332 Common Science 



Since the light cannot get through the black part of 

 a negative, the coating on the paper behind that part 

 is not affected and it stays light colored ; and since the 

 light can get through the clear parts of the negative, the 

 coating on the paper back of those parts is affected 

 and becomes dark. Therefore, the print is " right side 

 out," there is a light place on the print for every white 

 place on the object photographed, and there is a dark 

 place on the print for every black place on the object. 



Moving-picture films are printed from one film to 

 another, just as you printed from a negative to a piece 

 of paper. The negative is taken on one film, then this 

 is printed on another film. The second film is " right 

 side out." 



Light and the manufacture of food in plants. Much 

 the most important chemical effect of light, however, 

 is not in making photographs, in bleaching things, 

 or in " burning " your skin. It is in the putting 

 together of carbon and water to make sugar in plants. 

 Plants get water (H 2 0) from the earth and carbon 

 dioxid (C02) from the air. When the sun shines on 

 chlorophyll, the green substance in plants, the chloro- 

 phyll puts them together and makes sugar. The plant 

 changes this sugar into starch and other foods, and into 

 the tissues of the plant itself. Nothing in the world can 

 put carbon dioxid and water together and make food 

 out of them except certain bacteria and the chlorophyll 

 of plants. And light is absolutely necessary for this 

 chemical action. Try this experiment : 



Experiment 102. Pin together two pieces of cork on op- 

 posite sides of a leaf that is exposed to the sun. The next 



