Acetylene is an "endothermic" compound, as has been mentioned in Chapter II., where the meaning of the expression endothermic is explained. It has there been indicated that by reason of its endothermic nature it is unsafe to have acetylene at either a temperature of 780° C. and upwards, or at a pressure of two atmospheres absolute, or higher. If that temperature or that pressure is exceeded, dissociation (i.e., decomposition into its elements), if initiated at any spot, will extend through the whole mass of acetylene. In this sense, acetylene at or above 780° C., or at two or more atmospheres pressure, is explosive in the absence of air or oxygen, and it is thereby distinguished from the majority of other combustible gases, such as the components of coal-gas. But if, by dilution with another gas, the partial pressure of the acetylene is reduced, then the mixture may be subjected to a higher pressure than that of two atmospheres without acquiring explosiveness, as is fully shown in Chapter XI. Thus it becomes possible safely to compress mixtures of acetylene and oil-gas or coal-gas, whereas unadmixed acetylene cannot be safely kept under a pressure of two atmospheres absolute or more. In a series of experiments carried out by Dupré on behalf of the British Home Office, and described in the Report on Explosives for 1897, samples of moist acetylene, free from air, but apparently not purified by any chemical process, were exposed to the influence of a bright red-hot wire. When the gas was held in the containing vessel at the atmospheric pressure then obtaining, viz., 30.34 inches (771 mm.) of mercury, no explosion occurred. When the pressure was raised to 45.34 inches (1150 mm.), no explosion occurred; but when the pressure was further raised to 59.34 inches (1505 mm., or very nearly two atmospheres absolute) the acetylene exploded, or dissociated into its elements.
Acetylene readily polymerises when heated, as has been stated in Chapter II., where the meaning of the term "polymerisation" has been explained. The effects of the products of the polymerisation of acetylene on the flame produced when the gas is burnt at the ordinary acetylene burners have been stated in Chapter VIII., where the reasons therefor have been indicated. The chief primary product of the polymerisation of acetylene by heat appears to be benzene. But there are also produced, in some cases by secondary changes, ethylene, methane, naphthalene, styrolene, anthracene, and homologues of several of these hydrocarbons, while carbon and hydrogen are separated. The production of these bodies by the action of heat on acetylene is attended by a reduction of the illuminative value of the gas, while owing to the change in the proportion of air required for combustion (see Chapter VIII.), the burners devised for the consumption of acetylene fail to consume properly the mixture of gases formed by polymerisation from the acetylene. It is difficult to compare the illuminative value of the several bodies, as they cannot all be consumed economically without admixture, but the following table indicates approximately the maximum illuminative value obtainable from them either by combustion alone or in admixture with some non- illuminating or feebly-illuminating gas:
________________________________________________
| | | |
| | | Candles per |
| | | Cubic Foot |
|______________|___________________|_____________|
| | | |
| | | (say) |
| Acetylene | C_2H_2 | 50 |
| Hydrogen | H_2 | 0 |
| Methane | CH_4 | 1 |
| Ethane | C_2H_6 | 7 |
| Propane | C_3H_8 | 11 |
| Pentane | C_5H_12 (vapour) | 35 |
| Hexane | C_6H_14 " | 45 |
| Ethylene | C_2H_4 | 20 |
| Propylene | C_3H_6 | 25 |
| Benzene | C_6H_6 (vapour) | 200 |
| Toluene | C_7H_8 " | 250 |
| Naphthalene | C_10H_8 " | 400 |
|______________|___________________|_____________|
It appears from this table that, with the exception of the three hydrocarbons last named, no substance likely to be formed by the action of heat on acetylene has nearly so high an illuminative value--volume for volume--as acetylene itself. The richly illuminating vapours of benzene and naphthalene (and homologues) cannot practically add to the illuminative value of acetylene, because of the difficulty of consuming them without smoke, unless they are diluted with a large proportion of feebly- or non-illuminating gas, such as methane or hydrogen. The practical effect of carburetting acetylene with hydrocarbon vapours will be shown in Chapter X. to be disastrous so far as the illuminating efficiency of the gas is concerned. Hence it appears that no conceivable products of the polymerisation of acetylene by heat can result in its illuminative value being improved--even presupposing that the burners could consume the polymers properly--while practically a considerable deterioration of its value must ensue.
The heat of combustion of acetylene was found by J. Thomson to be 310.57 large calories per gramme-molecule, and by Berthelot to be 321.00 calories. The latest determination, however, made by Berthelot and Matignon shows it to be 315.7 calories at constant pressure. Taking the heat of formation of carbon dioxide from diamond carbon at constant pressure as 94.3 calories (Berthelot and Matignon), which is equal to 97.3 calories from amorphous carbon, and the heat of formation of liquid water as 69 calories; this value for the heat of combustion of acetylene makes its heat of formation to be 94.3 x 2 + 69 - 315.7 = -58.1 large calories per gramme-molecule (26 grammes) from diamond carbon, or -52.1 from amorphous carbon. It will be noticed that the heat of combustion of acetylene is greater than the combined heats of combustion of its constituents; which proves that heat has been absorbed in the union of the hydrogen and carbon in the molecule, or that acetylene is endothermic, as elsewhere explained. These calculations, and others given in Chapter IX., will perhaps be rendered more intelligible by the following table of thermochemical phenomena:
_______________________________________________________________
| | | | |
| Reaction. | Diamond | Amorphous | |
| | Carbon. | Carbon. | |
|________________________________|_________|___________|________|
| | | | |
| (1) C (solid) + O . . . | 26.1 | 29.1 | ... |
| (2) C (solid) + O_2 . . . | 94.3 | 97.3 | ... |
| (3) CO + O (2 - 1) . . . | ... | ... | 68.2 |
| (4) Conversion of solid carbon | | | |
| into gas (3 - 1) . . . | 42.1 | 39.1 | ... |
| (5) C (gas) + O (1 + 4) . . | ... | ... | 68.2 |
| (6) Conversion of amorphous | | | |
| carbon to diamond . . | ... | ... | 3.0 |
| (7) C_2 + H_2 . . . . | -58.1 | -52.1 | ... |
| (8) C_2H_2 + 2-1/2O_2 . . | ... | ... | 315.7 |
|________________________________|_________|___________|________|
W. G. Mixter has determined the heat of combustion of acetylene to be 312.9 calories at constant volume, and 313.8 at constant pressure. Using Berthelot and Matignon's data given above for amorphous carbon, this represents the heat of formation to be -50.2 (Mixter himself calculates it as -51.4) calories. By causing compressed acetylene to dissociate under the influence of an electric spark, Mixter measured its heat of formation as -53.3 calories. His corresponding heats of combustion of ethylene are 344.6 calories (constant volume) and 345.8 (constant pressure); for its heat of formation he deduces a value -7.8, and experimentally found one of about -10.6 (constant pressure).
THE ACETYLENE FLAME.--It has been stated in Chapter I. that acetylene burnt in self-luminous burners gives a whiter light than that afforded by any other artificial illuminant, because the proportion of the various spectrum colours in the light most nearly resembles the corresponding proportion found in the direct rays of the sun. Calling the amount of monochromatic light belonging to each of the five main spectrum colours present in the sun's rays unity in succession, and comparing the amount with that present in the light obtained from electricity, coal-gas, and acetylene, Münsterberg has given the following table for the composition of the several lights mentioned:
______________________________________________________________________
| | | | | |
| | Electricity | Coal-Gas | Acetylene | |
| |________________|__________________|_______________|_______|
| Colour | | | | | | | |
| in | | | | | | With | |
| Spectrum.| Arc. | Incan- | Lumin- | Incan- | Alone.| 3 per | Sun- |
| | | descent.| ous. | descent.| | Cent. | light.|
| | | | | | | Air. | |
|__________|______|_________|________|_________|_______|_______|_______|
| | | | | | | | |
| Red | 2.09 | 1.48 | 4.07 | 0.37 | 1.83 | 1.03 | 1 |
| Yellow | 1.00 | 1.00 | 1.00 | 0.90 | 1.02 | 1.02 | 1 |
| Green | 0.99 | 0.62 | 0.47 | 4.30 | 0.76 | 0.71 | 1 |
| Blue | 0.87 | 0.91 | 1.27 | 0.74 | 1.94 | 1.46 | 1 |
| Violet | 1.08 | 0.17 | 0.15 | 0.83 | 1.07 | 1.07 | 1 |
| Ultra- | | | | | | | |
| Violet | 1.21 | ... | ... | ... | ... | ... | 1 |
|__________|______|_________|________|_________|_______|_______|_______|
These figures lack something in explicitness; but they indicate the greater uniformity of the acetylene light in its proportion of rays of different wave-lengths. It does not possess the high proportion of green of the Welsbach flame, or the high proportion of red of the luminous gas- flame. It is interesting to note the large amount of blue and violet light in the acetylene flame, for these are the colours which are chiefly concerned in photography; and it is to their prominence that acetylene has been found to be so very actinic. It is also interesting to note that an addition of air to acetylene tends to make the light even more like that of the sun by reducing the proportion of red and blue rays to nearer the normal figure.