638 GAS a small quantity of the last named gas remain- ing. The luminosity of a gas flame depends both upon the percentage of heavy hydrocar- bons it contains, and the amount of atmospheric air or oxygen mixed with it. Sometimes in pass- ing it through many purifying processes a small amount of air is absorbed, the oxygen of which, combining with the carbon at the moment of ignition, causes an increased production of heat but diminution of light, on the principle of a Bunsen's burner. The illuminating power may therefore be estimated by analysis; but the practical method is to burn it in comparison with some light-producing body of known power, as a spermaceti candle. This test is made with an instrument called a photometer, FIG. 8. Bunsen's Photometer. a common form of which is shown in fig. 8. An argand burner which consumes a certain number of cubic feet per hour (in experiments usually five feet) is placed at a, and a candle at c. Between them there is a horizontal graduated scale which supports a slide, 5, bearing a ground glass screen having a figure in the centre more transparent than the rest of the plate. When this screen is moved to a point on the scale where the figure appears equally bright on each side, the light received from each source will be equal. If two can- dles are used, placed side by side, and the dis- tance between them and the screen is one eighth that between the latter and the gas- burner, it will show that the light from the. burner is equal to that of 16 candles. A sim- ple screen may be used, or it may have a mir- ror placed upon each side at the further edge, at such an angle that the two will reflect images of the figure toward the observer, so that a comparison may be made at the same instant. Gas as usually furnished is estima- ted, when burning at the rate of five cubic feet per hour, to produce a light equal to that given in the same time by 16 or 18 standard sperm candles, each burning at the rate of 120 grains per hour. The illuminating power of gas depends much upon the form of the burner. It is a matter of common observation that gas may be nearly deprived of its illumi- nating power if made to issue from the burner with great velocity, or if burned in a tall chim- ney which produces a very rapid current of air. Very small or thin flames also do not afford conditions of economical expenditure. The smaller or thinner the flame, the greater is its exposure to the oxygen of the air, and consequently the more rapid the consumption of the solid particles of carbon ; in other words, the more nearly are the conditions present which cause the flame of a Bunsen's burner to be nearly non-luminous. The conditions to be sought for in an illuminating flame are those which are most conducive to high heating of the carbon particles and to the keeping of them for the longest possible time in an uncombined state, but eventually insuring their complete combustion. A certain thickness of flame is therefore desirable. A poor gas, if burned in a jet issuing from a wide slit, may be made to yield a better light than a rich gas burned in a very thin flame, which is frequently the fault of the fish-tail burner. According to Prof. Silliman, the illuminating power of a given sample of gas burned in an argand burner is not in the proportion of gas consumed, but more nearly in the proportion of the square of the quantity. A ton of 2,000 Ibs. of good cannel coal, carefully distilled, will yield about 8,000 cubic feet of purified illuminating gas. Other bituminous coals yield from 6,000 cubic feet up to this amount. Every section of a gas works has one or more meters for measuring the volume of gas before it passes into the reservoirs. They are in the form of cylinders, usually about 12 ft. in diameter and from 8 to 12 ft. long. Fig. 9 is a transverse section showing the principle upon which they act. The outer cylinder or drum is stationary. The in- ner cylinder, turning upon a hollow axis, is divided by the par- titions a, a, a, a into five compartments, one in the centre of the hollow axis, and four, d, d, d, d, spiral in direction, exterior to this ; slits being left open at , e, e, e for the passage of the gas from the inner cylinder to the space be- tween it and the outer one, from which it has exit. The apparatus is a little more than half filled with water. A tube, c, passing through the axis of the cylinder, rises a little above the surface of the water, and delivers gas to the central compartment, from which it passes into each outer compartment successively through the openings <7, g, g, g. The movement of the inner cylinder is in the direction of the hands of a clock, and it will be seen that the gas can only pass through the slit e when it is above the water. The gas constantly passes into that compartment which is on the left in the figure. As this fills it raises that side, and consequently the opening of the compartment above, out of the water, from which the gas flows till it is submerged and emptied at the opposite side. A large pipe conveys the measured gas to the reservoirs or gas-holders, the large cylindrical structures so conspicuous about gas works, and which are constructed much upon the principle of the gas-holder for experimental purposes FIG. 9. Gas Meter.
