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 subst.i.tutions 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 cla.s.s 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 subst.i.tuting 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 HH H H | | || | | (1) H-C-O-C-H.(2) H-C-C-O-C-C-H.

| | || | | H H HH H H

301. Subst.i.tutions.--A large number of other subst.i.tutions can be made in each symbol, thus giving rise to as many different compounds.

In CH4, by subst.i.tuting 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 subst.i.tute the radical CH3, giving the graphic and common formulae. Also subst.i.tute 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 subst.i.tutions. 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 c.o.ke 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 pa.s.ses 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 c.o.ke; M, purifiers-ferric oxide or lime; G, gas-holder. In C remain the c.o.ke 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. c.o.ke 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.

The FeS is revivified by exposure to the air. 2 FeS + O3 = Fe2O3 + 2S. It can then be used again. H2S, if not separated, burns with the gas, forming H2S03, which oxidizes in the air to H2SO4; hence the need of removing it. CO2 diminishes the illuminating power.

306. Composition.--Even when freed from its impurities coal-gas is a very complex mixture, the chief components being nearly as follows:--

Percent Diluents, having little C, give H45) very little light. Notice the small CH,41) diluents. percentage of luminants, or light- CO5 ) giving compounds, also the proportion C,HB1.3) of C to H in them.