PHOTOGRAPHIC PROCESSES. 81 



should be allowed to reach them. The dark slide 

 should be carefully examined, and if of the double 

 dry form, two sheets of zinc the same size as the 

 plate, each covered with black velvet on one side and 

 having a spring between them, should be placed 

 with the velvet side next the back of the plates, in 

 the dark slide. 



The following formula for the preparation of dry 

 plates, having given almost perfect results; our 

 opinion as to their purchase, has been considerably 

 modified since the publication of the last edition. 

 Its production is due to Mr. Hartley of Chicago, and 

 is here inserted in almost his own words. 



FORMULA. 



Take any hard gelatine Swiss or Heinrich's 

 soak it for twelve hours in water, changing the 

 water three times during the twelve hours. Do not 

 cut it up. Take it out of the water and lay it on 

 clean paper to dry. No matter how much of this 

 you fix in this way, as when dry it will keep as 

 before. 



Take 75 grains of carbonate of soda (not bicar- 

 bonate) and 60 grains of citric acid, and put into 

 three ounces of warm water in a quart pitcher; 

 when the citric acid and carbonate of soda are dis- 

 solved and all effervescence ceases and carbonic acid 

 gas has passed off, add 16 ounces cold water and 720 

 grains of the soaked and dried gelatine and let it 

 stand thirty minutes ; now dissolve 720 grains of 



G 



