130 THE BOOK OF THE LANTERN. 



portion of the silver solution at a time, until all of it is 

 transferred to the earthenware pot. Wash out the silver 

 glass with half an ounce of water and add that too. The 

 newly-formed emulsion should now look like cream. Place 

 the cover on the jar, and put it back once more in the 

 warm water (without any flame beneath it) for fifteen 

 minutes. At the end of that time pour it out into a dish 

 or plate to set, and cover it over so that neither light nor 

 dust can trouble it. This will complete the first stage of 

 plate-making. 



In a few hours, according to the general temperature, 

 the emulsion will have set into a firm jelly. It must 

 now be washed to get rid of certain extraneous matter 

 which has been formed and which is not wanted. The 

 silver nitrate has combined with the bromide of potassium 

 to- form silver bromide, the required salt, which is sensi- 

 tive to light, but at the same time nitrate of potassium 

 (saltpetre) has also been formed, and must be eliminated. 

 As this latter is soluble in water, while the silver bromide 

 is insoluble, the matter is not a difficult one to accomplish. 

 By dividing the jelly into shreds, and putting it into 

 several changes of water, this universal solvent gets to 

 every side of it, so to speak, and the saltpetre is quickly 

 got rid of. Scrape up the jelly with a silver spoon, or a 

 slip of glass, and place it in the middle of the square of 

 canvas, which has been previously wrung out in water. 

 Gather up the ends so that the jelly forms a ball confined 

 within the canvas. Now place in a large pan of water, 

 and squeeze and twist the canvas with the hands (under 

 water) so that the jelly is forced through the meshes of the 



