122 INTRODUCTION TO GENERAL SCIENCE 



and examine in twenty-four hours. Repeat with barium 

 sulphate and whiting. 



Those who wish to make an exact test of paint may use the 

 following method : 



To a small amount of the paint add ten times its volume of 

 gasoline and shake well. Allow solids to settle, pour off the 

 liquid, and repeat twice. Collect ail of the liquid and allow 

 it to evaporate, or heat it over water. Look out for fire. 

 After cooling the resulting oily residue, add a few drops of 

 concentrated sulphuric acid. Brown rings indicate linseed 

 oil. Fish oil will not give the same test, and it may also be 

 detected by its odor. 



Evaporate to dryness the solids which were left, and add 

 acetic acid. Both lead and zinc will dissolve. A residue indi- 

 cates clay or barium sulphate (barites). Pass hydrogen sul- 

 phide (made by the action of hydrochloric acid upon ferrous 

 sulphide in a hydrogen generator) into the clear solution. 

 A black precipitate indicates lead. Filter, if there is a 

 precipitate, and add ammonium hydrate. A white precipi- 

 tate indicates zinc. 



89. CARBON DIOXIDE 



Carbon dioxide, although existing usually in an amount 

 which varies from three parts in ten thousand to eight parts in 

 ten thousand, nevertheless possesses thousands of tons of 

 weight. The use of carbon dioxide is the maintenance of 

 vegetable life. Most of the material contained in trees and 

 coal once existed as carbon dioxide in the atmosphere. Under 

 the effect of sunlight, the green coloring matter in the leaves, 

 called chlorophyll, has the power of absorbing the carbon 

 dioxide, retaining the carbon, and giving forth the oxygen. 



