A B C D E F G H I J K L M N O P R S T U V W X Z

An Introduction to Chemical Science

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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
stratified appearance. The rocks of the second class are
recognized by their resemblance to the products of modern
volcanoes and their non-stratified appearance. Rocks of the third
class are composed of crystals, which, though often very minute,
are minerals having a definite chemical composition. Examples of
the third class are gneiss, slate, schist, and marble. The last
two classes abound on the Eastern sea-board, while the interior
of our continent is composed almost exclusively of stratified
sedimentary rocks.

289. Composition.--Rocks are not definite compounds, but variable
mixtures of minerals. Some, however, are tolerably pure, as
limestone (CaCO3) and sand-stone.

Granite is mainly made up of three minerals,--quartz, feldspar,
and mica. Quartz, when pure, is SiO2. Feldspar is a mixed
silicate of K and Al, and often several other metals, K2Al2Si6O16
(=K2O, Al2O3, 6 SiO2) symbolizing one variety, while a variety of
mica is H8Mg5Fe7Al2Si3O18.

The pupil should learn to distinguish the different minerals in
granite. Quartz is glassy, mica is in scales, usually white or
black, and feldspar is the opaque white or red mineral.

290. Importance of Siliceous Rocks.--Slate and schist are also
mixed silicates. Pure sandstone is SiO2, the red variety being
colored by iron. Igneous rocks are always siliceous. Obsidian is
a glassy silicate. A mountain of very pure glass, obsidian, two
hundred feet high, has lately been found in the Yellow-stone
region. We see how important Si is, in the compounds Si02 and the
silicates, as a constituent of the terrestrial crust. Limestone
is the only extensive rock from which it is absent. Always
combined with O, it is, next to the latter, the most abundant of
elements. Silicates of Al, Fe, Ca, K, Na, and Mg are most common,
and these metals, in the order given, rank next in abundance.

291. Soils.--Beds of sand, clay, etc., are disintegrated rock.
Sand is chiefly SiO2; clay is decomposed feldspar, slatestone,
etc. Soils are composed of these with an added portion of
carbonaceous matter from decaying vegetation, which imparts a
dark color. The reddish brown hue so often observed in soils and
rocks results from ferric salts.

292. Minerals, of which nearly 1000 varieties are now known, may
be simple substances, as graphite and sulphur, or compounds, as
galena and gypsum. Only seven systems of crystallizations are
known, but these are so modified as to give hundreds of forms of
crystals. See Physics. A given chemical substance usually occurs
in one system only, but we saw in the case of S that this was not
always true.

Crystals of some substances deliquesce, or take water from the
air, and thus dissolve themselves. Some compounds cannot exist in
the crystalline form without a certain percentage of water. This
is called "water of crystallization"; if it passes into the air
by evaporation, the crystal crumbles to a powder- and is then
said to effloresce.

293. The Earth's Interior.--We are ignorant of the chemistry of
the earth's interior. The deepest boring is but little more than
a mile, and volcanic ejections probably come from but a very few
miles below the surface. The specific gravity of the interior is
known to be more than twice that of the surface rock. From this
it has been imagined that towards the center heavy metals like Fe
and Au predominate; but this is by no means certain, since the
greater pressure at the interior would cause the specific gravity
of any substance to increase.

294. Percentage of Elements.--Compute the percentage of O in the
following rocks, which compose a large proportion of the earth's
crust: SiO2, Al2SiO4, CaCO3. Find the percentage of O in pure
water. In air. Taking cellulose, C16H30O15, as the basis, find
the percentage of O in vegetation.

An estimate, based on Bunsen's analysis of rocks, of the chief
elements in the earth's crust, is as follows:--


O, 46 per cent Ca, 3 per cent
Si, 30 per cent Na, 2 per cent
Al, 8 per cent K, 2 per cent
Fe, 6 per cent Mg, 1 per cent


More than half the elements are known to exist in sea-water, and
the rest are thought to be there, though dissolved in such small
quantity as to elude detection. What four are found in the
atmosphere?CHAPTER LIV.

ORGANIC CHEMISTRY.

295. General Considerations.--Inorganic chemistry is the
chemistry of minerals, or unorganized bodies. Organic chemistry
was formerly defined as the chemistry of the compounds found in
plants and animals; but of late it has taken a much wider range,
and is now defined as the chemistry of the C compounds, since C
is the nucleus around which other elements centre, and with which
they combine to form the organic substances. New organic
compounds are constantly being discovered and synthesized, so
that nearly 100,000 are now known. The molecule of organic matter
is often very complex, sometimes containing hundreds of atoms.

In organic as in inorganic chemistry, atoms are bound together by
chemical affinity, though it was formerly supposed that an
additional or vital force was instrumental in forming organic
compounds. For this reason none of these substances, it was
thought, could be built up in the laboratory, although many had
been analyzed. In 1828 the first organic compound, urea, was
artificially prepared, and since then thousands have been
synthesized. They are not necessarily manufactured from organic
products, but can be made from mineral matter.

296. Molecular Differences.--Molecules may differ in three ways:
(1) In the kind of atoms they contain. Compare CO2 and CS2. (2)
In the number of atoms. Compare CO and CO2. (3) In the
arrangement of atoms, i.e. the molecular structure. Ethyl alcohol
and methyl ether have the same number of the same elements,
C2H6O, but their molecular structure is not the same, and hence
their properties differ.

Qualitative analysis shows what elements enter into a compound;
quantitative analysis shows the proportion of these elements;
structural analysis exhibits molecular structure, and is the
branch to which organic chemists are now giving particular
attention. `

A specialist often works for years to synthesize a series of
compounds in the laboratory.

297. Sources.--Some organic products are now made in a purer and
cheaper form than Nature herself prepares them. Alizarine, the
coloring principle of madder, was until lately obtained only from
the root of the madder plant; now it is almost wholly
manufactured from coal-tar, and the manufactured article serves
its purpose much better than the native product. Ten million
dollars' worth is annually made, and Holland, the home of the
plant, is giving up madder culture. Artificial naphthol-scarlet
is abolishing the culture of the cochineal insect. Indigo has
also been synthesized. Certain compounds have been predicted from
a theoretical molecular structure, then made, and afterwards
found to exist in plants. Others are made that have no known
natural existence. The source of a large number of artificial
organic products is coal-tar, from bituminous coal. Saccharine, a
compound with two hundred and eighty times the sweetening power
of sugar, is one of its latest products. Wood, bones, and various
fermentable liquids are other sources of organic compounds.

298. Marsh-Gas Series.--The chemistry of the hydro-carbons
depends on the valence of C, which, in most cases, is a tetrad.
Take successively 1, 2, and 3 C atoms, saturate with H, and note
the graphic symbols:--


H H H H H H
| | | | | |
H-C-H, or CH4. H-C-C-H, or? H-C-C-C-H, or ?
| | | | | |
H H H H H H

Write the graphic and common symbols for 4, 5, and 6 C atoms,
saturated with H. Notice that the H atoms are found by doubling
the C atoms and adding 2. Hence the general formula for this
series would be CnH2n+2. Write the common symbol for C and H with
ten atoms of C; twelve atoms; thirteen. This series is called the
marsh-gas series. The first member, CH4 methane, or marsh gas,
may be written CH3H, methyl hydride, CH3 being the methyl
radical. C2H6, ethane, the second one, is ethyl hydride, C2H5H.
Theoretically this series extends without limit; practically it
ends with C35H72.

In each successive compound of the following list, the C atoms
increase by unity. Give the symbols and names of the compounds,
and commit the latter to memory:--


Boiling-point.
1. CH4 methane, or CH3H, methyl hydride, gas.
2. C2H6 ethane, C2H5H, ethyl hydride, gas
3. C3H8 propane, C3H7H, propyl hydride, gas
4. ? butane, ? ? 1 degree
5. ? pentane ? ? 38 degrees
6. ? hexane, ? ? 70 degrees
7. ? heptane, ? ? 98 degrees
8. ? octane, ? ? 125 degrees
9. ? nonane, ? ? 148 degrees
10.? dekane, ? ? 171 degrees


Note a successive increase of the boiling-point of the compounds.
Crude petroleum contains these hydro-carbons up to 10.
Petroleumissues from the earth, and is separated into the
different oils by fractional distillation and subsequent
treatment with H2SO4, etc. Rhigoline is mostly 5 and 6; gasoline,
6 and 7; benzine, 7; naphtha, 7 and 8; kerosene, 9 and 10. Below
10 the compounds are solids. None of those named, however, are
pure compounds. Explosions of kerosene are caused by the presence
of the lighter hydro-carbons, as naphtha, etc. Notice that, in
going down the list, the proportion of C to H becomes much
greater, and the lower compounds are the heavy hydro-carbons. To
them belong vaseline, paraffine, asphaltum, etc.

299. Alcohols.--The following replacements will show how the
symbols for alcohols, ethers, etc., are derived from those of the
marsh-gas series. Notice that these symbols also exhibit the
molecular structure of the compound. In CH3H by replacing the
last H with the radical OH, we have CH3OH, methyl hydrate. By a
like replacement C2H5H becomes C2H5OH, ethyl hydrate. These
hydrates are alcohols, and are known as methyl alcohol, ethyl
alcohol, etc. The common variety is C2H5OH. How does this symbol
differ from that for water, HOH? Notice in the former the union
of a positive, and also of a negative, radical.

Complete the table below, making a series of alcohols, by
substitutions as above from the previous table.



1. CH3OH, methyl hydrate, or methyl alcohol.
2. C2H5OH, ethyl hydrate, or ethyl alcohol.
3. ? ? ?
4. ? ? ?
5. ? ? ?

Continue in like manner to 10.

The graphic symbol for CH3OH is---

H
|
H-C-OH;
|
H

for C2H5OH it is--

H H
| |
H-C-C-OH.
| |
H H

Write it for the next two.


300. Ethers.--Another interesting class of compounds are the
oxides of the marsh-gas series. In this series, O replaces H.
CH3H becomes (CH3)2O, and C2H5H becomes (C2H5)2O. Why is a double
radical taken? These oxides are ethers, common or sulphuric ether
being (C2H5)2O. Complete this table, by substituting O in place
of H, in the table on page 176.


1. (CH3)2O, methyl oxide, or methyl ether.
2. (C2H5)2O, ethyl oxide, or ethyl ether.
3. ? ? ?
4. ? ? ?
5, etc. ? ? ?

Graphically represented the first two are:--

H H H H H H
| | | | | |
(1) H-C-O-C-H. (2) H-C-C-O-C-C-H.
| | | | | |
H H H H H H


301. Substitutions.--A large number of other substitutions can be
made in each symbol, thus giving rise to as many different
compounds.


In CH4, by substituting 3 Cl for 3 H,--


H Cl
| |
H-C-H becomes H-C-CI, or CHCl3,the symbol for chloroform.
| |
H Cl


Replace successively one, two, and four atoms with Cl, and write
the common symbols. Make the same changes with Br. For each atom
of H in CH4 substitute the radical CH3, giving the graphic and
common formulae. Also substitute C2H5. Are these radicals
positive or negative? From the above series of formulae, of which
CH4 is the basis, are derived, in addition to the alcohols and
ethers, the natural oils, fatty acids, etc.

302. Olefines.--A second series of hydro-carbons is represented
by the general formula CnH2n. The first member of this series is
C2H4 or, graphically,--


H H
| |
C = C.
| |
H H

Compare it with that for C2H6, in the first series, noting
the apparent molecular structure of each.

H H
| |
C = C - C - H, or C3H6 is the second member.
| | |
H H H


Write formulae for the third and fourth members.

Write the common formulae for the first ten of this series. This
is the olefiant-gas series, and to it belong oxalic and tartaric
acids, glycerin, and a vast number of other compounds, many of
which are derived by replacements.

303. Other Series.--In addition to the two series of hydro-
carbons above given, CnH2n+2 and CnH2n, other series are known
with the general formulm CnH2n-2, CnH2n-4, CnH2n-6, CnH2n-8,
etc., as far as CnH2n-32, or C26H2O. Each of these has a large
number of representatives, as was found in the marsh-gas series.
Not far from two hundred direct compounds of C and H are known,
not to mention substitutions. The formula CnH2n-6 represents a
large and interesting group of compounds, called the benzine
series. This is the basis of the aniline dyes, and of many
perfumes and flavors.

Chapter LV.

ILLUMINATING GAS.

304. Source.--The three main elements in combustion are O, H, C.
Air supplies O, the supporter; C and H are usually united, as
hydro-carbons, in luminants and combustibles. H gives little
light in burning; C gives much. The fibers of plants contain
hydro-carbons, and by destructive distillation these are
separated, as gases, from wood and coal, and used for
illuminating purposes. Mineral coal is fossilized vegetable
matter; anthracite has had most of the volatile hydro-carbons
removed by distillation in the earth; bituminous and cannel coals
retain them. These latter coals are distilled, and furnish us
illuminating gas.

Experiment 129.--Put into a t.t. 20 g. of cannel coal in fine
pieces. Heat, and collect the gas over H2O. Test its
combustibility. Notice any impurities, such as tar, adhering to
the sides of the t.t., or of the receiver after combustion. Try
to ignite a piece of cannel coal by holding it in a Bunsen flame.
Is it the C which burns, or the hydrocarbons? Distil some wood
shavings in a small ignition-tube, and light the escaping gas.

305. Preparation and Purification.--To make illuminating gas,
fire-clay retorts filled with coal are heated to 1100 degrees or
more, over a fire of coke or coal. Tubes lead the distilled gas
into a horizontal pipe, called the hydraulic main, partly filled
with water, into which the ends of the gas-pipe dip. The gas then
passes through condensers consisting of several hundred feet of
vertical pipe, through high towers, called washers, in which a
fine spray Fig. 60. Gas Works.

A, furnace; C, retorts containing coal; T, gas-tubes leading to
B, the hydraulic main; D, condensers; O, washers, with a spray of
water, and sometimes coke; M, purifiers-ferric oxide or lime; G,
gas-holder. In C remain the coke and gas carbon. At B, D, E, and
O, coal tar, H2O, NH3, CO2, and SO2 are removed. At M are taken
out H2S and CO2.of water falls, into chambers with shelves
containing the purifiers CaO or hydrated Fe2O3, and finally into
a gas-holder, whence it is distributed. At the hydraulic main,
condensers, washers, and purifiers, certain impurities are
removed froth the gas. Coke is the solid C residue after
distillation. Gas-carbon, also a solid, is formed by the
separation of the heavier hydro-carbons at high temperature, and
is deposited on the sides of the retort.

Coal gas, as it leaves the retort, has many impurities. It is
accompanied with about 3 its weight of coal tar, 1/2 its weight
of H2O vapor, 1/50 NH3, 1/20 CO2, 1/20 to 1/50 H2S, 1/300 to
1/600 S in other forms. The tar is mostly taken out at the
hydraulic main, which also withdraws some H2O with other
impurities in solution. The condensers remove the rest of the
tar, and the H2O, except what is necessary to saturate the gas.
At the main, the condensers, and the washers, NH3 is abstracted,
CO2 and H2S are much reduced, and the other S compounds are
diminished. Lime purification removes CO2 and H2S, and, to some
extent, other S compounds. Iron purification removes H2S. Fe2O3 +
3 H2S = 2 FeS + S + 3 H2O.

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