~The grouping of compounds in classes.~ The study of organic chemistry is much simplified by the fact that the large number of bodies included in this field may be grouped in classes of similar compounds. It thus becomes possible to study the properties of each class as a whole, in much the same way as we study a group of elements. The most important of these classes are the _hydrocarbons_, the _alcohols_, the _aldehydes_, the _acids_, the _ethereal salts_, the _ethers_, the _ketones_, the _organic bases_, and the _carbohydrates_. A few members of each of these classes will now be discussed briefly. THE HYDROCARBONS Carbon and hydrogen combine to form a large number of compounds. These compounds are known collectively as the _hydrocarbons_. They may be divided into a number of groups or series, each being named from its first member. Some of the groups are as follows: METHANE SERIES CH_{4} methane C_{2}H_{6} ethane C_{3}H_{8} propane C_{4}H_{10} butane C_{5}H_{12} pentane C_{6}H_{14} hexane C_{7}H_{16} heptane C_{8}H_{18} octane ETHYLENE SERIES C_{2}H_{4} ethylene C_{3}H_{6} propylene C_{4}H_{8} butylene BENZENE SERIES C_{6}H_{6} benzene C_{7}H_{8} toluene C_{8}H_{10} xylene ACETYLENE SERIES C_{2}H_{2} acetylene C_{3}H_{4} allylene Only the lower members (that is, those which contain a small number of carbon atoms) of the above groups are given. The methane series is the most extensive, all of the compounds up to C_{24}H_{50} being known. It will be noticed that the successive members of each of the above series differ by the group of atoms (CH_{2}). Such a series is called an _homologous series_. In general, it may be stated that the members of an homologous series show a regular gradation in most physical properties and are similar in chemical properties. Thus in the methane group the first four members are gases at ordinary temperatures; those containing from five to sixteen carbon atoms are liquids, the boiling points of which increase with the number of carbon atoms present. Those containing more than sixteen carbon atoms are solids. ~Sources of the hydrocarbons.~ There are two chief sources of the hydrocarbons, namely, (1) crude petroleum and (2) coal tar.