In order to make clear to those readers who have not studied modern organic chemistry, what we do know on the subject, a few words of introduction will be necessary. All organic compounds contain carbon, in combination with hydrogen, and very frequently also with oxygen, nitrogen, and other elements. A single atom of carbon is able to combine with 4 atoms of hydrogen, as it does to form marsh gas, or methyl hydride, CH_{4}. Other elements may be substituted for the hydrogen; for instance, if we replace 3 of the hydrogen atoms with chlorine, we obtain chloroform, CHCl_{3}. Again an atom of oxygen may be inserted between the carbon atom and one of the hydrogen atoms, producing methyl hydroxide or wood spirit. The group CH_{3} is called methyl, and we may substitute in wood spirit this entire methyl group for one of the atoms of hydrogen, when we shall have ordinary alcohol, C_{2}H_{5}OH. This building-up process may be repeated almost _ad infinitum_, producing a whole series of alcohols of higher and higher boiling point as the atoms of carbon become more numerous. Again, if in wood spirit we substitute an atom of oxygen for 2 of the remaining atoms of hydrogen we obtain formic acid, CHO.OH, the first of a long series of acids, of which the second, corresponding to ordinary alcohol, is acetic acid, and the highest members, such as stearic acid, C_{18}H_{35}O.OH, are solid fats. Hence the whole series are commonly called the fatty acids. A few structural formulæ will serve to make these points clearer, but it may be well to say that such formulæ must be taken simply as indicating the order in which the different atoms are united, and in no sense their actual position in space. The atoms in a molecule are held together by attractions and are in continual motion, so that they are more comparable to the planets of the solar system than to a rigid shape. _Methyl Hydride._ _Methyl Alcohol._ _Common Alcohol._ H H H H │ │ │ │ H──C──H H──C──O──H H──C──C──O──H │ │ │ │ H H H H _Chloroform._ _Formic Acid._ _Acetic Acid._ Cl O H O │ ║ │ ║ H──C──Cl H──C──O──H H──C──C──O──H │ │ Cl H In benzene, C_{6}H_{6} we have a compound of another type. There is reason to think that the carbon atoms in this case are united in a ring, as shown, H H H │ │ │ C──C══C ║ │ C──C══C │ │ │ H H H