AgNO3 in indelible inks depends on the same property. This may be
due to a reduction of AgNO3 to Ag4O. Stains can be removed from
the skin or from linen by a solution of Kl, or of CuCl2 followed
by sodium hyposulphite. Lunar caustic is made by fusing AgNO3
crystals, and is used for cauterizing (burning) the flesh. Much
AgCN finds use in electroplating.

Experiment 128.--Put 5 cc. AgNO3 solution in each of three t.t.
To the first add 3 cc. HCl, to the second 3cc.NaCl solution, and
to the third 3 cc. KBr solution. Write the reaction for each
case, and notice that the first two give the same ppt., as in
fact any soluble chloride would. Filter the second and third, on
separate filter papers, and expose half the residue to direct
sunlight, observing the change of color by occasionally stirring.
Solar rays reduce AgCl and AgBr, it is thought, to Ag2Cl and
Ag2Br. Try to dissolve the other half in Na2S2O3, sodium
thiosulphate solution. This experiment illustrates the main facts
of photography.

CHAPTER LI.

PHOTOGRAPHY.

284. Descriptive.--The silver halogens, AgCI, AgBr, AgI, are very
sensitive to certain light rays. Red rays do not affect them;
hence ruby glass is used in the "dark room."

Photography involves two processes. The negative of the picture
is first taken upon a prepared glass plate, and the positive is
then printed on prepared paper. The negative shows the lights and
shades reversed, while the positive gives objects their true
appearance.

Few photographers now make their own plates, these being prepared
at large manufactories. The glass is there covered on one side
with a white emulsion of gelatine and AgBr, making what are
called gelatine-bromide plates. This is done in a room dimly
lighted with ruby light. The plates are dried, packed in sealed
boxes, and thus sent to photographers. The artist opens them in
his dark room, similarly lighted, inserts the plates in holders,
film side out, covers with a slide, adjusts to the camera,
previously focused, and makes the exposure to light. The time of
exposure varies with the kind of plate, the lens, and the light,
from several  seconds, minutes, or hours, to 1/250 part of a
second in some instantaneous work. In the dark room the plates
are removed and can be at once developed, or kept for any time
away from the light. No change appears in the plate until
development, though the light has done its work.

To develop the plate, it is put into a solution of pyrogallic
acid, the developer, and carbonate of sodium, the motive power in
the process. Other developers are often used. The chemical action
here is somewhat obscure, but those parts of the plates which
were affected by the light are made visible, a part of the AgzBr
being reduced to Ag by the affinity which sodium pyrogallate has
for Br. Ag2Br = 2 Ag + Br. Br is dissolved and Ag is deposited.
When the rather indistinct image begins to fade out, the plate is
dipped for a minute into a solution of alum to harden the
gelatine and prevent it from peeling off (frilling). It is
finally soaked in a solution of sodium thiosulphate (hyposulphite
or hypo), Na2S208. This removes the AgBr that the light has
failed to reduce. The processis called fixing, as the plate may
thereafter be exposed to the light with impunity. It must be left
in this bath till all the white part, best seen on the back of
the plate, disappears. 2AgBr + 3Na2S2O3 = Ag2Na4(S2O3) + 2 NaBr.
Both products are dissolved. It is then thoroughly washed. Any
dark objects become light in the negative, and vice versa. Why?

For the positive, the best linen paper is covered on one side
with albumen, soaked in NaCl solution, dried, and the same side
laid on a solution of AgNO3. What reaction takes place? What is
deposited on the paper, and what is dissolved? This sensitized
paper, when dry, is placed over a negative, film to film, and
exposed in a printing frame to direct sunlight till much darker
than desired in the finished picture. What is dark in the
negative will be light in the positive. Why? The reducing action
of sunlight is similar to that in the negative. Explain it.

After printing, the picture is toned and fixed. Toning consists
in giving it a rich color by replacing part of the Ag2Cl with
gold from a neutral solution of AuCl3. 3 Ag2Cl+ AUCl3 = 6AgCI +
Au. Fixing removes the unaffected AgCl, as in the negative, the
same substance being used. Describe the action. 2 AgCI + 3
Na2S203 = Ag2Na4(S203) + 2 NaCl. Both the positive and the
negative must be well washed after each process, particularly
after the last. The picture is then ready for mounting. In fine
portrait work both the negative and the positive are retouched.
This consists in removing blemishes with colored pencils or India
ink.

The negative--No. 1. Dissolve: sulphite soda crystals, 2 oz. (57
g) in 8 oz. (236 cc.) water (distilled); citric acid, 60 grains
(4 g) in 1/2 oz. (15 cc.) water; bromide ammonium, 25 grains (1
1/2 g) in 1/2 oz. water; pyrogallic acid, 1 oz. (28 g) in 3 oz.
(90 cc.) water. After dissolving, mix in the order named, and
filter. No. 2. Dissolve: sulphite soda, 2 oz. (57 g) in 4 oz.
(118 cc.) water; carbonate potash, 4 oz. (113 g) in 8 oz. (236
cc.) water. Dissolve separately, mix, and filter. To develop
plates, mix 1 dram (3 2/3 cc.) of No. 1 and 1 dram of No. 2 with
2 oz. (60 cc.) water. Cover the plate with the mixture, and leave
as long as the picture increases in distinctness. Remove, wash,
and put it into a saturated solution of alum for a minute or two,
then wash and put it into a half-saturated solution of hypo.
Leave till no white AgCl is seen through the back of the plate.
Wash it well.

The positive.--1. Dissolve 30 grains (2 g.) pure gold chloride in
15 oz. (450 cc.) water. This forms a stock solution. 2. Make a
saturated solution of borax. 3. Prepare a toning bath by adding
1/2 oz. (15 cc.) of the gold chloride solution and 1 oz. (30 cc.)
of the borax solution to 7 oz. (210 cc.)  water. After printing
the picture, wash it in 3 or 4 waters, put it into the toning
bath, and leave it till considerably darker than desired; wash,
and put it for 15 minutes into a hypo solution that has been,
after saturation, diluted with 3 or 4 volumes of water. Then wash
repeatedly.

CHAPTER LII.

PLATINUM AND GOLD.

PLATINUM.

Examine platinum foil and wire.

285. Platinum is much rarer than gold, and is about two-thirds as
costly as the latter. It is found alloyed with other metals, as
An, and is obtained from sand, in which it occurs, by washing.
Aqua regia is the only acid which dissolves it, and the action is
much slower than with Au. Pt is one of the heaviest metals,
having a specific gravity three times that of Fe, or twenty-one
and a half times that of water. Its fusing-point is about 1600
degrees, or just below the temperature of the oxy-hydrogen flame.
Like Au it has little affinity for other elements, but alloys
with many metals. Pt is so tenacious that it can be drawn into
wire invisible to the naked eye, being drawn out in the center of
a silver wire, which is afterwards dissolved away from the Pt by
HNO3. Noting its valences, 2 and 4, write the symbols for the ous
and ic chlorides and oxides.

286. Uses.--Pt is much used in chemistry in the form of foil,
wire, and crucibles. On what properties does this use depend?
Describe its use in making H2SO4.

PtCl4 is made by dissolving Pt in aqua regia, and evaporating the
liquid. On heating PtCl4, half of its Cl is given up, leaving
PtCl2. If it be still more strongly heated, the Cl all passes
off, leaving spongy Pt. By fusing this in the oxy-hydrogen flame,
ordinary Pt is obtained. Spongy Pt has a remarkable power of
absorbing, or occluding, O without uniting with it. This O it
gives up to some other substances, and thus becomes indirectly an
oxidizing agent. What other element has this property of
occluding gases?

GOLD.

Examine auriferous quartz, gold chloride, yellow and ruby glass
colored with gold. 287. Gold is rarely found combined, and has
small affinity for other elements, though forming alloys with Cu,
Ag, and Hg. Its source is usually either quartz rock, called
auriferous quartz, or sand in placer mines. The element is widely
distributed, occurring in minute quantities in most soils, sea
water, etc. California and Australia are the two greatest gold-
producing countries. That from California has a light color, due
to a slight admixture of Ag. Australian gold is of a reddish hue,
due to an alloy of Cu. Gold-bearing quartz is pulverized, and
treated with Hg to dissolve the precious metal, which is then
separated from the alloy by distillation. Compare this with the
preparation of Ag.

Such is the malleability of Au that it has been hammered into
sheets not over one-millionth of an inch thick; it is then as
transparent as glass. Gold does not tarnish or change below the
melting-point. On account of its softness it is usually alloyed
with Cu, sometimes with Ag. Pure gold is twenty-four carats fine.
Eighteen carat gold has eighteen parts Au and six Cu. Gold coin
has nine parts Au to one part Cu. The most important compound is
AuCl3. Describe a use of it. This metal is much employed in
electroplating, and somewhat in coloring glass.

CHAPTER LIII.

CHEMISTRY OF ROCKS.

288. Classification.--Rocks may be divided, according to their
origin, into three classes: (1) Aqueous rocks. These have been
formed by deposition of sedimentary material, layer by layer, on
the bottoms of ancient oceans, lakes, and rivers, from which they
have gradually been raised, to form dry land. (2) Eruptive or
volcanic rocks. These have been forced, as hot fluids, through
rents and fissures from the interior of the earth. (3)
Metamorphic rocks. These, by the combined action of heat,
pressure, water, and chemical agents, have been crystallized and
chemically altered. The rocks of the first class, such as chalk,
limestone, shale, and sandstone, are distinguished by the
existence of fossils in them, or by the successive layers of the
material which goes to make up their structure and to give them a

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