Chemical Change and Energy 329 



FIG. 173. The silver salt on the paper remains white where it was shaded by 



the key. 



on the film or plate, and wherever there is a dark spot on 

 the object, there will be no change on the film or plate. 



As a matter of fact, the film or plate is exposed such 

 a short time that there is not time for the change to be 

 completed. So the photographer develops the negative ; 

 he washes it in some chemicals that finish the process 

 which the light started. 



If he exposed the whole plate to the light now, how- 

 ever, all the unchanged parts of the silver salt would also 

 be changed by the light, and there would be no picture 

 left. So before he lets any light shine on it, except red 

 light which has no effect on the silver salt, he dissolves 



