Thus it appears that past an orifice or constriction 0.5 mm. in diameter no explosion of acetylene can proceed, whatever may be the proportions between the gas and the air in the mixture present.
With every gas the explosive limits and the range of explosibility are also influenced by various circumstances, such as the manner of ignition, the pressure, and other minor conditions; but the following figures for mixtures of air and different combustible gases were obtained by Eitner under similar conditions, and are therefore strictly comparable one with another. The conditions were that the mixture was contained in a tube 19 mm. (3/4-inch) wide, was at about 60° to 65° F., was saturated with aqueous vapour, and was fired by electric spark.
Table giving the Percentage by volume of Combustible Gas in a Mixture of that Gas and Air corresponding with the Explosive Limits of such a Mixture.--(Eitner.)
____________________________________________________________________
| | | | |
| Description of | Lower | Upper | Difference between the |
| Combustible Gas. | Explosive | Explosive | Lower and Upper Limits, |
| | Limit. | Limit. | showing the range |
| | | | covered by the |
| | | | Explosive Mixtures. |
|__________________|___________|___________|_________________________|
| | | | |
| | Per Cent. | Per Cent. | Per Cent. |
| Carbon monoxide | 16.50 | 74.95 | 58.45 |
| Hydrogen | 9.45 | 66.40 | 57.95 |
| Water-gas | | | |
| (uncarburetted) | 12.40 | 66.75 | 54.35 |
| ACETYLENE | 3.35 | 52.30 | 48.95 |
| Coal-gas | 7.90 | 19.10 | 11.20 |
| Ethylene | 4.10 | 14.60 | 10.50 |
| Methane | 6.10 | 12.80 | 6.70 |
| Benzene (vapour) | 2.65 | 6.50 | 3.85 |
| Pentane " | 2.40 | 4.90 | 2.50 |
| Benzoline " | 2.40 | 4.90 | 2.50 |
|__________________|___________|___________|_________________________|
These figures are of great practical significance. They indicate that a mixture of acetylene and air becomes explosive (i.e., will explode if a light is applied to it) when only 3.35 per cent. of the mixture is acetylene, while a similar mixture of coal-gas and air is not explosive until the coal-gas reaches 7.9 per cent. of the mixture. And again, air may be added to coal-gas, and it does not become explosive until the coal-gas is reduced to 19.1 per cent. of the mixture, while, on the contrary, if air is added to acetylene, the mixture becomes explosive as soon as the acetylene has fallen to 52.3 per cent. Hence the immense importance of taking precautions to avoid, on the one hand, the escape of acetylene into the air of a room, and, on the other hand, the admixture of air with the acetylene in any vessel containing it or any pipe through which it passes. These precautions are far more essential with acetylene than with coal-gas. The table shows further how great is the danger of explosion if benzene, benzoline, or other similar highly volatile hydrocarbons [Footnote: The nomenclature of the different volatile spirits is apt to be very confusing. "Benzene" is the proper name for the most volatile hydrocarbon derived from coal-tar, whose formula is C_6H_6. Commercially, benzene is often known as "benzol" or "benzole"; but it would be generally advantageous if those latter words were only used to mean imperfectly rectified benzene, i.e., mixtures of benzene with toluene, &c., such as are more explicitly understood by the terms "90.s benzol" and "50.s benzol." "Gasoline," "carburine," "petroleum ether," "benzine," "benzoline," "petrol," and "petroleum spirit" all refer to more or less volatile (the most volatile being mentioned first) and more or less thoroughly rectified products obtained from petroleum. They are mixtures of different hydrocarbons, the greater part of them having the general chemical formula C_nH_2n+2 where n = 5 or more. None of them is a definite chemical compound as is benzene; when n = 5 only the product is pentane. These hydrocarbons are known to chemists as "paraffins," "naphthenes" being occasionally met with; while a certain proportion of unsaturated hydrocarbons is also present in most petroleum spirits. The hydrocarbons of coal-tar are "aromatic hydrocarbons," their generic formula being C_nH_2^n-6, where n is never less than 6.] are allowed to vaporise in a room in which a light may be introduced. Less of the vapour of these hydrocarbons than of acetylene in the air of a room brings the mixture to the lower explosive limit, and therewith subjects it to the risk of explosion. This tact militates strongly against the use of such hydrocarbons within a house, or against the use of air-gas, which, as explained in Chapter I., is air more or less saturated with the vapour of volatile hydrocarbons. Conversely, a combustible gas, such as acetylene, may be safely "carburetted" by these hydrocarbons in a properly constructed apparatus set up outside the dwelling-house, as explained in Chapter X., because there would be no air (as in air-gas) in the pipes, &c., and a relatively large escape of carburetted acetylene would be required to produce an explosive atmosphere in a room. Moreover, the odour of the acetylene itself would render the detection of a leak far easier with carburetted acetylene than with air-gas.
N. Teclu has investigated the explosive limits of mixtures of air with certain combustible gases somewhat in the same manner as Eitner, viz.: by firing the mixture in an eudiometer tube by means of an electric spark. He worked, however, with the mixture dry instead of saturated with aqueous vapour, which doubtless helps to account for the difference between his and Eitner's results.
Table giving the Percentages by volume of Combustible Gas in a Dehydrated Mixture of that Gas and Air between which the Explosive Limits of such a Mixture lie.--(Teclu).
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| | | |
| | Lower Explosive Limit. | Upper Explosive Limit. |
| Description of |________________________|________________________|
| Combustible Gas. | | |
| | Per Cent. of Gas. | Per Cent. of Gas. |
|__________________|________________________|________________________|
| | | |
| ACETYLENE | 1.53-1.77 | 57.95-58.65 |
| Hydrogen | 9.73-9.96 | 62.75-63.58 |
| Coal-gas | 4.36-4.82 | 23.35-23.63 |
| Methane | 3.20-3.67 | 7.46- 7.88 |
|__________________|________________________|________________________|
Experiments have been made at Lechbruch in Bavaria to ascertain directly the smallest proportion of acetylene which renders the air of a room explosive. Ignition was effected by the flame resulting when a pad of cotton-wool impregnated with benzoline or potassium chlorate was fired by an electrically heated wire. The room in which most of the tests were made was 8 ft. 10 in. long, 6 ft. 7 in. wide, and 6 ft. 8 in. high, and had two windows. When acetylene was generated in this room in normal conditions of natural ventilation through the walls, the volume generated could amount to 3 per cent. of the air-space of the room without explosion ensuing on ignition of the wool, provided time elapsed for equable diffusion, which, moreover, was rapidly attained. Further, it was found that when the whole of the acetylene which 2 kilogrammes or 4.4 lb. of carbide (the maximum permissible charge in many countries for a portable lamp for indoor use) will yield was liberated in a room, a destructive explosion could not ensue on ignition provided the air-space exceeded 40 cubic metres or 1410 cubic feet, or, if the evolved gas were uniformly diffused, 24 cubic metres or 850 cubic feet. When the walls of the room were rendered impervious to air and gas, and acetylene was liberated, and allowed time for diffusion, in the air of the room, an explosion was observed with a proportion of only 2-1/2 per cent. of acetylene in the air.
Solubility of Acetylene in Various Liquids.