of light on color. A living plant owes its brilliant hues to the
sunshine; but a dead one, or the tints extracted from it, will fade
in the same rays which clothe the tulip in crimson and gold,--as our
lady-readers who have rich curtains in their drawing-rooms know full
well. The sun, then, is a master of _chiaroscuro_, and, if he has a
living petal for his pallet, is the first of colorists.--Let us walk
into his studio, and examine some of his painting machinery.

* * * * *

1. THE DAGUERREOTYPE.--A silver-plated sheet of copper is resilvered by
electro-plating, and perfectly polished. It is then exposed in a glass
box to the vapor of iodine until its surface turns to a golden yellow.
Then it is exposed in another box to the fumes of the bromide of lime
until it becomes of a blood-red tint. Then it is exposed once more, for
a few seconds, to the vapor of iodine. The plate is now sensitive to
light, and is of course kept from it, until, having been placed in the
darkened camera, the screen is withdrawn and the camera-picture falls
upon it. In strong light, and with the best instruments, _three
seconds'_ exposure is enough,--but the time varies with circumstances.
The plate is now withdrawn and exposed to the vapor of mercury at 212 deg..
Where the daylight was strongest, the sensitive coating of the plate has
undergone such a chemical change, that the mercury penetrates readily to
the silver, producing a minute white granular deposit upon it, like
a very thin fall of snow, drifted by the wind. The strong lights are
little heaps of these granules, the middle lights thinner sheets of
them; the shades are formed by the dark silver itself, thinly sprinkled
only, as the earth shows with a few scattered snow-flakes on its
surface. The precise chemical nature of these granules we care less
for than their palpable presence, which may be perfectly made out by a
microscope magnifying fifty diameters or even less.

The picture thus formed would soon fade under the action of light, in
consequence of further changes in the chemical elements of the film
of which it consists. Some of these elements are therefore removed by
washing it with a solution of hyposulphite of soda, after which it is
rinsed with pure water. It is now permanent in the light, but a touch
wipes off the picture as it does the bloom from a plum. To fix it, a
solution of hyposulphite of soda containing chloride of gold is poured
on