Mr. R. Hunt on Hydriodates as Photographic Agents, 203 



experiments are, the establishment of fixed principles, which 

 remove most of the uncertainty attending the use of the hy- 

 driodates, an explanation of many of the anomalous results 

 they give, and the discovery of some very remarkable pro- 

 perties not before noticed. 



The uncertainty attending the application of the hydrio- 

 dates, has greatly circumscribed their use, and it is the de- 

 sire of forwarding the progress of a beautiful art, which now 

 makes me solicit a few pages of your valuable Journal. 



Sir John Herschel, in his very excellent memoir "On the Che- 

 mical Action of the Rays of the Solar Spectrum," &c.* particu- 

 larly notices the inconstancy of the effects exhibited by the 

 hydriodates. " Nothing," says that talented and indefatigable 

 inquirer, " can be more variable and capricious than the re- 

 sults obtained according to the different intensities of the so- 

 lutions applied ; the qualities of the paper ; the degree of 

 darkening induced on the paper before the application of the 

 ioduretted solution, the state of the paper as to moisture or 

 dryness, and other circumstances." 



That the various positions I wish to establish may be com- 

 pletely understood, and to ensure the same results in other 

 hands, it will be necessary to enter into a somewhat detailed 

 account of several kinds of paper which have been used, and to 

 give tolerably full directions for successfully using the same, 

 either in the camera obscura, or for drawings by application. 



1. The preparation of the paper. The variable texture of 

 even the finest kinds of paper occasioning irregularities of im- 

 bibition, is a constant source of annoyance, deforming the 

 drawings with dark patches, which are very difficult to re- 

 move ; consequently my first endeavours were directed to the 

 formation of a surface on which the photographic prepara- 

 tions might be spread with perfect uniformity. 



2. A variety of sizes were tried with very variable results. 

 Nearly all the animal glutens appear to possess a colorific 

 property, which may render them available in many modifi- 

 cations of the processes published byMr. Talbot, but they all 

 seem to protect the darkened silver from the action of the 

 hydriodic solutions. The gums are acted on by the nitrate 

 of silver and browned, independent of light, which browning 

 considerably mars the effect of the finished picture. It is a 

 singular fact that the tragacanth and acacia gums render 

 the drawings much less permanent. I therefore found it ne- 

 cessary for general practice to abandon the use of all sizes, 

 except such as enter into the composition of the paper in the 

 manufacture. 



[* An abstract of Sir J. Herschel's paper appeared in vol. xvi. p. 331.] 



