as Photographic Agents. 209 



should be separated by filtration, when the solution is com- 

 posed of the hydriodate of baryta and iron. By now adding 

 a drop or two of very dilute sulphuric acid, more baryta is 

 precipitated and hydriodic acid left free. The clear solution 

 must be decanted off, as the filtering through paper decom- 

 poses the acid. By this means a photographic fluid of great 

 value is formed. It should be prepared in small quantities, 

 as it suffers decomposition under the influence of the atmo- 

 sphere and of light. It is always easy to set hydriodic acid 

 free by precipitating sulphate of baryta. 



28. Directions for talcing Photographs. — For drawings by 

 application less care is required than for the camera obscura. 

 With a very soft flat brush apply the hydriodic solution on 

 both sides of the prepared paper until it appears equally ab- 

 sorbed, place it in close contact with the object to be copied, 

 and expose to sunshine. The exposure should continue until 

 the light parts of the picture (iodide of silver (54?.)) are seen 

 to brown. The observance of this simple rule will ^be found 

 of very great advantage in practice. Immersion for a short 

 time in soft water removes the brown hue, and renders the 

 bright parts of the picture more clear than they would other- 

 wise have been. 



29. 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 is apparent from the chemical action on the silver; it 

 is then to be stretched on a frame, and not allowed to touch 

 in any part but at the edges ; placed in the dark chamber of 

 the camera at the proper focus, and submitted to luminous in- 

 fluence. 



If the wetted paper is placed upon any porous body, it 

 will be found, owing to the capillary communication esta- 

 blished between different points, that the solution is removed 

 from some parts to others, and different states of sensitive- 

 ness induced. Another advantage of the frame is, the paper 

 being by the moisture rendered semi-transparent, the light 

 penetrates and acts to a greater depth, thus cutting out fine 

 lines which would otherwise be lost. However, if the camera 

 is large, there is an objection to the frame ; the solution is apt 

 to gather into drops, and act intensely on small spots to the 

 injury of the general effect. When using a large sheet, the 

 safest course is to spread it out when wetted upon a piece of 

 very clean wet glass, great care being taken that the paper 

 and glass are in every part in close contact. The picture is 

 not formed so quickly when the glass is used as when the 

 paper is extended on a frame, owing to the evaporation being 

 slightly retarded ; the additional time required, about one- 



PhiU Mag. S. 3. Vol. 17. No. 109. Sept. 18*0. ? 



