[24] Wagner (1888) showed that when unsaturated hydrocarbons are shaken with a weak (1 p.c.) solution of potassium permanganate, KMnO4, at ordinary temperatures, they form glycols—for example, C2H4 yields C2H6O2.
[25] My article on this subject appeared in the Journal of the St. Petersburg Academy of Sciences in 1861. Up to that time, although many additive combinations with hydrocarbons and their derivatives were known, they had not been generalised, and were even continually quoted as cases of substitution. Thus the combination of ethylene, C2H4, with chlorine, Cl2, was often regarded as a formation of the products of the substitution of C2H5Cl and HCl, which it was supposed were held together as the water of crystallisation is in salts. Even earlier than this (1857, Journal of the Petroffsky Academy) I considered similar cases as true compounds. In general, according to the law of limits, an unsaturated hydrocarbon, or its derivative, on combining with rX2, gives a substance which is saturated or else approaching the limit. The investigations of Frankland with many organo-metallic compounds clearly showed the limit in the case of metallic compounds, which we shall constantly refer to later on.
[26] The conception of homology has been applied by Gerhardt to all organic compounds in his classical work, ‘Traité de Chimie Organique,’ finished in 1855 (4 vols.), in which he divided all organic compounds into fatty and aromatic, which is in principle still adhered to at the present time, although the latter are more often called benzene derivatives, on account of the fact that Kekulé, in his beautiful investigations on the structure of aromatic compounds, showed the presence in them all of the ‘benzene nucleus,’ C6H6.
[27] This is always true for hydrocarbons, but for derivatives of the lower homologues the law is sometimes different; for instance, in the series of saturated alcohols, CnH2n+1(OH), when n = 0, we obtain water, H(OH), which boils at 100°, and whose specific gravity at 15° = 0·9992; when n = 1, wood spirit CH3(OH), which boils at 66°, and at 15° has a specific gravity = 0·7964; when n = 2, ordinary alcohol, C2H5(OH), boiling at 78°, specific gravity at 15° = 0·7936, and with further increase of CH2 the specific gravity increases. For the glycols CnH2n(OH)2 the phenomenon of a similar kind is still more striking; at first the temperature of the boiling point and the density increase, and then for higher (more complex) members of the series diminish. The reason for this phenomenon, it is evident, must be sought for in the influence and properties of water, and that strong affinity which, acting between hydrogen and oxygen, determines many of the exceptional properties of water (Chapter [I].).
[28] As, for example, in the saturated series of hydrocarbons CnH2n+2, the lowest member (n = 0) must be taken as hydrogen H2, a gas which (t.c. below -190°) is liquefied with great difficulty, and when in a liquid state has doubtless a very small density. Where n = 1, 2, 3, the hydrocarbons CH4, C2H6, C3H8 are gases, more and more readily liquefiable. The temperature of the absolute boiling point for CH4 = -100°, and for ethane C2H6, and in the higher members it rises. The hydrocarbon C4H10, liquefies at about 0°. C5H12 (there are several isomers) boils at from +9° (Lvoff) to 37°, C6H14 from 58° to 78°, &c. The specific gravities in a liquid state at 15° are:—
| C5H12 | C6H14 | C7H16 | C10H22 | C16H34 |
| 0·63 | 0·66 | 0·70 | 0·75 | 0·85 |
[29] If, at the ordinary temperature (assuming therefore that the water formed will be in a liquid state) a gram molecule (26 grams) of acetylene, C2H2, be burnt, 310 thousand calories will be emitted (Thomsen), and as 12 grams of charcoal produce 97 thousand calories, and 2 grams of hydrogen 69 thousand calories, it follows that, if the hydrogen and carbon of the acetylene were burnt there would be only 2 × 97 + 69, or 263 thousand calories produced. It is evident, then, that acetylene in its formation absorbs 310–263, or 47 thousand calories.
For considerations relative to the combustion of carbon compounds, we will first enumerate the quantity of heat separated by the combustion of definite chemical carbon compounds, and then give a few figures bearing on the kinds of fuel used in practice.
For molecular quantities in perfect combustion the following amounts of heat are given out (when gaseous carbonic anhydride and liquid water are formed), according to Thomsen's data (1) for gaseous CnH2n+2: 52·8 + 158·8n thousand calories; (2) for CnH2n: 17·7 + 158·1n thousand calories; (3) according to Stohmann (1888) for liquid saturated alcohols, CnH2n+2O: 11·8 + 156·3n, and as the latent heat of evaporation = about 8·2 + 0·6n, in a gaseous state, 20·0 + 156·9n; (4) for monobasic saturated liquid acids, CnH2nO2:—95·3 + 154·3n, and as their latent heat of evaporation is about 5·0 + 1·2n, in a gaseous form, about—90 + 155n; (5) for solid saturated bibasic acids, CnH2n-2O4:—253·8 + 152·6n, if they are expressed as CnH2nC2H2O4, then 51·4 + 152·6n; (6) for benzene and its liquid homologues (still according to Stohmann) CnH2n-6:—158·6 + 156·3n, and in a gaseous form about—155 + 157n; (7) for the gaseous homologues of acetylene, CnH2n-2 (according to Thomsen)—5 + 157n. It is evident from the preceding figures that the group CH2, or CH3 substituted for H, on burning gives out from 152 to 159 thousand calories. This is less than that given out by C + H2, which is 97 + 69 or 166 thousand; the reason for this difference (it would be still greater if carbon were gaseous) is the amount of heat separated during the formation of CH2. According to Stohmann, for dextroglucose, C6H12O6, it is 673·7; for common sugar, C12H22O11, 1325·7; for cellulose, C6H10O5, 678·0; starch, 677·5; dextrin, 666·2; glycol, C2H6O2, 281·7; glycerine, 397·2, &c. The heat of combustion of the following solids (determined by Stohmann) is expressed per unit of weight: naphthalene, C10H8, 9,621; urea, CN2H4O, 2,465; white of egg, 5,579; dry rye bread, 4,421; wheaten bread, 4,302; tallow, 9,365; butter, 9,192; linseed oil, 9,323. The most complete collection of arithmetical data for the heats of combustion will be found in V. F. Longinin's work, ‘Description of the Various Methods of Determining the Heats of Combustion of Organic Compounds’ (Moscow, 1894).
The number of units of heat given out by unit weight during the complete combustion and cooling of the following ordinary kinds of fuel in their usual state of dryness and purity are:—(1) for wood charcoal, anthracite, semi-anthracite, bituminous coal and coke, from 7,200 to 8,200; (2) dry, long flaming coals, and the best brown coals, from 6,200 to 6,800; (3) perfectly dry wood, 3,500; hardly dry, 2,500; (4) perfectly dry peat, best kind, 4,500; compressed and dried, 3,000; (5) petroleum refuse and similar liquid hydrocarbons, about 11,000; (6) illuminating gas of the ordinary composition (about 45 vols. H, 40 vols. CH4, 5 vols. CO, and 5 vols. N), about 12,000; (7) producer gas (see [next Chapter]), containing 2 vols. carbonic anhydride, 30 vols. carbonic oxide, and 68 vols. nitrogen for one part by weight of the whole carbon burnt, 5,300, and for one part by weight of the gas, 910, units of heat; and (8) water gas (see [next chapter]) containing 4 vols. carbonic anhydride, 8 vols. N2, 24 vols. carbonic oxide, and 46 vols. H2, for one part by weight of the carbon consumed in the generator 10,900, and for one part by weight of the gas, 3,600 units of heat. In these figures, as in all calorimetric observations, the water produced by the combustion of the fuel is supposed to be liquid. As regards the temperature reached by the fuel, it is important to remark that for solid fuel it is indispensable to admit (to ensure complete combustion) twice the amount of air required, but liquid, or pulverised fuel, and especially gaseous fuel, does not require an excess of air; therefore, a kilogram of charcoal, giving 8,000 units of heat, requires about 24 kilograms of air (3 kilograms of air per thousand calories) and a kilogram of producer gas requires only 0·77 kilogram of air (0·85 kilo. of air per 1,000 calories), 1 kilogram of water gas about 4·5 of air (1·25 kilo. of air per 1,000 calories).