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. "Little need be said of them, as they fall within the domain of the companies and the public inspector of gas. Under favorable conditions gas-meters will remain in order for ten years or more; and when they become defective they as often favor the consumer, probably, as they do the gas company. Their defects do not often occasion inconvenience; and when they once get out of order they run so wild that their condition is soon detected, when the errors in previous bills should be corrected by estimate of other seasons." "You haven't mentioned the apparatus (carburetters) for increasing the richness of the gas, which can be applied by the consumer upon his own premises," said the old gentleman. "There is little need. The burners should be adjusted to the quality of gas furnished. If there were any real gain in this method of enrichment, the gas companies are the parties who could make the most of it: indeed, many of them do to such an extent as can be made profitable. But whenever the temperature of the atmosphere falls, the matter added to the gas is deposited in the pipes, sometimes choking them entirely at the angles. No: arrange your burners and regulators to suit the gas that is furnished, demand of the company that it fulfil the law and the contract in regard to the quality of the gas, and give all gas-improving machines the go-by.[3] "Light having, perhaps, been sufficiently considered for the present needs, we have now to note the effects of the combustion of gas upon the atmosphere, and through this upon the furnishing of rooms and the health of the persons living therein," said the chemist, again taking up his manuscript. "The usual products from the combustion of common illuminating gas are carbonic acid, sulphuric acid, ammonia and water-vapor. Every burner consuming five cubic feet of gas per hour spoils as much air as two full-grown men: it is therefore evident that the air of a room thus lighted would soon become vitiated if an ample supply of fresh air were not frequently admitted.