382 GENERAL SCIENCE 



Project. Have one member of the class explain how pictures are taL 

 negative, positive, developing, and printing. 



Experiment to Show the Chemical Action of Light. 



paper is often used by architects and draftsmen for printing diagrams. 

 With a camel's hair brush carefully paint a piece of glazed or shiny 

 paper with the solution given on this page. Do this in the dark or 

 before a red light. Place the paper in a drawer or some dark place 

 to dry. 



Place a piece of some opaque object over the blueprint paper 

 after it has dried in a dark room, and expose the paper to the sunlight 

 for a minute. Wash thoroughly in water. An image will form on the 

 paper. Be sure that the paper is thoroughly washed so that no 

 yellow stains are left. Try some of the paper with a negative 

 which you may have at home. 



Blueprint Solution. 



Solution A: Ferric ammonium citrate (brown), 80 grams; water, 1 ounce. 



Solution B: Potassium ferricyanide, 60 grams; water, 1 ounce. Do not 

 mix until ready to use. Keep in the dark; mix equal portions of A and B; filter 

 before using. 



Another solution may be made by: 



Solution A: Ferric ammonium citrate (green), 110 grams; water, 1 ounce. 



Solution B: Potassium ferricyanide, 40 grams; water, 1 ounce. Mix and 

 use as above. 



Be sure not to use potassium ferrocyanide. The potassium ferricyanide 

 should be in clear ruby red crystals; if otherwise, rinse with water and dry 

 between blotting papers before weighing. 



The solutions may be made more sensitive by adding a few drops of oxalic 

 acid. The solutions also keep better if one-half gram per ounce of potassium 

 bichromate is added to the mixed solution. 



Chemical Action of Light in Nature. It is believed that the ultra- 

 violet ray is the agent which changes water and carbon dioxide into 

 starch in the leaves of plants. Probably the coat of tan so often 

 obtained at the seashore is due to the ultra-violet ray. Physicians 

 are treating some diseases with this ray. 



Light is the most powerful germicide known, 



