78

MISCELLANEOUS PROCESSES.

The colour of these photographs may be very agreeably varied, bysoaking the finished drawing in a weak solution of the nitrate of silver,by which the chromate of silver is formed,—a salt of a bright red col-our—or in a solution of the bichloride of mercury, by which a chromateof mercury is formed, which is of a rich purple colour. When thedrawings are again dry, they must be washed in water having a verysmall portion of common salt in it, to remove tho silver or mercuryfrom the white parts of the paper.

The most interesting variety of photographic paper, prepared withthe bichromate of potash, is a kind described by M. E. Becquerel. liestates,—It is sufficient to steep a paper prepared in Mr. Ponton’s man-ner, and upon which there exists a faint copy of a drawing, in a solutionof iodine in alcohol, to wash this paper in alcohol, and then dry it; thenthe parts which were white become blue, and those which were yellowremain more or less clear.

M. E. Becquerel has pursued his investigations into the action of thechromic acid on organic compounds, and has shown that the mode ofsizing the papers influences their colouration by light, and that with junsized paper, colouration is effected only after a long time. Per- :coiving that the principal reaction resulted from the chromic acid con-tained in the bichromate of potash, on the starch in the size of thepaper, it occurred to M. E. Becquerel, that, as starch has the propertyof forming with iodine a combination of a very fine blue colour, itshould produce deep shades of that tint, whilst the lights still remainedan orange-yellow.

His method of proceeding is to spread a size of starch, very uniformlyover the surface of the paper. It is then steeped in a weak alcoholicsolution of iodine, and afterwards washed in a great quantity of water.

By this immersion it should take a very fine blue tint. If this is uni-form, the paper is considered fit for the experiment: in the contrarycase it is sized again. It is then steeped in a concentrated solution ofbichromate of potash, and pressed between folds of blotting paper, anddried near the fire. To be effective, it should be very dry.

It is now fit for use. When the copy is effected, which requires in sun-shine about five minutes, the photograph is washed and dried. Whendry, it is steeped in a w r eak alcoholic solution of iodine, and afterwards,when it has remained in it some time, it is washed in water, and care-fully dried in blotting paper, but not at tho fire, for at a little below100° Fain the combination of iodino and starch discolours.

If it be considered that the drawing is not sufficiently distinct, thisimmersion may be repeated several times; by this means may be ob-tained the intensity of tone that is desired, which intensity cannot bechanged at will by employing a more concentrated solution of iodine.

When the paper is damp, the shades are of a very fine blue, but whenit is dry, the colour becomes deep violet If while the drawing is stillwet it be covered with a layer of gum arabic, the colour of the drawingis greatly preserved, and more beautiful when it is dry. When a paper