16 Our Surroundings 



form. Since his day, many experiments have tended to confirm 

 the truth of this discovery, and now it is universally admitted that 

 matter cannot actually be destroyed. This fact is known as the 

 conservation of matter. The law may be stated as follows: 

 "Whenever a change in the composition of a substance takes 

 place, the amount of matter after the change is the same as before 

 the change/' The changes in matter which you see every day, 

 such as the burning 1 of wood or coal, in no way increase or de- 

 crease the amount of matter in the world. 



The most striking characteristic of matter is that it is con- 

 stantly subject to change in appearance and in composition. This 

 change may be rapid, as when the foundations of a building give 

 way, or slow, as when a body breaks up piece by piece, like the 

 wearing out of the floors on which we walk. These are examples of 

 physical changes, because the material is not changed. The fine 

 particles that are torn away are the same material. The wood that 

 disappears from the floor in the form of very fine particles still 

 remains wood. 



There are changes, however, which affect the nature of the 

 material. These changes, also, may be very rapid, as in the case 

 of a burning building, or slow, as in the case of gradual decay. 

 These are examples of chemical changes because the material is 

 actually changed to form new substances. When wood is merely 

 broken it is still wood, but when it is burned it is no longer 

 wood. All living matter is especially liable to chemical change. 



Forms of Energy. Any change is always the result of the 

 action of some force, and every force is dependent on some form 

 of energy. Energy is the ability to do work, or to cause something 

 to happen. Like matter, it exists in different forms. Thus, for 

 example, it may appear in the forms of motion, heat, light, or 

 electricity. Any one of these forms of energy may be changed 

 into other forms, but no energy is either created or destroyed. 

 This fact is known as the conservation of energy. It was one 

 of the great discoveries of the nineteenth century, and is con- 

 stantly applied in various ways. 



The form of energy which is most apparent to us is the energy 

 of motion, as shown in a speeding train, a moving football, a 



