16 EXPERIMENTAL GENERAL SCIENCE 



amorphous. Some are transparent and allow light to pass 

 through them; others are opaque and stop the light. All kinds 

 of matter are said to be impenetrable in that they take up a 

 certain amount of space to the exclusion of everything else, 

 and they are also indestructible that is, they cannot be 

 destroyed. It is true that we may appear to destroy matter, 

 as when we burn a block of wood or explode gunpowder, but no 

 matter is really destroyed. In the instances mentioned, mat- 

 ter has merely changed its composition and now exists as gas or 

 ashes, or both. 



22. Physical and Chemical Change. The changes which 

 occur in matter may affect either its form or its composition. 

 When the change does not affect the composition of matter, as 

 when iron is melted or water is turned to steam, it is called a 

 physical change. The science that deals with such changes is 

 called physics. When the composition of matter is changed, 

 as when wood burns, iron rusts, or quicklime slacks, it is called 

 a chemical change. Such changes are studied in chemistry. In 

 physical changes the original substance may usually be recov- 

 ered again by reversing the process by which it was changed. 

 Thus, if ice be heated until it becomes water, it may be turned 

 to ice again by withdrawing the heat. In chemical changes, 

 one or more new substances are formed and the original sub- 

 stance is not usually to be obtained again by reversing the 

 process. Wood once burned cannot be had as wood again 

 until the plant has built it up once more from the gases and 

 ashes into which it was turned by burning. 



Practical Exercises 



1. Why does one have difficulty in pouring a liquid into a bottle 

 through a close-fitting funnel? 



2. Dissolve a single crystal of potassium permanganate in a test-tube 

 of water. What gives the color to the water? 



