D.—DIRECTIONS FOR TAKING PHOTOGRAPHS.

41

quantities, as it suffers decomposition under the influences of the atmos-phere and light.

Hydriodic Acid, if used on paper which will not decompose its aque-ous solution, which is rather difficult to find, acts very readily on thedarkened silver. A portion of this acid free in any of the solutions,most materially quickens the action. From the barytic solution it isalways easy to set free the required portion, by precipitating the barytesby sulphuric acid. As the hydriodate of barytes is rarely kept by theretail chemist, it may be useful to give an easy method of preparing thesolution of the required strength.

Put into a Florence flask one ounce of iodine, and cover it with onefluid ounce and half of distilled water; to this add half a drachm ofphosphorus cut into small pieces; apply a very gentle heat until theyunite, and the liquid becomes colourless; then add another fluid ounceand a half of water. It is now a solution of hydriodic acid and phos-phoric acid. By adding carbonate of barytes to it, a phosphate ofbarytes is formed, which, being insoluble, falls to the bottom, whilstthe soluble hydriodate of barytes remains dissolved. Make up thequantity of the solution to nine ounces with distilled water, and care-fully preserve it in a green glass stoppered bottle.

D.—Directions for taking Photographs.

For drawings by application, less care is required than for thecamera obscura. With a very soft flat brush apply the hydriodic solu-tion on both sides of the prepared paper, until it appears equally ab-sorbed ; place it in close contact with the object to bo copied, and ex-pose it to sunshine. The exposure should continuo until the parts ofthe paper exposed to uninterrupted light, which first change to a paleyellow, are seen to brown a little. The observance of this simplo rulewill be found of very great advantage in practice. Immersion for a *short time in soft water removes the brown hue, and renders thebright parts of the picture clearer than they would otherwise have been.

Engravings to be copied by this process,—which they are most beauti-fully ,—should be soaked in water and superimposed on the photographicpapers, quite wet. If the paper is intended to be used in the camera,it is best to soak it in the hydriodic solution until a slight change isapparent, from chemical action on the silver; it is then to be stretchedon a slight frame of wood, which is made to fit the camera, and notallowed to touch in any part but at the edges; placed in the darkchamber of the camera at the proper focus, and pointed to the object ofwhich a copy is required, which, with good sunshine, is effected in abouttwenty minutes, varying of course with the degree of sensibility which ismanifested by the paper. If the wetted paper is placed upon any

G