HAMILTON SCIENTIFIC ASSOCIATION. 81 



as readily as sugar in water, and pointed out a fact that to convert 

 the silver bromide, or chloride, a considerable excess of hypo must 

 be used. He also showed that the latter reaction excess of silver 

 salt on hyposulphite constituted a most delicate test; so much so 

 that he calculated that he could detect one part of hypo in 97,800 

 parts of water. It is also clear that although not applied he was 

 the first to discover a process analogous to coating clear glass with 

 silver emulsions. It is also to Hershall that we owe the blue print. 

 Thus, he experimented much with mercuric salts in devising a pro- 

 cess for using them in his cyanotype process. He discovered that 

 an ordinary blue print placed in a solution of mercurous nitrate is 

 gradually bleached. The paper having been dried, the picture can 

 be restored by passing a smooth hot iron over it when it reappears, 

 not blue but brown, but soon fades out. 



To summarize : Hershall gave us thiosulphate fixation. He 

 independently discovered a process almost exactly similar to that 

 devised by Talbot some five years previously. He was the first to 

 use the terms "positive" and "negative," and to obtain a nega- 

 tive on glass. 



It is interesting to note that Hershall's experiments with the 

 platinum salt is in a sense the forerunner of the platinotype pro- 

 cess, to-day so widespread and popular for the more artistic sorts of 

 renderings of photographic subjects. The "blue-prints" that 

 have played such ari important role in the present-day developm^ent 

 of all the vast industrial enterprises that involve mechanical and 

 engineering drawings, enabling hosts of duplicates to be placed in 

 the hands of workmen, superintendents and the like, are also the 

 direct result of his investigations of the iron salts. 



His career is thus a most instructive example of the world-wide 

 benefits that a man of science can confer on humanity and progress. 



Thus in looking back we find one great characteristic, namely, 

 that it was the custom of each of these pioneers to take something 

 already in practice and to brace something different of his own onto 

 it. Dagurre used iodine ; Talbot the hypo prescribed by Hershall • 

 St. Victor, a cousin of the late Niepce, used glass on which to take 



