All hydrocarbons, whether gaseous, liquid or solid, are combustible substances sparingly soluble or insoluble in water. The liquefied gaseous hydrocarbons, as well as those which are liquid at ordinary temperatures, and those solid hydrocarbons which have been liquefied by fusion, have the appearance and property of oily liquors, more or less viscid, or fluid. The solid hydrocarbons more or less resemble wax in their properties, although ordinary oils and wax generally contain oxygen in addition to carbon and hydrogen, but in relatively small proportion. There are also many hydrocarbons which have the appearance of tar—as, for instance, metacinnamene and gutta-percha. Those liquid hydrocarbons which boil at a high temperature are like oils, and those which have a low boiling point resemble ether, whilst the gaseous hydrocarbons in many of their properties are akin to hydrogen. All this tends to show that in hydrocarbons physically considered the properties of solid non-volatile charcoal are strongly modified and hidden, whilst those of the hydrogen predominate. All hydrocarbons are neutral substances (neither basic nor acid), but under certain conditions they enter into peculiar reactions. It has been seen in those hydrogen compounds which have been already considered (water, nitric acid, ammonia) that the hydrogen in almost all cases enters into reaction, being displaced by metals. The hydrogen of the hydrocarbons, it may be said, has no metallic character that is to say, it is not directly displaced by metals, even by such as sodium and potassium. On the application of more or less heat all hydrocarbons decompose forming charcoal and hydrogen. The majority of hydrocarbons do not combine with the oxygen of the air or oxidise at ordinary temperatures, but under the action of nitric acid and many other oxidising substances most of them undergo oxidation, in which either a portion of the hydrogen and carbon is separated, or the oxygen enters into combination, or else the elements of hydrogen peroxide enter into combination with the hydrocarbon. When heated in air, hydrocarbons burn, and, according to the amount of carbon they contain, their combustion is attended more or less with a separation of soot—that is, finely divided charcoal—which imparts great brilliancy to the flame, and on this account many of them are used for the purposes of illumination—as, for instance, kerosene, coal gas, oil of turpentine. As hydrocarbons contain reducing elements (that is, those capable of combining with oxygen), they often act as reducing agents—as, for instance, when heated with oxide of copper, they burn, forming carbonic anhydride and water, and leave metallic copper. Gerhardt proved that all hydrocarbons contain an even number of hydrogen atoms. Therefore, the general formula for all hydrocarbons is C n H 2 m where n and m are whole numbers. This fact is known as the law of even numbers . Hence, the simplest possible hydrocarbons ought to be: CH 2 , CH 4 , CH 6 ... C 2 H 2 , C 2 H 4 , C 2 H 6 , C 2 H 8 ... but they do not all exist, since the quantity of H which can combine with a certain amount of carbon is limited, as we shall learn directly.
Some of the hydrocarbons are capable of combination, whilst others do not show that power. Those which contain less hydrogen belong to the former category, and those which, for a given quantity of carbon, contain the maximum amount of hydrogen, belong to the latter. The composition of those last mentioned is expressed by the general formula C