360 THE BEGINNINGS OF PHOTOGRAPHY. 



the nitrate iiiid the nmriate, a portion of the metallic oxide abandons 

 its acid to (Miter into union with the animal or vegetable su])stance, so 

 as to form with it an insolu])le compound. If so, it was not improl)al)lc 

 that substances might be found capable of destroying this compound 

 either by simple or comi)Ound afHnities. He had imagined some 

 experiments on the subject and hoped to publish them later." In con- 

 clusion he says: "Nothing but a method of preventing the unshaded 

 parts of the delineation from l)eing colored l)v exposure to the day'is 

 wanting to rendei" the process as useful as it is elegant.'' From this it 

 is very evident that he fully appreciated the value of the process if only 

 the difficulty of rendering the images permanent could be overcome. 



It is easy to understand that the want of sensitiveness on the one 

 hand and on the other the difficulty of iixing the images were sufficient 

 to render Wedgwood's original idea of reproducing objects in the 

 camei'a quite inipractical)le with papers prepared with the nitrate or 

 chloride of silver. Davy does not mention having tried any chemical 

 method of fixing l)eyond repeated washings. In the case of the nitrated 

 papers, washing with warm distilled water should have l)een sufficient 

 to render the pictures fairly permanent; but it would not answer with 

 the muriate, for which the only method available at the time would be 

 treatment with solution of salt or of ammonia, both of which are 

 unsatisfactory. Davy, being well acquainted with the previous work 

 of Scheele, as is shown by his note in the paper, would have known of 

 the solvent action of ammonia on the unexposed chloride, but, as 

 Berthollet showed, it also attacks the exposed and darkened parts. 

 Roliert Hunt says it can be used etiectively as a fix'ing agent for silver 

 chloride or nitrate prints, but requires very great care in its use to 

 avoid the solution of the reduced image. With chloride prints on 

 paper prepared as described above, without any free silver, by brush- 

 ing on the chloride (Talbot seems to have been the first to use the 

 method of preparing the muriated paper by double decomposition with 

 excess of silver) I found that the image was weak and only loosely 

 adherent to the paper. 



After treatment with dilute solution of ammonia the coating dis- 

 solved oil', leaving a faint gray image, formed, as Davy describes, 

 b}' the combination of some of the silver with the organic material of 

 the paper. A 10 per cent solution of connuon salt did not have the 

 same solvent and clearing action as the anuuonia, and the paper dark- 

 ened again readily in the light. The solvent action of hyposulphite of 

 soda on salts of silver wwis not known till 1819, when Sir John Hcr- 

 schel first drew attention to it. Under these circumstances one can 

 not be surprised at Davy not following up the subject, he being fully 

 occupied with his Royal Institution lectures, besides investigations 

 and researches of greater importance at the time. He published the 

 results of Wedgwood's invention with his own observations, as far as 



