The chemist now read from his manuscript again: "Carburetted hydrogen of a passably good quality requires two volumes of pure oxygen for its complete combustion and conversion into carbonic acid and water. Atmospheric air contains, in its pure state, about twenty per cent. of oxygen; therefore, one cubic foot of gas requires for its perfect combustion ten cubic feet of air. If less be admitted to the flame, a quantity of free carbon will escape, and be deposited in the form of black smoke. If an excess of air be admitted, we shall find that the quantity of nitrogen accompanying this excess has a tendency to extinguish the flame, while it takes no part in the elective affinity constantly going on between the other elements—namely, hydrogen, oxygen and the vapor of carbon.
"Again," said he, turning down the gas, "if the flame be reduced to a consumption of two feet per hour, its light will be equal to that of one candle only; but on raising the chimney, thus, about half an inch from the gallery or support the light is greatly increased, or by simply placing a disk on top of the chimney the light is increased ninefold; both of which effects seem to result from a diminished current of air, while at the same time there is an ample supply. Lastly, with the ordinary glass moon-globe so generally used in dwellings with the fishtail burner little difference can be perceived between the light given from the flame by four feet and that from six feet of gas per hour, in consequence of the strong current of air passing up through the globe; but if the top of the glass be enclosed by a talc cover having an orifice in the centre about an inch in diameter, then the conditions of the burner are completely changed. The light is greatly increased, because the highest economical advantage is then approached."[2]
"Smoke from the aperture and lamp-black on the cover must result from such an arrangement," objected the old gentleman.
"There need be very little of either," responded the chemist. "From some burners there is little light without smoke. A smoky flame may arise from too much carbon, but the gas companies in this part of the country are not apt to make their product too rich; and such a condition is not likely to occur except with vapor-gas when warm weather quickly succeeds to a cold spell in the winter season. The consumer's immediate remedy in any case is to use a smaller tip with the fishtail and batwing burners, and a taller chimney with the argand; which devices will give a quicker movement to the gas in one case and to the air in the other. The smoking, however, may be caused by carbonic acid, which checks combustion. There is always more or less of this in gas, arising from a partial combustion in the retorts when charging them with coal or while withdrawing the exhausted charge. But it is only by excessively slow and careless work that this can happen to a serious extent. Only an expert can tell when this condition exists, though if the symptoms do not yield to manipulations of the chimney and tap, it may be suspected. There is no effective remedy for this adulteration which can be applied by the consumer except a vigorous complaint against the company which supplies the stuff.
"There remains one burner or lamp to be mentioned, contrived with special reference to health," he continued—"the ventilating standard lamp of Doctor Faraday, used in the House of Lords. In this there is an outer glass by which the vitiated air passes away through the pipe communicating with the external air. The lamp is interesting, but there is a question whether there is any practical advantage in its use. Rutter's ventilating lamp is of different form, having a globe instead of an outer cylinder, the gas and air coming in from above. Some of the best dwellings now being erected in the vicinity of New York are provided with tin pipes leading from the burners to the open air. In some the pipe receives the foul air from an open metallic or mineral shade over the burner; others have a larger pipe enclosing the gas-pipe for ventilation, the tops of the two pipes (including the burner) being enclosed by a globe pierced with holes for fresh air. There is said to result a good ventilation, with economy of gas, an increased steadiness of the flame and power of light. A better arrangement is a third pipe enclosing the gas-pipe and enclosed in the ventilating-pipe, opening to the air, instead of the holes in the globe, which in this case should be air-tight. This plan is said to have reached its perfection when the three pipes are filled with wire gauze to some extent. This, being heated by the escape of hot gases in the ventilating-pipe, sends both the air and the gas to the flame already highly heated. The result is said to be admirable as regards ventilation, steadiness and power of the light and economy of gas.
"With these lamps the pressure of the gas-current is of great importance; and I now turn to that subject. It is a general complaint in buildings whose rooms are high that the flow of gas on the lower floor is deficient, while on the upper floors there is a greater supply than is necessary. This inconvenience arises from the upper stories being subjected to less atmospheric pressure than the lower, every rise of ten feet making a difference in the pressure of about one-tenth of an inch of water; and, consequently, a column of gas acquires that amount of pressure additional. The following table, recording an experiment of Mr. Richards, will show the result in respect to light:
| Gas | issuing | from | the | burner | at | a | pressure | pressure | of— |
| 1/10 | inch | of | water | gave | the | light | of | 12 | candles |
| 5/10 | " | " | " | " | " | " | " | 6 | " |
| 10/10 | " | " | " | " | " | " | " | 2 | " |
| 40/10 | " | " | " | " | " | " | no | appreciable | light. |
Suppose a building of six floors is supplied from the gas-mains at a pressure of six-tenths, and that the difference of altitude between the highest and lowest light is equal to fifty feet: the gas in the highest or sixth floor will issue from the burners at a pressure of eleven-tenths; the fifth floor, at ten-tenths; and so on. In order to secure an entirely equable flow and economical light a regulator is necessary on each floor above the first. The gas companies are frequently obliged to supply mills at a much greater pressure than is stated above as necessary, in order that the ground floors may have sufficient light."
"How about incorrect meters?" asked the traveller.