Chapter 91

Chapter 912,273 wordsPublic domain

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:--

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:--

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.