RECENT ADVANCES IN PHOTOGRAPHY. m 



which has not been acted upon by the light. It has been usually said 

 that alkaline development is only available for bromide of silver, but 

 my experience has taught me that iodide of silver is as amenable 

 to alkaline development as the bromide, although not so rapidly, 

 and that chloride is very amenable to it, and will give most beautiful 

 pictures. 



Another mode of development, now very much in vogue, is that 

 with ferrous oxalate. In this case we have an organic salt of iron in 

 the ferrous state, which is capable of reducing silver bromide, iodide, 

 and chloride to the metallic state, while itself is reduced to the ferric 

 state. This process also requires a restrainer. 



I have found a kindred developer, the use of which I consider one 

 of the most recent advances in photography. It is an iron developer, 

 which is capable of being used without any restrainer whatever. I 

 call it ferrous citro-oxalate. It is made by adding to a solution of 

 citrate of potash ferrous oxalate till no more will dissolve ; the result- 

 ing compound is probably citrate of iron, but in a stronger form than 

 is usually found. 



Mr. Berkeley has lately introduced an improvement in the ordinary 

 alkaline developer, in which he mixes with the pyrogallic-acid solution 

 four times the weight of sulphite of soda. The action, apparently, is 

 that the sulphite of soda absorbs the oxygen with greater avidity than 

 does the pyrogallic acid, thus leaving that agent to do its work ; con- 

 sequently, we have a developer which remains uncolored for a very 

 long period. 



Another developer, which is competent to work also without a re- 

 strainer, but has not been used to a very great extent on account of 

 its high price, is hydro-kinone. It is a much more powerful absorber 

 of oxygen than pyrogallic acid, to such a degree that one grain of it, 

 is as active as two grains of that substance. Not requiring any re- 

 strainer, even when so troublesome a salt as silver chloride is used, it 

 is able to give a better detail and allow a shorter exposure in the cam- 

 era than when the ordinary alkaline developer is used. It is applicable 

 to any plate with which you can work. 



The next point to which I wish to call attention is the action of 

 sensitizers. It may be proper first to explain what a sensitizer is. 

 When you have chloride of silver exposed to light, you have a new 

 compound formed, which is called subchloride, or argentous chloride 

 (Ag 2 Cl 2 = Ag 2 Cl + Cl), and chlorine is liberated. This chlorine is 

 very difficult to eliminate, if you do not give it something that can 

 take it up ; for instance, if you place perfectly pure chloride of silver 

 in vacuo, without any trace of organic matter present, you will find 

 that you get no darkening action, even if it is exposed to brilliant 

 sunlight for months. If a white powder of the kind was submitted 

 to you, to determine its character, you would say at once that it was 

 not chloride of silver, because it was not darkened, since one of the 



