COMPOSITION OF MATTER 31 



the peculiar property of giving out heat and always remaining 

 about five degrees warmer than its surroundings. The energy 

 of light may, like heat, effect chemical changes. Hydrogen 

 and chlorine, mixed in darkness, give no reaction, but a ray of 

 strong light will cause them to unite with explosive violence. 

 It is also the chemical energy of light that affects the silver 

 salts of photographic films and plates and causes the pictures to 

 appear. Light also causes the disintegration of hydrogen per- 

 oxide which accounts for its being kept in brown bottles and in 

 darkness. Energy of a similar nature causes our skin to tan in 

 strong sunlight. 



Practical Exercises 



1. Consult the table of chemical elements and make a list of all of 

 those with which you are familiar. 



2. Make a list of all the uncombined chemical elements that you can 

 find in the school room. 



3. Write after the names in the following list the correct chemical 

 symbols : 



Iodine Potassium Iron 



Carbon Oxygen Zinc 



Mercury Gold Chlorine 



Tungsten Sodium Calcium 



4. Write after the names of the following, the chemical elements of 

 which they are composed : 



Limestone (CaCO 3 ) Washing soda (Na 2 CO 8 ) 



Table salt (NaCl) Calcium chloride (CaCl 2 ) 



Sulphuric Acid (H 2 SO 4 ) 



6. Thoroughly mix one-quarter of a teaspoonful of powdered zinc and 

 an equal amount of powdered sulphur. Place these on a piece of asbestos 

 paper and apply heat from above by means of a bunsen burner held at 



