182 
During an investigation into the early history of photo- 
graphy, I met with the statement that Daguerre used 
hyposulphite of soda in his process for fixing the pictures, 
and also that in Mr. Talbot’s patent the use of that substance 
was included. I was under the impression that Sir John 
Herschel had pointed out that hyposulphite of soda would 
fix the photographic image, but was unable to ascertain 
where or when the discovery jvas first published. In order 
to determine this point I wrote to Sir John Herschel, 
requesting him to inform me whether the discovery was his, 
and the date when it was published. To these questions I 
received the following reply : — 
Colingwood, October 29, 1864. 
Sir, 
I think I may very fairly claim the discovery of the 
hyposulphites as fixing agents, as I believe I was the first to call 
the attention of chemists to that class of salts and their peculiar 
habitudes, especially in relation to the insoluble salts of silver. 
In my paper “On the Hyposulphurous Acid and its Com- 
pounds,” which bears date Jan. 8, 1819, and which appeared in 
Brewster and Jamieson’s Edinb. Phil. Journal , 1819, occur these 
words : — 
“ One of the most singular characters of the hyposulphites is 
the property their solutions possess of dissolving muriate of silver 
and retaining it in considerable quantities in permanent solution.” 
(page 11.) 
“ Hyposulphite of potash. — It dissolves muriate of silver even 
when very dilute, with great readiness.” (p. 19.) 
“ Hyposulphite of soda. ...... Muriate of silver newly precipi- 
tated dissolves in this salt when in a somewhat concentrated 
solution in large quantity and almost as readily as sugar in water.” 
(p. 19.) 
“ Hyposulphite of strontia Like the rest of the hypo- 
sulphites it readily dissolves muriate of silver, and alcohol preci- 
pitates it as a sweet syrup.” (p. 21.) 
