and Laboratory Methods. 2337 



" This solution does not injure the silver image or the gelatine film in any 

 way. It will entirely remove a few stains, especially if applied before the stain 

 has time to dry in thoroughly, and will lighten the color even of those produced 

 by hydro-quinone to a considerable extent. But, as a rule, it will be found that 

 such a solution as 



Bleaching powder, ..... 30 grains, 



Concentrated hydrochloric acid, . . 40 minims, 



Water, to 5 ounces, 



is necessary for complete stain elimination. 



" It is important when making up the bleaching powder solution to see that 

 no particles are left undissolved. I always dissolve the bleaching powder first 

 in the water, and, after stirring it about, filter ; then I add the acid. I believe 

 that the above solution is capable of bleaching out entirely any stain which could 

 possibly be formed by any developer, if sufficient time be given. It will bleach 

 the stain images produced by pyro and hydro-quinone due to too little sulphite. 



" The weak point is, of course, that it attacks the silver image, forming silver 

 chloride, and on account of this property, I have found it a very excellent solu- 

 tion for indirect sulphuretting purposes. The obvious way of dealing with the 

 bleached negative when it has been quite freed from stain is to redevelop the 

 image with any suitable developer, which, since the image is of silver chloride, 

 must not contain much sulphite. Great care must be taken to wash the negative 

 very thoroughly between the bleaching powder solution and development, as 

 otherwise, when using a phenolic developer, a brown stain is produced all over 

 the negative, owing to the oxidation of the phenol by the last traces of the 

 bleaching solution. However, this method unfortunately restores most stains to 

 some extent. Ferous oxalate appears to do so least. It is better to treat the 

 plate with a sulphuretting agent, such as a solution of sodium sulphite or sul- 

 phuretted hydrogen ; of course, washing between the bleaching and sulphuret- 

 ting solutions. But even this method appears to restore some kinds of stains to 

 a small extent. Perhaps, without blackening, the negative could be backed 

 with black paper and then photographed ; or, if it had been very considerably 

 intensified with silver (mind, silver — not mercury) before the application of the 

 bleaching powder bath, it might be printed from as it is, that is, with a silver 

 chloride image, but I have not tried experiments on these lines. 



" Very similar to the acid bleaching powder solution are solutions of chlorine 

 and bromine, or an acidified solution of a hypobromite, but iodine solution is 

 not so efficient as a bleaching solution for stains as the above. 



" Alkaline hypochlorite and hypobromite solutions will bleach a stain com- 

 pletely if allowed to act for sufficient time. When the stain is of such a nature 

 as to be bleached quickly, these solutions can be so mixed as not to attack the 

 silver image to any appreciable extent. The slight amount of silver chloride or 

 bromide formed (that is, if any at all is formed) can be removed by hypo solu- 

 tion without any restoration of the stain. The following is the best solution of 

 the kind I have tried : 



Bleaching powder, 47 grains. 



Caustic soda, 8 grains. 



Water, to ..... • 9 ounces. 



