ing an exceedingly sensitive
surface. The pictures on this paper are easily fixed by salt or weak
ammonia.

NITRATE OF SILVER.--This salt in a state of purity, does not appear to
be sensibly affected by light, but the presence of the smallest portion
of organic matter renders it exceedingly liable to change under
luminous influence.

If a piece of nitrated paper is placed upon hot iron, or held near the
fire, it will be found that at a heat just below that at which the
paper chars, the salt is decomposed. Where the heat is greatest, the
silver is revived, and immediately around it, the paper becomes a deep
blue; beyond this a pretty decided green color results, and beyond the
green, a yellow or yellow brown stain is made. This exhibits a
remarkable analogy between heat and light,--before spoken of in chap.
II--and is of some practical importance in the preparation of the paper.

PRISMATIC ANALYSIS.--The method of accomplishing the prismatic
decomposition of rays of light by the spectrum has already been
described on pages 22 and 23. The color of the impressed spectrum, on
paper washed with nitrate of silver, is at first, a pale brown, which
passes slowly into a deeper shade; that portion corresponding with the
blue rays becoming a blue brown; and under the violet of a peculiar
pinkey shade, a very decided green tint, on the point which corresponds
with the least refrangible blue rays, may be observed, its limits of
action being near the centre of the yellow ray, and its maximum about
the centre of the blue, although the action up to the edge of the
violet ray is continued with very little diminution of effect; beyond
this point the action is very feeble.

When the spectrum is made to act on paper which has been previously
darkened, by exposure to sunshine under cupro-sulphate of ammonia, the
phenomena are materially different. The photographic spectrum is
lengthened out on the red or negative side by a faint but very visible
red portion, which extends fully up to the end of the red rays, as seen
by the naked eye. The tint of the general spectrum, too, instead of
brown is dark grey, passing, however, at its most refracted or positive
end into a ruddy brown.

In its Photographic application, the nitrate of silver is the most
valuable of the salts of that metal, as from it most of the other
argentine compounds can be prepared, although it is not of itself
sufficiently sensible to light to render it of much use.