84 THE WOMBAT. 



colour is not affected, as the oxide is insoluble in these 

 re-agents. This is why a reddish print loses less in fixing - 

 than a purple one. The nitrate in the paper assists also in 

 the reduction of the chloride. 



Toning. You all know that the object of toning is to get 

 rid of the unpleasant reddish tint by substituting for it the 

 pleasant purple tint of gold. It is found that if chloride of 

 gold (Au CI3 ) is dissolved in a neutral or slightly alkaline 

 solution, and brought into contact with the reduced silver in 

 the print, the gold is deposited and chlorine is liberated. 

 This chlorine must unite with something, and if nothing else 

 be present it will act on the reduced silver chloride and form 

 once more chloride of silver, which will be dissolved out in the 

 hypo. An absorbent for the chlorine must therefore be added 

 to the solution of gold, and for this purpose acetate of soda or 

 ammonium sulphocyanide is used. We get the equation : — 

 2 AUCI3 + NaC, H 3 0>=Au 2 + Na G> Cl 3 O. + 3 HC1. 

 chloride of gold plus acetate of soda give purple gold plus 

 sodium trichloracetate plus hydrochloric acid. 



A. The nitrate of silver present in the paper will form 

 a rather insoluble double salt with the acetate of sodium, and 

 must therefore be removed by liberal washing or decomposed 

 with chloride of sodium (common salt) or ammonium chloride. 

 The latter is the principle involved in " pretoning." 



B. Ag NO ;i + NHi CU Ag CI > NH» N0 3 . 



It will be noticed that hydrochloric acid is liberated during 

 toning ; if the paper be transferred direct to the fixing bath 

 there is a slight elimination of sulphurous acid, which 

 immediately splits into sulphurous acid and sulphur. 



Na 2 S> O3 + 2HCU Ho SO3 + 2Na CI +S;— Hyposulphite 

 of soda and hydrochloric acid give sulphurous acid, sodium 

 chloride and sulphur. This deposit of sulphur will cause 

 yellowing, and therefore the print must be well washed be- 

 tween toning and fixing, and also if a trace of nitrate of silver 

 is allowed to remain in the print when it is put into the fixing; 

 bath, an insoluble hyposulphite of silver is formed. 



2 Ag NO3 + Na. S» 0:i = Ag, S> 5 i 2 Na NO3 ;— 

 nitrate of silver and hyposulphite of soda give hyposulphite of 

 silver and nitrate of soda. 



This hyposulphite being insoluble, it remains in the print,. 

 and though not perceptible at first, being white, it is ulti- 

 mately decomposed by the moisture of the air. Ag> Si O3 + 

 H-_> O = Ag> S I H> S O4 ; — hyposulphite of silver and water 

 give black sulphide of silver and sulphuric acid. This trace 

 of black sulphide of silver gives the picture a yellow tint. 



