Method of preserving the Sensitiveness of Collodion Plates. 335 



glass vessel in tbe light, while very slight, if any, evidence of 

 reducing action was appreciable in that kept in the dark, even 

 after the expiration of a month. Finding the action of light to 

 exercise this influence, we determined to keep separately, as far 

 as possible, these two necessary ingredients in the process. With 

 regard to the degree of concentration of the glycerine, the sample 

 made use of has a specific gravity of 1 '23 ; this we have employed 

 in its original state, and mixed with various proportions of water ; 

 we perceive no great difference in the results, but are inclined to 

 prefer its employment with but little dilution with watei-. 



The process we have been led to adopt is the following : — The 

 glass plate, cleaned with especial care (by treatment, first, with 

 a hot solution of common washing soda, and subsequently with 

 sti'ong nitric acid), is coated with iodide of ammonium collodion 

 in the usual way, and made sensitive by immersion in the ordi- 

 nary silver bath (30 grains of the nitrate to 1 ounce of water ; 

 perfectly saturated with iodide of silver*, so that the plate may 

 be left its full time without fear of dissolving the sensitive film) ; 

 after remaining here three or foiu' minutes, the excited plate is 

 transferred to, and immersed for an equal time in, a washing 

 bath of pui'e distilled water ; or instead of this bath we have 

 sometimes used a stream of water from the " syringe bottle," the 

 object being to remove the great excess of free nitrate of silver 

 from the sensitive film. 



So prepared, the plate is ready to receive the glycerine treat- 

 ment. For this purpose we require, besides Price's glycerine, 

 sp. gr. 1'23 or thereabouts, a dilute solution of nitrate of silver 

 (one grain of nitrate to the ounce of watei'). When about to be 

 used, an intimate mixture is made in the proportion of three 

 parts by volume of glycerine to one of the silver solution, and 

 poured on to the surface of the washed collodion plate, its action 

 being assisted by transferring, some two or three times, to and 

 fi'om the measure glass ; after five minutes' contact the plate has 

 to be well drained, and placed in a nearly vertical position on 

 blotting-paper, to absorb the large excess of glycerine from its 

 surface. It will then be in a fit state for receiving the impres- 

 sion in the camera, a process which may either be performed 

 immediately or deferred for a period of at least twenty-four days, 

 the longest trial to which we have as yet submitted the plates. 



* This is readily effected Vjy dissolving the total weight of nitrate of sil- 

 ver in one-fourth of the bulk of water to be ultimately employed ; a grain 

 or so of iodide of |)otassium dissolved in a little water is now added, to 

 precipitate an equivalent amount of iodide of silver, with which the solution 

 will be saturated on stirring; the remaining bulk of the water is then 

 added. After allowing time for subsidence, the solution may be filtered 

 without difficulty. The addition of a few drops of glacial acetic acid to a 

 large bath is an improvement. 



