FIXING AND CLEARING 125 
Finally, add: 
Chrome alum 20 g 154 gr. 
Hot water 50 ccm 34 Oz. 
Make the total bulk up to 1000 ccm or 16 oz. 
Rapid ammonia fixing bath.—It is frequently recommended 
to add ammonium chloride to the fixing bath on the supposi- 
tion that ammonium hyposulphite is formed, that this fixes 
quicker, and is washed out more easily. If a 20 per cent solu- 
tion of hypo is used, the addition of from 2% to 5 per cent 
of ammonium chloride increases the rapidity of fixing. With 
a 40 per cent solution, it has no effect. The addition of 
10 per cent of ammonia water to a 20 per cent solution of 
hypo acts as quickly as the ammonium chloride. 
ReEsIDUES.—To precipitate metallic silver from old hypo 
baths, from 6 to 8 g per liter (420 to 560 grains per gallon) 
of sodium hydrosulphite, Na,S,O,, should be added, with 
about half the quantity of caustic soda, and the bath heated 
to boiling to decompose excess of hydrosulphite. This treat- 
ment regenerates the hypo, and the bath can be used again. 
Addition of a saturated solution of sulphurated potash, the 
so-called liver of sulphur, may be used for precipitating the 
silver from old hypo baths. This should be done out of 
doors, as sulphuretted hydrogen is evolved. The mixture 
should be well stirred and the silver sulphide allowed to 
settle down; then some of the clear supernatant liquid should 
be collected in a graduate, and tested with a drop or two of 
liver of sulphur solution. A deep brown colour or blackish 
precipitate shows the presence of silver in solution, and then 
more sulphur compound should be added. 
A much more cleanly precipitant is zinc. This can be used 
either in scrap or mossy form. The liquid should be well 
stirred and allowed to settle; this operation should be re- 
peated three times at intervals of 24 hours. The clear liquid 
