172 OUTLINE OF CHEMICAL STUDY 



upon that there is some change in the kind of matter, a 

 chemical change is said to take place. 



Experiment 107. Dissolve some common salt in water until 

 no more can be taken up. Has any change occurred ? Now boil 

 the salt water to dry ness. Does anything remain ? Was this a 

 physical or a chemical change ? 



Experiment 108. Place a bit of sulphur in an old spoon and 

 heat it over an alcohol lamp or gas burner. The sulphur melts 

 and, if still heated, vaporizes. Hold a saucer just above the 

 spoon ; sulphur collects on the saucer in tiny particles. What 

 sort of change ? 



Now burn a bit of sulphur in the spoon, holding the saucer 

 above it as before. No. sulphur collects on the dish. In burning, 

 the sulphur unites with oxygen from the air, forming a different 

 substance, that passes off as a gas. Is this a physical or a chemi- 

 cal change ? 



196. Composition and Decomposition. In a general 

 way, any chemical change falls under one of two classes, 

 composition and decomposition. Composition is the 

 process of uniting two or more substances to form an- 

 other. When a substance is broken up into the two or 

 more substances that compose it, the process is called 

 decomposition, and the body is said to be decomposed. 



197. Kinds of Substances We do not need to be 



told that there is an almost countless number of differ- 

 ent substances on earth. Many of these we know can 

 be made from simple substances, by processes which 

 man has devised. Others are found in the rock and 

 soil of the earth, having been made by natural processes 

 long ages ago. And a still larger number, perhaps, are 

 made by the growth and action of living matter, plant 



