Chemical Change and Energy 331 



FIGS. 174 and 175. Where the negative is dark, the print is 



light. 



down. Wash for a couple of minutes. Turn it over and 

 examine it. If it has been exposed to the light too long, it 

 will be dark ; if it has been exposed too short a time, it will 

 be too light; in either case, if the print is not clear, repeat 

 with a fresh piece of blueprint paper, altering the time of 

 exposure to the sunlight to improve the print. 



You can make pretty outline pictures of leaves and pressed 

 flowers, or of lace, by laying these on the blueprint paper in 

 place of the negative and in other respects doing as directed 

 above. 



In making blueprints you are changing an iron salt 

 instead of a silver salt, by the action of light. Regular 

 photographic prints are usually made on paper treated 

 with a silver salt rather than with iron salt, and some- 

 times a gold or platinum salt is used. But these other 

 salts have to be washed off with chemicals since they 

 do not come off in water, as the unchanged part of the 

 iron salt comes off when you fix the blueprint paper in 

 the water bath. 



