Lastly, the proofs are fixed in a solution of Hyposulphite of Soda, one part to four of water; which may be used many times successively. This Bath alters the tone very little if the deposit of Gold be well fixed on the Print; but the writer has often observed in the case of Albumen paper and paper prepared with Citrate (Formula II.) that if removed too quickly from the Sel d'or, the purple tones change by immersion in the Hyposulphite to a chocolate-brown. Ammonio-Nitrate Prints are less liable to alter in this way. In order that the fixing may be properly performed, the time of immersion should not be less than ten minutes with a porous paper, plain salted; or fifteen minutes in the case of an English or albuminized paper. Ammonia may be used for fixing plain paper Prints; about one part of the Liquor Ammoniæ, to four of water. Ten minutes' immersion will usually be sufficient, and the tone is very little affected. This process is a good one, but the pungent smell of the Ammonia is an objection, and the Bath discolours by use. Some care too is required in order to ensure a proper fixing of the prints (see the remarks at page 131). For directions to wash and mount the proofs, see page 255. It will sometimes happen in the Sel d'or process, from the toning Bath having but little solvent action on the light shades, that the Prints, after being washed and dried, appear too dark; this may be remedied by laying them for a few minutes in _a very dilute solution_ of Chloride of Gold (five or six drops of the yellow solution of the Chloride to a few ounces of water) and washing for an additional quarter of an hour. Or an over-printed Positive may be saved by toning it with Chloride of Gold instead of Sel d'or. In that case, after proper removal of the free Nitrate of Silver, a few drops of a lemon-yellow solution of Chloride of Gold (with a fragment of Carbonate of Soda added to remove acidity, p. 132), should be poured over the Print, which is to be subsequently fixed in the usual way. _Advantages of toning by Sel d'or._--This process will be found especially useful by those who print large Positives. The solutions may be mixed in a few minutes, and, being very dilute, are economical. It is not even necessary to employ a _Bath_ for toning, but if the Sel d'or solution be prepared of about twice or three times the strength given in the formula, it will be sufficient to pour a few drachms upon the surface of the print. As the Gold solution is always used soon after mixing, a uniform and permanent tint can be obtained; whereas the single fixing and toning Bath of Gold and Hyposulphite loses much of its efficacy by keeping, and _over-printing_ of the proof is required in proportion as the Bath becomes older. SECTION IV. _On a mode of Printing enlarged and reduced Positives, Transparencies, etc., from Collodion Negatives._