68 THE WOMBAT. 



only sensitive material in the paper is the pure chloride of 

 silver ; this is not strictly true, for the paper — that is, the 

 organic vehicle that contains the sensitive substance — -contains 

 a considerable quantity of other matter in addition, but it will 

 be sufficient for our purpose to start with the assumption that 

 we have nothing but the pure chloride of silver which has 

 been unaffected by light to remove. When the solution of 

 hypo-sulphite of soda comes into contact with the prints, 

 chemical energy is manifest at once ; the chloride of silver is 

 iusoluble in water, but is dissolved and decomposed by the 

 hyposulphite of soda, the two chemicals acting upon each 

 other, with the result that what is known in chemistry as 

 double decomposition takes place, the resulting products being 

 hyposulphite of silver and chloride of sodium. Now there 

 are two forms of these decomposition products ; one is not 

 soluble in water, and though it may be formed in the film, all 

 the washing in the world will never take it out; it is, however, 

 readily soluble in a moderately strong solution of hyposulphite 

 of soda, and having been dissolved by this a double salt is 

 formed which is freely soluble in water, and it is on the 

 presence of this double salt that proper fixing depends. The 

 whole of the products of decomposition are then easily 

 removed by ordinary washing, and all proceeds well. 



Now it will be evident from what has been said above 

 that if there has been an insufficient quantity of the fixing 

 solution employed for a batch of prints, or if the solution was 

 not strong enough, nor allowed to act for a sufficient time, or 

 prevented from reaching all parts of each individual print, 

 there will be a danger of the first form of decomposition 

 product alone being formed, and this, being insoluble in water, 

 withstands the washing operation after fixing, and remains in 

 the film. If it were unalterable there would be nothing to 

 fear, but unfortunately this material is very sensitive to light 

 and other agents, and sooner or later it makes its presence 

 felt by discolouring, and as this discolouring is more evident 

 in the whites and half tones of the print than in the deepest 

 shadows, the picture is ruined. Prints that have been over- 

 taken by this complaint can be found in great numbers, being 

 recognisable by the yellow or brown colouration of those parts 

 that should be white and which, when the prints were first 

 mounted, were white. These prints are only mentioned to 

 combat the common idea that the fading or discolouring of 

 silver prints is the result of imperfect wishing, whereas, in 90% 

 of the cases in which such occurs, the complaint can be laid 

 at the door of imperfect fixing. As a rule our prints receive 

 quite sufficient washing, that is, provided that the water has 

 been allowed free access to all parts of every print whilst the 

 washing operation is in progress ; the researches of Haddon and 

 Grundy shewed that washing of only a few minutes duration 



