96 Mr. Talbot on the History of Photography. 
nitrate of mercury, and then rubbed very bright, and placed in 
a closed box along with a small cup containing iodine, The 
result was a formation of Newton’s rings of the greatest splen- 
dour and of a large size. But they did not appear to be in 
any degree sensitive to light. 
The next experiment related was as follows :—Ifa piece of 
silver leaf is exposed to the vapour of iodine, however uniform 
the tension of the vapour may be, yet it does not combine uni- 
formly with the metal, but the combination commences at the 
edge of the leaf and spreads inwards, as is manifested by the 
formation of successive bands of colour parallel to the edge. 
Perhaps this is due to the powerful electrical effect which the 
sharp edges and points of bodies are known to possess, so that 
electricity may be either the cause, or the attending consequence 
of the combination of vapour with a metallic body. 
Again :—if a minute particle of iodine is laid on a steel 
plate, it liquefies, forming an iodide of iron, and a dew spreads 
around the central point. Now, if this dew is examined in a 
good microscope, its globules are seen not to be arranged 
casually, but in straight lines along the edges of the minute 
strize or scratches which the microscope detects even on po- 
lished surfaces. This is another proof how vapour is attracted 
by sharp edges, for the sides of those strize are such. 
The above extracts from the Athenseum will I think suffi- 
ciently show that I was acquainted with the effects of iodine 
vapour on silver surfaces at the time of the Birmingham Meet- 
ing, and consequently, prior to the publication of Daguerre’s 
secret. For it will be in the recollection of men of science, 
that the publication of that important discovery was first re- 
ceived in England during the very week in which the meeting 
was held. 
I am desirous to point out this circumstance, because it is 
connected with the early history of the Photographic art ;— 
as I will explain. 
Having in the year 1834 discovered the principles of Pho- 
tography on paper, I some time afterwards made experiments 
on metal plates ; and in the year 1838 I discovered the method 
of rendering a silver plate sensitive to light by exposing it to 
iodine vapours. I was at that time therefore treading in the 
steps of Daguerre, without knowing that he, or indeed that 
any other person, was pursuing, or had even commenced or 
thought of, the art which we now term Photography. 
But as I was not aware of the power of mercurial vapour 
to bring out the latent impression, I found my plates of iodized 
silver deficient in sensibility, and therefore continued to use 
in preference my photogenic drawing paper. ‘This was in 1838, 
a -- 
