21 



dion, which does not consist of " dissolved paper " (as represented), but 

 of a new substance, complex and unstable as we have already seen, it 

 is more than probable that this substratum or base takes an active 

 chemical part in the decompositions which ensue. 



Witli respect to the developing agent, the attempt was made to 

 assume that pyrogallic acid was a mere modification of gallic acid, that 

 it was in fact gallic acid sublimed by heat, and exalted in properties. 

 But the fact stands otherwise : the modification includes the loss of 

 carbonic acid ; and its properties as an acid are more than doubtful ; 

 nor are they intimated by the name, which refers rather to its origin 

 than its properties, and simply means, "from gallic acid by fire." Now 

 it is an analogous body to other substances obtained from organic acid ; 

 and I know not why its name should not conform to that analogy, in 

 which case it would be called " gaUone," thus : — 



Gallic acid — 



Cj, H3, O5 

 Acetic acid — 



C4, H3, Og 

 Benzoic acid- 



Cu, H„-, O3 

 Butyric acid- 



Cg, H7, O3 



f 



By heat ' 

 lose I 



CO,= Ce, H3, O3 

 00^=03, H3, 

 C0„=C,3, H„ O 

 C03=Cj, Hj, 



( Pyrogallic acid, or 

 1 Gallone. 

 ( Pyroacetic acid, or 

 I Acetone. 



Benzone. 

 Butyrone. 



The chief difference, practically, between gallic and pyrogallicjacids as 

 reducing agents, is in time and energy — the reduction by the former 

 being gradual, so that the intermediate stages of oxidation are gone 

 through ere the reduction is complete ; the latter at once reduces the 

 metal as a black deposit. PyrogalHc acid is therefore quite inapplicable 

 in lieu of gallic acid as a sensitive agent, and cannot be employed in 

 Mr. Fox Talbot's process. But the process is by no means dependant 

 on the use of pyrogallic acid, as known to most Liverpool photographers ; 

 reduction by strong iron solutions answering the same purpose ; or weak 

 iron solutions, the deposit being further strengthened by salts of gold, 

 or mercury, or iodides, or sulphides of the alkalies, involving reactions 

 and manipulations quite inapplicable to any paper process. The depo- 

 sition of silver in a white granular condition by iron, forming positive 

 images, is certainly a process beyond the provision of any paper patent ; 

 and the fixing agents now in use, are as certainly an advance in know- 

 ledge since the date of that specification. Hence my conclusion is, 

 that while most photographic processes have one object in view, viz. the 

 reduction of silver from its salts, and in pursuance of tliat object apply 

 known jirinciples of chemical science in the clioice of reducing agents ; 



