298 



in toning, with the result that part of the silver image might 

 be brought into a soluble form analogous to that given in 

 equation ix., it would appear unnecessary to insist on the 

 complete elimination of the tetrathionates. At the same, 

 time, the writer would not suggest that any salts which can 

 be removed should be left. The whole object of the treat- 

 ment is to allow toning to commence with a pure silver image. 

 From the foregoing, then, it would appear that : — 



1.. The trouble is due to the insoluble silver sodium, 

 thiosulphate (AgNaS203). 



2. That this becomes completely soluble by treatment- 

 with an oxidiser. 

 The writer is fully aware that their exists no proof as- 

 to the correctness of his hypothesis, and that the questions 

 which might arise from the fact that the reactions take place 

 in the presence of a colloid have not been investigated at all. 

 At the sam^e time, the results obtained appear to lend support 

 to the hypothesis which is tentatively advanced. 



The tones obtained are wholly homogenous; there is na 

 loss in density values, and there is no uneven toning. Quite- 

 delicate prints have been toned after such treatment, with 

 excellent results, quite unobtainable otherwise. The richness- 

 of the tones obtained generally is quite an outstanding feature. 

 The method adopted in practice is to wash the prints for, 

 approximately, half an hour in running water, treat them 

 .for five minutes in an 0"6 per cent, solution of potassium per- 

 carbonate, and continue washing for a further half an hour. 

 No attempt is made to curtail the time of washing. The potas- 

 sium percarbonate treatment is designed to do what prolonged 

 washing apparently does not do. If the soluble sodium 

 thiosulphate and the soluble silver sodium thiosulphate have 

 not been wholly removed by normal washing, the treatment 

 with the potassium percarbonate will considerably facilitate 

 their removal, as well. as remove the insoluble silver sodium 

 thiosulphate. 



Concerning the sulphide method of toning, it should be 

 observed that when using the chlorine "bleacher," the pre- 

 liminary oxidising treatment is quite unessential in order to 

 obtain good tones, since there is no potassium ferricyanide 

 employed in the "bleacher." Hence equations viii. and ix. 

 cannot function, even if some thiosulphates do remain in the 

 film. With the chlorine "bleacher," toning can commence 

 with only a brief rinse after taking the prints from the 

 fixing bath. At the same time, this is not recommended^ 

 and the prints should preferably be well washed after 

 "fixing," and then dried before they are toned. 



