Actually comparisons of the effect of additions of various proportions of a richly illuminating gas, such as acetylene, on the illuminative value of a gas which has little or no inherent illuminating power, are largely vitiated by the want of any systematic method for arriving at the representative illuminative value of any illuminating gas. A statement that the illuminating power of a gas is x candles is, strictly speaking, incomplete, unless it is supplemented by the information that the gas during testing was burnt (1) in a specified type of burner, and (2) either at a specified fixed rate of consumption or so as to afford a light of a certain specified intensity. There is no general agreement, even in respect of the statutory testing of the illuminating power of coal-gas supplies, as to the observance of uniform conditions of burning of the gas under test, and in regard to more highly illuminating gases there is even greater diversity of conditions. Hence figures such as those quoted above for the enrichment value of acetylene inevitably show a certain want of harmony which is in reality due to the imperfection or incompleteness of the modes of testing employed. Relatively to another, one gas appears advantageously merely in virtue of the conditions of assessing illuminating power having been more favourable to it. Therefore enrichment values, such as those given, must always be regarded as only approximately trustworthy in instituting comparisons between either different diluent gases or different enriching agents.
ACETYLENE MIXTURES FOR RAILWAY-CARRIAGE LIGHTING.--In modern practice, the gases which are most commonly employed for diluents of acetylene, under the conditions now being considered, are cannel-coal gas (in France) and oil-gas (elsewhere). Fowler has made a series of observations on the illuminating value of mixtures of oil-gas and acetylene. 13.41 per cent. of acetylene improved the illuminating power of oil-gas from 43 to 49 candles. Thirty-nine-candle-power oil-gas had its illuminating power raised to about 60 candles by an admixture of 20 per cent. of acetylene, to about 80 candles by 40 per cent. of acetylene, and to about 110 candles by 60 per cent. of acetylene. The difficulty of employing mixtures fairly rich in acetylene, or pure acetylene, for railway- carriage lighting, lies in the poor efficiency of the small burners which yield from such rich gas a light of 15 to 20 candle-power, such as is suitable for the purpose. For the lighting of railway carriages it is seldom deemed necessary to have a flame of more than 20 candle-power, and it is somewhat difficult to obtain such a flame from oil-gas mixtures rich in acetylene, unless the illuminative value of the gas is wasted to a considerable extent. According to Bunte, 15 volumes of coal-gas, 8 volumes of German oil-gas, and 1.5 volumes of acetylene all yield an equal amount of light; from which it follows that 1 volume of acetylene is equivalent to 5.3 volumes of German oil-gas.
A lengthy series of experiments upon the illuminating power of mixtures of oil-gas and acetylene in proportions ranging between 10 and 50 per cent. of the latter, consumed in different burners and at different pressures, has been carried out by Borck, of the German State Railway Department. The figures show that per unit of volume such mixtures may give anything up to 6.75 times the light evolved by pure oil-gas; but that the latent illuminating power of the acetylene is less advantageously developed if too much of it is employed. As 20 per cent. of acetylene is the highest proportion which may be legally added to oil- gas in this country, Borck's results for that mixture may be studied:
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| | | | | | | |
| | | | | | | Propor- |
| | | | Consump- | | Consump- | tionate |
| Kind of | No. of | Pres- | tion per | Candle- | tion per | Illum- |
| Burner. | Burner | sure. | Hour. | Power. | Candle- | inating |
| | | mm. | Litres. | | Hour. | Power |
| | | | | | Litres. | to Pure |
| | | | | | | Oil-Gas.|
|___________|________|_______|__________|_________|__________|_________|
| | | | | | | |
| Bray | 00 | 42 | 82 | 56.2 | 1.15 | 3.38 |
| " | 000 | 35 | 54 | 28.3 | 1.91 | 4.92 |
| " | 0000 | 35 | 43.3 | 16 | 2.71 | 4.90 |
| Oil-gas | | | | | | |
| burner | 15 | 24 | 21 | 7.25 | 2.89 | 4.53 |
| " " | 30 | 15 | 22 | 10.5 | 2.09 | 3.57 |
| " " | 40 | 16 | 33.5 | 20.2 | 1.65 | 3.01 |
| " " | 60 | 33 | 73 | 45.2 | 1.62 | 3.37 |
| |
| The oil-gas from which this mixture was prepared showing: |
| |
| Bray | 00 | 34 | 73.5 | 16.6 | 4.42 | ... |
| " | 000 | 30 | 48 | 6.89 | 6.96 | ... |
| " | 0000 | 28 | 39 | 3.26 | 11.6 | ... |
| Oil-gas | | | | | | |
| burner | 15 | 21 | 19 | 1.6 | 11.8 | ... |
| " " | 30 | 14 | 21.5 | 2.94 | 7.31 | ... |
| " " | 40 | 15 | 33 | 6.7 | 4.92 | ... |
| " " | 60 | 25 | 60 | 13.4 | 4.40 | ... |
|___________|________|_______|__________|_________|__________|_________|
It will be seen that the original oil-gas, when compressed to 10 atmospheres, gave a light of 1 candle-hour for an average consumption of 7.66 litres in the Bray burners, and for a consumption of 7.11 litres in the ordinary German oil-gas jets; while the mixture containing 20 per cent. of acetylene evolved the same amount of light for a consumption of 2.02 litres in Bray burners, or of 2.06 litres in the oil-gas jets. Again, taking No. 40 as the most popular and useful size of burner, 1 volume of acetylene oil-gas may be said to be equal to 3 volumes of simple oil-gas, which is the value assigned to the mixture by the German Government officials, who, at the prices ruling there, hold the mixture to be twice as expensive as plain oil-gas per unit of volume, which means that for a given outlay 50 per cent. more light may be obtained from acetylene oil-gas than from oil-gas alone.
This comparison of cost is not applicable, as it stands, to compressed oil-gas, with and without enrichment by acetylene, in this country, owing to the oils from which oil-gas is made being much cheaper and of better quality here than in Germany, where a heavy duty is imposed on imported petroleum. Oil-gas as made from Scotch and other good quality gas-oil in this country, usually has, after compression, an illuminating duty of about 8 candles per cubic foot, which is about double that of the compressed German oil-gas as examined by Borck.
Hence the following table, containing a summary of results obtained by H. Fowler with compressed oil-gas, as used on English railways, must be accepted rather than the foregoing, in so far as conditions prevailing in this country are concerned. It likewise refers to a mixture of oil-gas and acetylene containing 20 per cent. of acetylene.
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| | | | | | |
| | | | | | Ratio of |
| | |Consumption| |Candles per| Illuminating |
| Burner. |Pressure.| per Hour. |Candle| Cubic Foot| Power to that |
| | Inches. |Cubic Feet.|Power.| per Hour. |of Oil-gas [1] |
| | | | | | in the same |
| | | | | | Burner. |
|_____________|_________|___________|______|___________|_______________|
| | | | | | |
| Oil-gas . . | 0.7 | 0.98 | 12.5 | 12.72 | 1.65 |
| Bray 000 . | 0.7 | 1.17 | 14.4 | 12.30 | 1.57 |
| " 0000 . | 0.7 | 0.97 | 10.4 | 10.74 | 1.41 |
| " 00000 | 0.7 | 0.78 | 5.6 | 7.16 | 1.08 |
| " 000000 | 0.7 | 0.55 | 1.9 | 3.52 | 1.14 |
|_____________|_________|___________|______|___________|_______________|
[Footnote 1: Data relating to the relative pecuniary values of acetylene (carburetted or not), coal-gas, paraffin, and electricity as heating or illuminating agents, are frequently presented to British readers after simple recalculation into English equivalents of the figures which obtain in France and Germany. Such a method of procedure is utterly incorrect, as it ignores the higher prices of coal, coal-gas, and especially petroleum products on the Continent of Europe, which arise partly from geographical, but mainly from political causes.]
The mixture was tried also at higher pressures in the same burners, but with less favourable results in regard to the duty realised. The oil-gas was also tried at various pressures, and the most favourable result is taken for computing the ratio in the last column. It is evident from this table that 1 volume of this acetylene-oil-gas mixture is equal at the most to 1.65 volume of the simple oil-gas. Whether the mixture will prove cheaper under particular conditions must depend on the relative prices of gas-oil and calcium carbide at the works where the gas is made and compressed. At the prevailing prices in most parts of Britain, simple oil-gas is slightly cheaper, but an appreciable rise in the price of gas- oil would render the mixture with acetylene the cheaper illuminant. The fact remains, however, that per unit weight or volume of cylinder into which the gas is compressed, acetylene oil-gas evolves a higher candle- power, or the same candle-power for a longer period, than simple, unenriched British oil-gas. Latterly, however, the incandescent mantle has found application for railway-carriage lighting, and poorer compressed gases have thereby been rendered available. Thus coal-gas, to which a small proportion of acetylene has been added, may advantageously displace the richer oil-gas and acetylene mixtures.