297 



other cause was operating which would be responsible for the 

 silver image losses. The only apparent cause is the possible 

 occurrence of the insoluble silver sodium thiosulphate 

 (AgNaSjOg) in the film, from which, in the presence of the 

 toning solutions, some sodium thiosulphate might be derived, 

 which would act with the potassium ferricyanide in the toning 

 solutions as a reducer of the silver image according to 

 equations viii. and ix. 



The results in the copper method would suggest; that the 

 copper ferrocyanide was also soluble, and that its removal 

 occurred before that of the silver, for this reason. Muir 

 and Morley (10) give the colour of copper ferrocyanide as 

 "brown-red," and of silver ferrocyanide as "white . 

 turning blue in air." Now the colour of the toned image 

 is a resultant of : — 



1. The black unaffected silver. 



2. The bluish-silver ferrocyanide. 



3. The brownish-red copper ferrocyanide. 

 Consequently if copper ferrocyanide passes into solution, 

 the resultant tone will be much colder than otherwise. The 

 effect would be most noticeable in that part of the image 

 where the original silver deposit was least. This is exactly 

 in accordance with results obtained, and accounts for the lack 

 of homogeneity of colour values. 



The Remedy. 



Many substances have been used as "hypo" eliminators 

 to avoid the washing of prints to any extent after "fixing." 

 The substances in most general use are strong oxidising agents, 

 and it is believed that "hypo" elimination is made to take place 

 more rapidly by conversion of the sodium thiosulphate to the 

 tetrathionate (Nsi^^^O^) (11).(2) Although the writer would 

 not desire to introduce niore complexities into the chemical 

 procedure of photography than necessary, yet the field invited 

 experiment. Tests were made with hydrogen peroxide, 

 potassium permanganate, and potassium persulphate, but most 

 successfully with potassium percarbonate (K2C20g). Since 

 the tetrathionates are easily soluble in water (12, 13) their 

 elimination would be a very simple matter. Failing any evi- 

 dence that the tetrathionates could combine with any salt used 



(2) The verdict from available literature on the subject of 

 the action of oxidisers on thiosulphates is not unanimous, e.g., 

 Muir and Morley, in "Watts' Dictionary of Chemistry," vol. iv., 

 p. 705, state: "Thiosulphates are converted into tetrathionates 

 by the action of iodine; oxidisers such as HNO3, KCIO3, and 

 HClAq, and KMnO^Aq produce sulphates." 



