Moissan (Paris, 1893) produced diamonds artificially by means of the high temperature attained in the electrical furnace[17] by dissolving carbon in molten cast iron, and allowing the solution with an excess of carbon, to cool under the powerful pressure exerted by rapidly cooling the metal.[17 bis] K. Chroustchoff attained the same end by means of silver, which dissolves carbon to the extent of 6 p.c. at a high temperature. Rousseau, for the same purpose, heated carbide of calcium in the electric furnace. There is no doubt that all these investigators obtained the diamond as a transparent body, which burnt into CO2, and possessed an exceptional hardness, but only in the form of a fine powder.
Judging from the fact that carbon forms a number of gaseous bodies (carbonic oxide, carbonic anhydride, methane, ethylene, acetylene, &c.) and volatile substances (for example, many hydrocarbons and their most simple derivatives), and considering that the atomic weight of carbon, C = 12, approaches that of nitrogen, N = 14, and that of oxygen, O = 16, and that the compounds CO (carbonic oxide) and N2C2 (cyanogen) are gases, it may be argued that if carbon formed the molecule C2, like N2 and O2, it would be a gas. And as through polymerism or the combination of like molecules (as O2 passes into O3 or NO2 into N2O4) the temperatures of ebullition and fusion rise (which is particularly clearly proved with the hydrocarbons of the CnH2n series), it ought to be considered that the molecules of charcoal, graphite, and the diamond are very complex, seeing that they are insoluble, non-volatile, and infusible. The aptitude which the atoms of carbon show for combining together and forming complex molecules appears in all carbon compounds. Among the volatile compounds of carbon many are well known the molecules of which contain C5 ... C10 ... C20 ... C30, &c., in general Cn where n may be very large, and in none of the other elements is this faculty of complexity so developed as in carbon.[18] Up to the present time there are no grounds for determining the degree of polymerism of the charcoal, graphite, or diamond molecules, and it can only be supposed that they contain Cn where n is a large quantity. Charcoal and those complex non-volatile organic substances which represent the gradual transitions to charcoal[19] and form the principal solid substances of organisms, contain a store or accumulation of internal power in the form of the energy binding the atoms into complex molecules. When charcoal or complex compounds of carbon burn, the energy of the carbon and oxygen is turned into heat, and this fact is taken advantage of at every turn for the generation of heat from fuel.[20]
No other two elements are capable of combining together in such variety as carbon and hydrogen. The hydrocarbons of the CnH2m series in many cases differ widely from each other, although they have some properties in common. 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.[21] 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[22] displaced by metals, even by such as sodium and potassium. On the application of more or less heat all hydrocarbons decompose[23] 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.[24] 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 CnH2m 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: CH2, CH4, CH6 ... C2H2, C2H4, C2H6, C2H8 ... 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 CnH2n + 2. These so-called saturated hydrocarbons are incapable of combination.[25] The hydrocarbons CH6, C2H8, C3H10, &c.... do not exist. Those containing the maximum amount of hydrogen will be represented by CH4 (n = 1, 2n + 2 = 4), C2H6 (n = 2), C3H8 (n = 3), C4H10, &c. This may be termed the law of limits. Placing this in juxtaposition with the law of even numbers, it is easy to perceive that the possible hydrocarbons can be ranged in series, the terms of which may be expressed by the general formulæ CnH2n+2, CnH2n, CnH2n-2, &c.... Those hydrocarbons which belong to any one of the series expressible by a general formula are said to be homologous0 with one another. Thus, the hydrocarbons CH4, C2H6, C3H8, C4H10, &c.... are members of the limiting (saturated) homologous series CnH2n+2. That is, the difference between the members of the series is CH2.[26] Not only the composition but also the properties of the members of a series tend to classification in one group. For instance, the members of the series CnH2n+2 are not capable of forming additive compounds, whilst those of the series CnH2n are capable of combining with chlorine, sulphuric anhydride, &c.; and the members of the CnH2n-6 group, belonging to the coal tar series, are easily nitrated (give nitro-compounds, Chapter [VI].), and have other properties in common. The physical properties of the members of a given homologous series vary in some such manner as this; the boiling point generally rises and the internal friction increases as n increases[27]—that is, with an increase in the relative amount of carbon and the atomic weight; the specific gravity also regularly changes as n becomes greater.[28]
Many of the hydrocarbons met with in nature are the products of organisms, and do not belong to the mineral kingdom. A still greater number are produced artificially. These are formed by what is termed the combination of residues. For instance, if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated, this latter absorbs the sulphur from both the compounds, and the liberated carbon and hydrogen combine to form a hydrocarbon, methane. If carbon be combined with any metal and this compound MCn be treated with an acid HX, then the haloid X will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon. Thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids. If a mixture of bromo-benzene, C6H5Br, and ethyl bromide, C2H5Br, be heated with metallic sodium, the sodium combines with the bromine of both compounds, forming sodium bromide, NaBr. From the first combination the group C6H5 remains, and from the second C2H5. Having an odd number of hydrogen atoms, they, in virtue of the law of even numbers, cannot exist alone, and therefore combine together forming the compound C6H5.C2H5 or C8H10 (ethylbenzene). Hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds, especially by heating—that is, by dry distillation. For instance, gum-benzoin contains an acid called benzoic acid, C7H6O2, the vapours of which, when passed through a heated tube, split up into carbonic anhydride, CO2, and benzene, C6H6. Carbon and hydrogen only unite directly in one ratio of combination—namely, to form acetylene, having the composition C2H2, which, as compared with other hydrocarbons, exhibits a very great stability at a somewhat high temperature.[29]
There is one substance known among the saturated hydrocarbons composed of 1 atom of carbon and 4 atoms of hydrogen; this is a compound containing the highest percentage of hydrogen (CH4 contains 25 per cent. of hydrogen), and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon. This saturated hydrocarbon, CH4, is called marsh gas or methane. If vegetable or animal refuse suffers decomposition in a space where the air has not free access, or no access at all, then the decomposition is accompanied with the formation of marsh gas, and this either at the ordinary temperature, or at a comparatively much higher one. On this account plants, when decomposing under water in marshes, give out this gas.[29 bis] It is well known that if the mud in bogs be stirred up, the act is accompanied with the evolution of a large quantity of gas bubbles; these may, although slowly, also separate of their own accord. The gas which is evolved consists principally of marsh gas.[30] If wood, coal, or many other vegetable or animal substances are decomposed by the action of heat without access of air—that is, are subjected to dry distillation—they, in addition to many other gaseous products of decomposition (carbonic anhydride, hydrogen, and various other substances), evolve a great deal of methane. Generally the gas which is used for lighting purposes is obtained by this means and therefore always contains marsh gas, mixed with dry hydrogen and other vapours and gases, although it is subsequently purified from many of them.[31] As the decomposition of the organic matter, which forms coal, is still going on underground, the evolution of large quantities of marsh gas frequently occurs in coal-mines.[32] When mixed with air it forms an explosive mixture, which forms one of the great dangers of coal mining, as subterranean work has always to be carried on by lamp-light. This danger is, however, overcome by the use of Humphry Davy's safety lamp.[33] Sir Humphry Davy observed that on introducing a piece of wire gauze into a flame, it absorbs so much heat that combustion does not proceed beyond it (the unburnt gases which pass through it may be ignited on the other side). In accordance with this, the flame of the Davy lamp is surrounded with a thick glass (as shown in the drawing), and has no communication whatever with the explosive mixture except through a wire gauze which prevents it igniting the mixture of the marsh-gas issuing from the coal with air. In some districts, particularly in those where petroleum is found—as, for instance, near Baku, where a temple of the Indian fire-worshippers was built, and in Pennsylvania, and other places—marsh gas in abundance issues from the earth, and it is used, like coal gas, for the purposes of lighting and warming.[34] Tolerably pure marsh gas[35] may be obtained by heating a mixture of an acetate with an alkali. Acetic acid, C2H4O2, on being heated is decomposed into marsh gas and carbonic anhydride, C2H4O2 = CH4 + CO2.
An alkali—for instance, NaHO—gives with acetic acid a salt, C2H3NaO2, which on decomposition retains carbonic anhydride, forming a carbonate, Na2CO3, and marsh gas is given off:
C2H3NaO2 + NaHO = Na2CO3 + CH4
Marsh gas is difficult to liquefy; it is almost insoluble in water, and is without taste or smell. The most important point in connection with its chemical reactions is that it does not combine directly with anything, whilst the other hydrocarbons which contain less hydrogen than expressed by the formula CnH2n + 2 are capable of combining with hydrogen, chlorine, certain acids, &c.