Methods of preserving the Sensitiveness of Collodion Plates. 337 



Bearing in mind the qualities requisite to fulfil iu the best 

 manner the functions of a preservative agent, it occurred to us 

 that it might be possible to find a body having in itself the 

 power of rendering the collodion film sensitive to light, or at 

 least of sustaining it in that condition, and at the same time 

 possessing dehquescent properties ; substances having these two 

 characters combined are presented in the fluoride and siUco- 

 fluoride of silver. To put this supposition to the test of expe- 

 riment, we prepared these compounds (by dissolving freshly pre- 

 cipitated carbonate of silver in hydrofluoric and hydrofluosilicic 

 acids respectively), and used their solutions, in place of the 

 ordinary nitrate of silver bath, for exciting the iodized collodion 

 film. Plates so treated readily became coated with a layer of 

 iodide of silver, which seemed to be equally sensitive to light, 

 whether produced by this or the method commonly employed ; 

 they had also the power of retaining a moist surface; but, un- 

 fortunately for the present object, it was found that a strong 

 solution of fluoride of silver, like that of the nitrate, has the pro- 

 perty of dissolving off the precipitated iodide of silver, destroy- 

 ing it by forming the small holes so well known in the ordinary 

 collodion process. Meeting with this result, we determined to 

 tiy their application in a more dilute form, after exciting the 

 plate in a preliminary nitrate of silver bath ; but by this mode 

 of treatment also we were unsuccessful, being unable to preserve 

 the sensitiveness by a quantity which was insufficient to exercise 

 a destructive influence on the film. This difficulty, added to 

 that experienced in the preparation of the fluoride in a neutral 

 condition, any excess of hydrofluoric acid being objectionable 

 on account of its property of etching the glass, and on the other 

 hand, the slightest alkaline reaction rendering it extremely diffi- 

 cult to obtain clear pictures on development, deterred us from 

 pursuing the subject further in this direction. 



Compelled, therefore, to return to the principle originally 

 adopted, viz. the use of nitrate of silver in conjunction with a 

 deliquescent salt, sufficient in amount to prevent its crystallizing 

 or even concentrating beyond a certain limit upon the plate, we 

 sought only to improve the processes already before the public. 



A longer experience with the nitrate of magnesia has demon- 

 strated the difficulty of preparing this substance on a large scale, 

 free from an impurity very inimical to its successful application 

 -—the nitrite of magnesia ; the presence of this latter, by giving 

 rise to the formation in the film of the nitrite of silver, a body 

 prone to spontaneous decomposition even at ordinary tempera- 

 tures, must necessarily introduce a condition unfavourable to the 

 ultimate result. To effect the conversion of the nitrite* into 



* The presence of nitrous aci<l is easily recognized by Dr. Price's test. 

 It is applied by adding a mixture of dilute hydrochloric acid and iodide of 



