THE WOMBAT. 85 



Fixing. The nitrate of silver is removed by washing in 

 water, but the chloride of silver is dissolved out by means of 

 hypo. 



Ag CI + Na> S. 0: 5 = Na CI t Na Ag Si 5 ;— silver 

 chloride and hypo, give sodium chloride and a double hypo- 

 sulphite cf silver and sodium (soluble). But if you do not use 

 enough hypo, an insoluble salt is formed. 2 Ag CI + 3 Naa 

 Si O, = Ag 2 Na 4 (S> 3 ) 3 t 2 Na CI. 



The soluble sodio-silver hyposulphite is sensitive to light, 

 and if not entirely removed by washing, it will ultimately 

 decompose, liberating sulphur compounds, the blacks being at 

 the same time reduced. Fixing a plate is precisely the same 

 process, the bromide being dissolved out by the hypo, in the 

 same manner as the chloride. Cyanide of potassium may be 

 used for fixing plates. 



Ag Br t 2 K Cn = K Cn Ag Cn + K Br ; — bromide of 

 silver and cyanide of potassium give a double cyanide of 

 potassium and silver (soluble) and bromide of potassium. 

 But cyanide cannot be used for fixing paper, as it dissolves 

 the oxide of silver. 



Ag, O + 2 K Cn + Ha O = 2 Ag Cn + 2 K HO. 



HINTS ON LANTERN SLIDE MAKING. 



By H. Crisp. 



It often happens that reduced negatives of unmounted 

 photographs are required for lantern slide making, the copy 

 having to be returned as received, viz., unmounted. A simple 

 and speed}' method of preparing for copying in this case is to 

 well soak the print in clean water, and then place, face up, on 

 a sheet of glass and pad off the surface moisture, when the 

 glass containing the print may be fixed in any desired position, 

 and the exposure made. It will be found that the print will 

 adhere quite long enough for the necessary operations to be 

 carried out, and when finished with may be stripped off and 

 dried. 



A handy appliance to use when copying illustrations from 

 books, etc., is made by using an L shaped arrangement of 

 two pieces of wood, the long upright being, say, 6 inches by 1 

 inch by 4 feet, and containing a groove in which a quarter 

 plate camera may slide up and down and may be secured at 

 any desired spot ; and the short horizontal piece 12 inches by 

 12 inches by 1 inch, firmly attached to the upright, and acting 

 as a table on which to lay the book when used vertically, or 

 as a copying board to pin prints, etc., to, if used horizontally. 



