Chemical Change and Energy 327 



cabinet or closet. The. next day compare the two cloths. 

 Which has kept its color the better ? If the difference is not 

 marked, repeat the experiment for 2 or 3 days in succession, 

 putting the same cloth, wet, in the sun each time. 



Bleaching is usually a very slow kind of burning. It 

 is the dye that is oxidized (burned), not the cloth. Most 

 dyes will combine with the oxygen in the air if they are 

 exposed to the sunlight. The dampness quickens the 

 action. 



Why some people freckle in the sun. When the sun- 

 light falls for a long time on the skin, it often causes 

 the cells in the under part of the skin to produce some 

 dark coloring matter, or pigment. This dark pigment 

 shows through the outer layer of skin, and we call the 

 little spots of it freckles. Some people are born with 

 these pigment spots ; but when the freckles come out 

 from long exposure to the sunlight, they are an example 

 right in our own skins of chemical change caused by the 

 action of light. Tan also is due to pigment in the skin 

 and is caused by light. 



The next experiments with their explanations will 

 show you how cameras can take pictures. If you are 

 not interested in knowing how photographs are made, 

 do the experiments and skip the explanations down to 

 the middle of page 332. 



Experiment 100. Dissolve a small crystal of silver nitrate 

 (AgNOa) in about half an inch of pure water in the bottom 

 of a test tube. Distilled water is best for this purpose. 

 Now add one drop of hydrochloric acid (HC1). The white 

 powder formed is a silver salt, called silver chlorid (AgCl) ; 

 the rest of the liquid is now a diluted nitric acid (HNOs). 



Pour the suspension of silver chlorid (AgCl) on a piece 



