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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.


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:--

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

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