POSITIVE PHOTOGRAPHS BY ONE PROCESS. 89

Papers darkened by the diffused light of a cloudy day arescarcely, if at all, acted on by these salts. Great care must betaken fo prevent the silver solution from flowing over the edgesof the paper, as thereby an extra quantity of darkened silver isformed on both sides, which requires a long-continued action ofthe hydriodates and light to bleach.

The kind of paper on which the silver is spread is an objectof much importance. A paper known to stationers as satinpost, double-glazed, bearing the mark of J. Whatman, TurkeyMill, is decidedly superior to every other kind I have tried. Thedark specks which abound in some sorts of paper must beavoided, and the spots made by flies very carefully guardedagainst. These are of small consequence during the darkeningprocess, but when the hydriodic wash is applied, they formcentres of chemical action, and the bleaching process goes onaround them, independently of light, deforming the drawingwith small rings, which are continually extending theirdiameters.;

The saline washes may be considerably varied, and combinedto an indefinite extent, with a continued change of effect, whichis singularly interesting. In their application we should beguided, as m the negative process, by their combining propor-tions. The following list of the salts which will give the besteffects, selected from upwards of seven hundred combinations,will show the variety of colours produced. They are placed inthe order of the sensitiveness they appear to maintain, whenused as nearly as possible under the same circumstances.

Colour of Picture.

MURIATE OF AMMONIA.. Red, changing to black in the sunshine.CHLORIDE OF SODIUM... Ditto. ditto.MURITATE OF STRONTIA.. 4 fine brown.

MURIATE OF BARYTA...d rich brown, inclining to purple.

Sor. CuroridE oF LIME Very red.

Sor. CHLORIDE OF SoDA 4 brick red.

Topipe oF PorasstoMm...Yellowisk brown.

. Variable, sometimes yellowish, often a

CHLORATE oF POTASH!steel blue.

PHOSPHATE OF SODA.... Mouse colour.

TARTRATE OF SODA..... Dark brown.

URATE OF SODA........ Yellowish brown.

MuRrIiaTE OF IRON...... Deep brown, which blackens.

BroMIDE OF SopIuM.... Red brown, of a peculiarly rich tint.

The change mentioned in the colour of the finished picture