24 



left free a certain, though excessively small, amount of silver, 

 which had been deserted for good by its last companion, 

 bromine^ and the tiny metallic particles thus existing seized 

 upon their kindred atoms as soon as these were separated 

 by the developer, and formed with them a solid but infinitely 

 delicate deposit on the plate. 



The other process, or chemical development, depends of 

 course on precisely the same action of light, but utilizes it in 

 a rather different way. It is only used with dry plates, in 

 which there is never any free silver nitrate, and what is 

 required is to obtain from the bromide compound itself the 

 free silver necessary to render visible the infinitesimal atoms 

 of that metal separated by the light. This is usually 

 effected by pyrogallol, or as it is inaccurately called pyro- 

 gallic acid. This substance has a strong affinity for bromine 

 as well as for oxygen, which is rendered very much more 

 intense by adding to it an alkali, and especially ammonia. 

 It then acts in two ways— the pyrogallol takes up the re- 

 maining particle of bromine left in the sub-bromide of silver, 

 and thus converts all the bromide which has been acted on by 

 light into free silver. But as even this amount of silver 

 would be imperceptible, the proportion of the bromide in 

 the film directly affected by the short exposure to light which 

 it has received being very small the ammonia is called into 

 play. This dissolves some of the unacted on silver bromide, 

 and renders it susceptible to the action of the pyrogallol, and 

 more silver is thus gradually added to the image, till in the 

 judgment of the operator sufficient has been deposited, when 

 the action is summarily stopped by a copious application of 

 cold water. 



Further than this I must not go in speaking of develop- 

 ment, though I have not even commenced to state all its 

 principles. I must pass on to the modification of the 

 dry plate process which now prevails. 



What I have said indeed has been chiefly with the object 

 of shewing how little the modern revolution in dry plates has 

 affected the now time honoured processes. With the single 



