while others
remain for weeks unimpaired, and require whole years for their total
obliteration. But though entirely faded out in appearance, the picture
is only rendered dormant, and may be restored, changing its character
from negative to positive, and its colors from brown to black, (in the
shadows), by the following process:--A bath being prepared by pouring a
small quantity of solution of pernitrate of mercury into a large
quantity of water, and letting the subnitrated precipitates subside,
the picture may be immersed in it, (carefully and repeatedly clearing
off all air bubbles,) and allowed to remain till the picture (if any
where visible,) is entirely destroyed; or if faded, till it is judged
sufficient from previous experience; a term which is often marked by
the appearance of a feeble positive picture, of a bright yellow hue, on
the pale yellow ground of the paper. A long time (several weeks) is
often required for this, but heat accelerates the action, and it is
often completed in a few hours. In this state the picture is to be
very thoroughly rinsed and soaked in pure warm water, and then dried.
It is then to be well ironed with a smooth iron, heated so as barely
not to injure the paper, placing it, for greater security against
scorching, between clean smooth paper. If then the process have been
successful, a perfectly black positive picture is at once developed.
At first it most commonly happens that the whole picture is sooty or
dingy to such a degree that it is condemned as spoiled, but on keeping
it between the leaves of a book, especially in a moist atmosphere, by
extremely slow degrees this dinginess disappears, and the picture
disengages itself with continually increasing sharpness and clearness,
and acquires the exact effect of a copper-plate engraving on a paper
more or less tinted with a pale yellow.

I ought to observe, that the best and most uniform specimens which I
have procured have been on paper previously washed with certain
preparations of uric acid, which is a very remarkable and powerful
photographic element. The intensity of the original negative picture
is no criterion of what may be expected in the positive. It is from
the production by one and the same action of light, of either a
positive or negative picture according to the subsequent manipulations,
that I have designated the process, thus generally sketched out, by the
term Amphitype,--a name suggested by Mr. Talbot, to whom I