ARGENTINE COMPOUNDS, SPREAD ON PAPER, SENSITIVE TO LIGHT, ETC. 333 
29. There are many inconveniences attending the use of fluid mercury, particularly 
to travellers ; I therefore propose a plan by which this may be avoided, and the mer- 
curialization very much facilitated. It is this : the box used for vaporization is so 
made as to admit of a piece of copper, rather more than half the size of the paper or 
plate, being placed upon the iron bottom. When required for use, the copper is 
rubbed over with the nitrate of mercury, washed, dried, and fixed in its place. As 
soon as the copper gets hot, which is very quickly, the mercury on its surface is vo- 
latilized, and the action on the photograph effected. I find this plan occupies only 
half the time of the original, with a capsule of mercury. 
30. The drawing is now to a certain extent complete, but I have found the use of 
the hyposulphite of soda to remove the iodine objectionable, on the ground of its 
tendency to darken the mercurial deposit. A warm solution of the muriate of soda 
is more efficacious. 
31. I have now to call particular attention to a phenomenon of a most remarkable 
character, opening a wide field for inquiry. The singular manner in which the mer- 
curial vapour arranges itself on Daguerre’s tablets has excited much attention, and 
given rise to numerous speculations, but even this appears to me far less curious than 
the following discovery. 
If one of the above papers, when removed from the mercurial vapour, be dipped into 
a solution of the bichloride of mercury, the drawing disappears, but after a few mi- 
nutes it is seen, as if by magic, unfolding itself, and gradually becoming far more 
beautiful and whiter than before* — delicate lines, before invisible or barely seen, are 
now distinctly marked, and a rare and singular perfection of detail given to the 
drawing. 
It may appear at first sight that the bichloride of mercury dissolves off the metal 
and deposits it again in the form of chloride (calomel). But this does not account 
for the fact, that if the paper has been prepared with the nitrate of silver, the mercury 
disappears, and the drawing vanishes, the deposit taking place only on those parts 
upon which light has acted but feebly, as, for instance, on the vernations of leaves, 
leaving those portions of surface exposed to full luminous influence without a particle 
of quicksilver. When the paper has been either a chloride or iodide, the effect is as 
above, and the thickness of the deposit is as the intensity of the light has been ; con 
sequently the semi-tints are beautifully preserved. If the drawing remains too long 
in the solution, the precipitate adheres to the dark parts and destroys the effect. The 
singularity of this operation will be more striking if the picture has been soaked for 
some time in a solution of the hyposulphite of soda, and then dipped into the bichlo- 
ride of mercury. As the drawing disappears, a series of circles, formed of a white 
powder, appear to arise from the paper, generally commencing at the centre, and 
slowly extending over its whole surface : the powder is afterwards deposited, and the 
sheet is buried in the precipitate ; but on taking the paper from the liquid and passing 
a stream of water over it, the precipitate is entirely removed from all the parts. 
