208 Mr. R. Hunt on the Use of Hydriodic Salts 



hydriodates, is the one generally employed. The strength at 



which I use these salts for most kinds of paper is thirty grains 



to an ounce of water. The following results will exhibit the 



different energies manifested by these solutions at several 



strengths, as tried on the same paper by the same light. 



120 grains of salt to an ounce of water"! ,„ • . 

 to . i . x . w2 minutes, 



took to whiten J 



100 do. to do. 10 — 



80 do. to do. 9 — 



60 do. to do. 7 — 



40 do. to do. 6 — 



30 do. to do. 4 — 



20 do. to do. 6 — 



10 do. to do. 12 — 



The other hydriodic salts correspond nearly with these in 



their action; a certain point of dilution is necessary with all. 



21. Hydriodate of ammonia, if used on unsized paper, has 

 some advantage as to quickness over either the salts of potassa 

 or soda. This preparation is, however, so readily decom- 

 posed, that the size of the paper occasions a liberation of 

 iodine, and the consequent formation of yellow-brown spots. 



22. Hydriodate of Iron, — This metallic hydriodate acts 

 with avidity on the darkened paper; but even in the shade its 

 chemical energy is too great, destroying the sharpness of out- 

 line and impairing the middle tints of the drawing. It also 

 renders the paper very yellow. 



23. Hydriodate of lime acts similarly to the iron, but less 

 energetically, and the paper is not rendered yellow by it. 



24. Hydriodate of manganese answers remarkably well 

 when it can be procured absolutely free of iron. When the 

 manganesic solution contains it, even in the smallest quantities, 

 light and dark spots are formed over the picture, which give 

 it a curious speckled appearance. 



25. Hydriodic acid, if used on paper which will not decom- 

 pose its aqueous solution, acts readily on the darkened silver. 

 Jt is difficult, however, to procure a paper which does not li- 

 berate the iodine. A portion of hydriodic acid, free, in any 

 of the saline solutions, greatly quickens the action. 



26. Hydriodate of baryta possesses advantages over every 

 other simple hydriodic solution, both as it regards quickness 

 of action, and the sharpness of the outline in the photograph. 



27. I find, however, the quickness of this solution may be 

 much increased. Forty grains of the hydriodate of baryta 

 being dissolved in one ounce of distilled water, thereto should 

 be added five grains of pure sulphate of iron, and allowed 

 slowly to dissolve* Sulphate of baryta is precipitated, which 



