"In the combustion of gas for heating purposes," continued the chemist, "seek the burner with free, rapid delivery through small holes. For light you want something different. Suppose you send a current of gas up into this sewing-thimble: it can find an exit only by turning backward. Then suppose it escapes from the thimble only to enter a larger cavity above it, whence it must issue through a burner-tip with an orifice of the usual size. The current, you perceive, is twice completely broken. It will be seen that only the expansive force of the gas, together with its buoyancy, acts upon the jets, instead of a direct current. Now, it will always be found that the burner which best carries out the principles just illustrated—other points being equal—will give more light with a less quantity of gas than any other. This also exhibits the chief principle of most of the governors or regulators.
"You will observe that this checking of the current is attained in various ways in different burners," continued the chemist as he unscrewed and dissected the samples before him. "In some it is done by a perforated metal disk in the orifice; in others, by a bit of wool, which checks slightly a slow current, and by the pressure of a strong one becomes compacted and forms a more effective obstacle. In most cases, however, it soon becomes solid with condensed matters from the gas. Another form of check is a small cap having perpendicular slits at the sides. The cylinder of the cap, being smaller than the orifice of the burner, screws down into it; the openings being shortened or lengthened according as the cylinder is screwed up or down. One objection to this is the trouble required in regulating. Here is another burner, in which the orifice ends in a cap whose sides, near the bottom, are pierced with four pin-holes directed downward. This reverses the direction of the current of gas, which then escapes through the pin-holes downward into a chamber, then turns upward along its sides to the tip, on entering which it again turns. Each burner is able to consume economically a flow of gas peculiar to itself, which can be ascertained by a minute's experiment, and then regulated by the tap in the pipe. But this requires much care, and is apt to be neglected. A very small tap in the burner (as in the Wood and Ellis burners), which can be adjusted so as to require no further attention, seems the best method of effecting this graduation."
The chemist now pulled a manuscript from his pocket and read from it as follows: "The quantity of light decreases with disproportionate rapidity by reduced consumption; for, as experiments have shown, when consuming only two feet per hour, eighty-five per cent. of the gas is lost; with two and a half feet the loss is sixty per cent.; and with three and a half feet it is thirty-four per cent. of that derived from the gas when burning the full quantity for which the burner is constructed. In some experiments made upon this matter under the direction of referees appointed by the London Board of Trade the loss at the other extreme is given. They report: 'Instead of the gas giving increased light as the rate of consumption is increased, it will be seen that in every case there is a point beyond which the light decreases relatively to the proportion of gas consumed. In every case, too, this point lies far below the maximum of gas-consumption, observing the turning-points in the case of the different burners.' Again, every burner has a certain amount of gas which it will consume to the greatest advantage as to both light and economy; which in a completely-regulated burner is quickly found, and the delivery fixed by the small tap. When the gas is issuing from the burner at so low a pressure that the flame is just on the point of smoking, the maximum effect for the quantity of gas consumed in that particular burner is attained, because in that case the quantity and intensity of the light are most advantageously balanced. For the same reason, the burner best suited for light is one in which the jet-openings are proportionately large, so as to prevent as much as possible too great contact with the air in the lower part of the flame. In case the air-currents disturb the light, it is necessary to turn on a stronger flow, which secures steadiness, but sets economy at naught."
"It would be a good thing," said the young fellow, interrupting him, "if some person would invent a burner that should heat the gas before its discharge. We could then get a perfect combustion of the carbon, and so greater brilliancy and economy."
"That is a very common error. Mr. Leslie's burner was designed on that very theory: the result was contrary to expectation."
"What was the form of the burner?" inquired our host.
"Leslie's burner is a form of the argand. The gas, instead of issuing from holes pierced in a solid ring, is conducted to the flame in separate small tubes upward of an inch long. Twenty-eight of these tubes are inserted in a ring two inches in diameter, and converge to one inch at the ends, where the gas escapes. These tubes become hot very quickly when the gas is lighted, and it issues at a high temperature. Here is the result of a test made by Mr. Clegg, and given on page 344 of his valuable work on coal gas:
COMMON ARGAND, FIFTEEN HOLES.
Consumption per hour in cubic feet:
6 feet, light = 17.4 standard candles.
5 feet, light = 13.64 standard candles
LESLIE'S BURNER, TWENTY-EIGHT HOLES.
6 feet, light = 14.73 standard candles.
5 feet, light = 11.28 standard candles.
"In experimenting with common burners, argand and others, it is found that, if the aperture in the tip is too small for the orifice in the body of the burner, the escaping gas is too highly heated and is consumed too quickly. So with Leslie's burner in an increased degree. Theories brought to the test of experiment are often disappointing."
The chemist now proceeded to illustrate his harangue with the argand upon the table, which he lighted and turned on full, without replacing the chimney. The dull-red flame streamed up to a height of eight inches or more, waving and smoking slightly. He now turned down the gas and replaced the chimney, then set the tap at the same angle as before. "Here," said he, "we have a flame barely four inches high—of brilliant white—which gives more light than the taller flame did. The cause of the shortening of the flame is the more rapid combustion of the gas, owing to the increased draught or air-supply in the chimney. From the greater intensity of this flame a much larger quantity of light is produced than by the longer flame. If too tall a chimney is used, the flame is shortened still more and its brilliancy increased, but not to a degree sufficient to compensate for the diminished surface. The light, you are doubtless aware, comes from the incandescence of the carbon, heated by the union of the hydrogen of the gas with a portion of the oxygen of the air."