ARGENTINE COMPOUNDS, SPREAD ON PAPER, SENSITIVE TO LIGHT, ETC. 327 
To prevent circumlocution in describing the results of the inquiries into which I 
was led, I would explain, that when I speak of a paper being rendered sensitive, I 
always mean, that it becomes, after exposure to light, capable of receiving the mer- 
curial vapour. 
7. Muriated photographic papers, variously prepared, pass, more or less rapidly, 
through the stages of oxidation, to a perfect reduction of the metal, by the operations 
of light. 
An examination of the effects of iodine, on all varieties of these papers, was insti- 
tuted. 
Any of the ordinary photographic papers allowed to darken to a full brown, be- 
come, by long exposure to iodine, of a steel-blue, or a violet colour. If exposed in 
this state to sunshine for a long period, their colours change from gray to a clear 
olive. 
Now, exposure to sunshine for a minute, or to diffused daylight for from five to 
ten minutes, produces no apparent change, but mercurial vapour speedily attacks the 
uncovered parts, and a faithful picture is given of whatever may have been superposed, 
but there is a want of sufficient contrast between the lights and shadows. 
8. By allowing the first darkening action to proceed until an olive coating is formed 
on the paper, I find, although it is not more speedily affected by the iodine, that it is 
more sensitive, and a better picture is formed than upon the brown paper. 
The kind of preparation used, appears to have but little effect upon the results. 
A chloride, iodide, or bromide of silver allowed to darken, answers equally well. 
There are, beside the faintness of the picture thus produced, defects which will, I 
fear, prevent our availing ourselves of this easy method of producing a moderately 
sensitive Daguerreotype paper. In the first place, the coat of oxide formed at the 
instant of exposure, protects the under layers of the salt, and the decomposing pro- 
cess proceeds irregularly ; and even the longest exposure appears insufficient to 
remove the whole of the chlorine, which obstinately, in the form of the sub-chloride 
of silver, remains in the paper. This occasions the iodidation to be less complete in 
some parts than in others, and, consequently, a variation in the photographic effect. 
Again, it appears, that upon the surface, long exposure effects a revival of the silver 
in minute points, around which the mercurial vapour is deposited in greater quantity 
than over the other parts of the paper. 
9. My next object was the production of a perfect oxide of silver, equally diffused 
over every portion of the paper. In this I have encountered many very great difficulties: 
irregularities, almost beyond detection by other means, became of serious consequence 
in the delicate process I was studying. 
The most effectual method I have discovered, is, first to soak the sheet in a weak 
solution of pure potassa, and after partially drying it, which must be done quickly, 
pass it through a solution of the nitrate of silver, and allow it to dry slowly in the 
dark. The paper is now a light brown ; but by heating it over a metallic plate, or 
