The quinones are compounds in which the hydrogen atoms of the benzene nucleus are replaced by a dyad group (O).

8. Aromatic Alcohols, Aldehydes, and Ketones.-The aromatic alcohols are isomeric with the phenols, but contain the OH in the side-group, so that they contain the group CH2OH, and hence are primary alcohols. The aromatic aldehydes are the products of oxidation of these alcohols.

The aromatic ketones, like the ketones of the methane series, contain the dyad group (CO)" linking together two hydrocarbon radicals, of which at least one contains the benzene nucleus.

9. Phenol Alcohols and Phenol Aldehydes.—When both hydrogen of the benzene nucleus and hydrogen of the side-group are replaced by OH groups, we have a phenol alcohol, and this by oxidation of the side-group yields a phenol aldehyde.

10. Aromatic Acids and Phenol Acids.-The product of the full oxidation of the aromatic alcohol is the aromatic acid. It contains the benzene nucleus joined with one or more COOH groups. If the benzene nucleus contain an OH group directly attached in addition, we have the phenol acid.

Under the head of aromatic compounds containing more than one nucleus we have several distinct cases to note.

1. Compounds with Several Uncondensed Nuclei.We have here included the diphenyl group, the diphenylmethane and triphenyl-methane groups, and the indigo group.

2. Compounds with Two Condensed Nuclei.-This includes naphthalene and its derivatives.

3. Compounds with Three Condensed Nuclei.-This includes anthracene, phenanthrene, and their derivatives.

4. Compounds containing Nitrogen in the Nucleus.This includes pyridine, quinoline, and acridine, the first of which corresponds to benzene, the second to naphthalene, and the third to anthracene, with one CH group of the nucleus replaced by nitrogen.

The pharmaceutically and medicinally important classes of alkaloids and ptomaines follow next. While these compounds have not as yet been sufficiently studied to enable us to classify them in all respects, they appear to be in large part complex derivatives of the bases pyridine and quinoline just referred

to.

The classes of terpenes and camphors are also important as present in the many naturally occurring essential oils. With these are also to be considered their products of oxidation, the

resins. Of these several groups are noted, such as oleo-resins, gum-resins, balsams, and hard resins.

The important class of glucosides are also considered as supplementary to the regularly classified compounds because of the variety of their composition, as shown by the decomposition products. With them are also noted a number of technically important dye woods and other vegetable principles.

Lastly, the protein substances are to be considered. We note in this connection,

1. The classification of cell-forming substances.

2. The decomposition of proteid matter.

3. Ferments and enzymes.

CHAPTER II.

OPEN-CHAIN OR ALIPHATIC HYDROCARBONS.

I. THE SATURATED HYDROCARBONS, OR PARAFFIN SERIES.

1. Composition, Nomenclature, and Molecular Constitution. We have shown that, while the single atom of tetrad carbon can take up four atoms of hydrogen to form CH4, two atoms of carbon, when linked together in the same molecule, can take up but six atoms of hydrogen, and three carbon atoms eight atoms of hydrogen. This is shown in the graphic formulas

We have stated under the heading of "Homologous Series" that the difference between any two members of such a series is uniformly CH2. We are, therefore, able to give a general formula for the entire series, by means of which the formula of any individual hydrocarbon in the series can be deduced. This general formula is CH2n+2; that is, for a given number of carbon atoms there will be needed for saturation twice as many hydrogen atoms plus two. More than that number, it is seen from the graphic formulas, cannot be attached.

The first three hydrocarbons of this series, CH, C2H ̧, CH ̧, can only be written structurally as represented above. The next one, however, C4H10, may be written in either one of two ways,—

In fact, these different graphic formulas represent two distinct compounds, one boiling at +1° C. and the other at -17° C.,

and known respectively as normal butane and iso-butane, which are isomeric with each other. In the case of C5H12, three isomeric compounds are possible and have been prepared, and with CH1, five isomers are possible and are known.

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The hydrocarbons of this series have been prepared as far as C35H72 They are named by taking the names first applied to the radicals in the case of the first four members of the series, and after that the Greek numerals, and applying the uniform terminaA table of the hydrocarbons of the paraffin series, with melting and boiling points, is attached.

SATURATED HYDROCARBONS-CH2n+2.

2. Occurrence, Preparation, and Description of the More Important. The hydrocarbons of the paraffin series occur abundantly, ready formed in nature, in various crude petroleums. The

lower members of the series, which are gaseous at ordinary temperatures, also occur as natural gas, which escapes from the earth in many localities; the middle members of the series make up the bulk of the petroleums, and hold dissolved, when first taken from the earth, both the gases and the higher members of the series, which are solids. These latter may occur by themselves also as ozokerite, or natural paraffine.

These hydrocarbons are also formed in the dry distillation of many naturally-occurring substances, such as bituminous coal, shales, wood, and from fish oils when distilled under strong pressure.

The first of the series, methane, is also found abundantly in nature as a product of decomposition, and owes its common name, "marsh gas, to such occurrence. Mixed with small quantities of carbon dioxide and nitrogen, it is formed whenever vegetable matter decomposes in the presence of water, as in the bottom of marshes and springs. Its formation here is due to the slow decomposition of the woody fibre under the special conditions of moisture, and probably the presence of micro-organisms, as it is known that cellulose may undergo a fermentative decomposition in their presence, with carbon dioxide and methane as sole products, according to the reaction: CH10O+ H2O = 3CO2+ 3CH4.

Methane also forms by the slow decomposition and change of bituminous coal, and hence is present in abundance in the galleries of coal-mines which are not properly ventilated. When mixed with air it constitutes the dangerous and explosive mixture known as "fire-damp." Under the name of "light carburetted hydrogen" it is known also in the distillation products of these coals. Thus, coal gas, as manufactured for illuminating purposes, contains from 30 to 40 per cent. of methane; "water gas," made by the action of steam on incandescent carbon, contains from 6 to 12 per cent. ; while natural gas, now used in large quantities for fuel purposes, contains from 90 to 95 per cent. of methane.

Methane may be artificially formed by a variety of methods: Thus, carbon monoxide and hydrogen may be made to unite under the influence of electricity: CO + 3H2=CH1 + H2O. Or a mixture of carbon disulphide vapor and hydrogen sulphide passed over ignited copper will react: CS2+ 2H2S + 8Cu CH1+4Cu,S. It is generally formed (although somewhat contaminated with ethylene and hydrogen) by the heating

=