ay through nearly its whole length.

In photographic drawing this salt is of the utmost importance. Mr.
Talbot's application of it will be given hereafter in another portion
of this work.

IODIDE OF SILVER--Perfectly pure, undergoes very little change under
the influence of light or heat; but if a very slight excess of the
nitrate of silver be added it becomes infinitely more sensitive than
the chloride.

The spectrum impressed upon paper prepared with a weak solution of the
hydriodate of potash presents some very remarkable peculiarities. The
maximum of intensity is found at the edge of the most refrangible
violet rays, or a little beyond it, varying slightly according to the
kind of paper used, and the quantity of free nitrate of silver present.
The action commences at a point nearly coincident with the mean red of
the luminous spectrum, where it gives a dull ash or lead color, while
the most refrangible rays impress a ruddy snuff-brown, the change of
tint coming on rather suddenly about the end of the blue or beginning
of the violet rays of the luminous spectrum. Beyond the extreme violet
rays, the action rapidly diminishes, but the darkening produced by
these invisible rays, extends a very small space beyond the point at
which they cease to act on the chloride of silver.

In its photographic application, it is, alone, of very little use; but
in combination with other reagents it becomes exquisitely sensitive.
With gallic acid and the ferrocyanate of potash it forms two of the
most sensitive photographic solutions with which we are acquainted.
These are used in the calotype process.

IODURET OF SILVER.--If upon a plate of polished silver we place a small
piece of iodine, and apply the heat of a lamp beneath the plate for a
moment, a system of rings is speedily formed. The first ring, which
spreading constantly forms the exterior of the circle, is of a bright
yellow color; within this, there arises, successively, rings of green,
red and blue colors, and then again a fine yellow circle, centred by a
greyish spot on the place occupied by the iodine. On exposing these to
the light, the outer yellow circle almost instantly changes color, the
others slowly, in the order of their position, the interior yellow
circle resisting for a long time the solar influence. These rings must
be regarded as films of the ioduret of silver, varying, not only in
thickness, but in the more or less perfect states of combination in