522 SMITH'S INTERMEDIATE CHEMISTRY 



In preparing the plate, silver bromide is first precipitated in 

 water containing gelatine. The mixture is kept warm, to permit 

 the precipitate to become more sensitive to light by acquiring a 

 coarser grain (" ripen "). The " emulsion " is applied to plates of 

 glass or strips of transparent celluloid (films). 



The brief exposure of the plate to the image of the object, 

 well-focussed in the camera, produces no visible effect. But the 

 bromide is thereafter more easily reduced to metallic silver, in 

 proportion to the intensity of the light that fell upon each part. 



Development consists in applying a reducing agent, of such 

 slight activity that its effect during the process on non-illumi- 

 nated parts of the bromide is practically zero. Ferrous oxalate, 

 or an alkaline solution of pyrogallol C 6 H 3 (OH) 3 or of quinol 

 CeHXOH^ (two substances belonging to the class of phenols, 

 see p. 353) may be used. The reduction goes fastest and de- 

 posits most silver where the illumination was most intense. Thus, 

 the plate becomes most opaque where the object was brightest, 

 and vice versa. On account of this reversal, the plate is called 

 a negative. With the potassium salt of quinol, quinone CeHA 

 is formed : 



2AgBr + C 6 H 4 (OK) 2 - 2Ag + 2KBr + C 6 H 4 O 2 . 



The foregoing processes are all carried out in a faint red light, 

 which is almost without action on silver bromide. To prevent 

 the gradual reduction of the remaining, unchanged bromide to 

 silver by daylight, it is dissolved out by soaking the plate in so- 

 dium thiosulphate (fixing). The plate is now clear where no silver 

 was deposited. The negative is finally washed thoroughly to re- 

 move all except the gelatine and the silver image, and is then dried. 



In printing, the prepared paper is illuminated through the nega- 

 tive, and light and dark are again reversed. The denser parts of 

 the negative protect the paper below them, and leave these parts 

 white. On printing paper, silver chloride suspended in egg al- 

 bumen is che sensitive substance, and the silver is liberated in a 



