the tree uses light energy, chemi- 

 cal energy, and heat energy, which it may change into mechanical 

 energy. Any one form of energy may be changed into another 

 form. We may observe such a change when we strike a nail with 

 a hammer and discover that the nail becomes hot. Our mechani- 

 cal energy has turned into heat energy. 



Demonstration 1. Show some elements, as carbon, iron, phosphorus, 

 and sulphur. 



