FIXING. 371 



light, preferably at a window, until the paper is of a deep chocolate 

 brown tint where directly exposed to the light. Usually the back 

 of the frame is made in two parts (as represented) or hinged 

 together, so that one half may be loosened and folded back and 

 the print examined as to its progress without disturbing its position 

 with reference to the negative when replaced, the glass and paper 

 being held together firmly by the other half of the back. When 

 the print is judged to be sufficiently deeply coloured, the paper is 

 removed from the frame, and the print " fixed " by immersing it in 

 a moderately strong solution of tliiosulpliate of sodium (hyposulphite 

 of soda) ; the effect of this is to dissolve out unaltered silver 

 chloride, &c., leaving behind particles of metallic silver in pro- 

 portion as the action of the light passing through the negative 

 has been more or less intense ; so that the dark parts of the 

 negative (corresponding with the light parts of the object photo- 

 graphed) furnish the light parts of the print, which is thus a 

 positive. 



If, however, the print to be prepared is not to be taken from a 

 glass negative but from an actual object (e.g., a skeleton leaf, a 

 dried fern, a piece of lace, &c.), the printing frame is fitted with a 

 piece of ordinary window glass ; on this is laid the leaf, &c., and 

 over this the sensitised paper, sensitive side next the object ; the 

 whole is then fixed in the frame by the wooden back and spring 

 clip, &c., and exposed for a sufficient time, after which the print 

 is fixed by steeping in thiosulphate of sodium solution, as before. 

 In all cases most copious washing of the fixed print is requisite ; 

 steeping in a succession of waters, and for several hours in each, is 

 desirable ; or for two or three days in a dish into which a continu- 

 ous small stream of fresh water runs from a tap. Silver prints are 

 always more or less liable to fade on keeping for some years, even 

 in a closed portfolio not exposed to light ; this fading takes place 

 far more rapidly when the prints have not been very thoroughly 

 washed, so that it is probable that the fading is mainly due to 

 slow chemical action taking place between the metallic silver and 

 minute quantities of thiosulphate not completely washed out, even 

 after long continued treatment with water, but retained in virtue 

 of a surface attracting action analogous to that described in Expt. 

 253. The attracted thiosulphate can be got rid of by chemical 

 means, e.g., soaking the print in a highly diluted solution of iodine, 

 and subsequently getting rid of the iodine by again soaking in a 

 solution of sodium sulphite, or by treating with solution of peroxide 

 of hydrogen or hypochlorite of sodium, &c. ; but the chemicals 

 thus used are rather apt to attack the metallic silver in the print 

 itself and thus damage it ; highly diluted hydrochloric acid solu- 



