and much light be lost, although, on account of their greater specific gravity, less of any of such gases would pass through the same burner in a given time.
Reference has been made to the large quantity of water-gas that is made by one ton of coal. In one of the processes employed, the average result of four months' working was 1,000 cubic feet of gas to 67.71 pounds of coal and 3.22 gallons of crude petroleum, or 33,082 cubic feet of about 19 candle gas to one ton of coal and 106.5 gallons petroleum: allowing 70 cubic feet for each gallon of petroleum, or 7,335 feet for the whole, there remains 25,745 feet for the one ton of coal. It will be remembered that the amount of gas obtained by distillation, from the very best coal, ranged between 10,000 and 16,000 cubic feet. Water-gas contains as a rule 40 to 50 per cent, of hydrogen, 30 to 40 per cent, of carbonic oxide, 10 per cent, of naphtha or petroleum gas, and a few per cent, of carbonic acid. The large proportion of heavy petroleum-gas (sp. gr. 0.600 to 0.700) and carbonic oxide (sp. gr. 0.967) makes its specific gravity much heavier than that of coal gas; but the hydrogen, which is the lightest known gas (sp. gr. 0.067), brings it down to between 0.500 and 0.600.
It has been seen that the gas of common coal is of comparatively poor illuminating power, unless enriched by the gas of other coals or of petroleum, and that water-gas of itself possesses no illuminating power whatever. Although, in considering the nature of the different gases, their relative values were incidentally compared, it is necessary to speak of them now more particularly. In regard to quality, it has been shown that naphtha-gas is the purest, since it contains no sulphur or ammonia, and that it is the richest, being from 60 to 80 candle-power, while common coal-gas is only from 15 to 20 candle-power. It is also the most economical, alike for producer and consumer: for the consumer, because, owing to its higher specific gravity, it burns much more slowly than the coal-gas, while it also gives a better light. The higher the specific gravity of the gas, the longer it will take to pass through a given orifice, and therefore the more slowly it will consume; and the higher the candle-power, the less gas is burned in giving the same amount of light. It is more economical for the producer, because, in the first place, there is a great saving in its manufacture, in the handling of the material. The retorts can be supplied continuously, and the frequent interruptions for recharging necessary, in the use of coal, are avoided. Each retort, too, can be made to produce a much larger amount of gas in the one case than in the other. About 10,000 feet of petroleum-gas can be made daily with a single retort, against about 5,000 feet of coal-gas; and 60,000 to 70,000 cubic feet per day, per stoker, against 25,000 to 30,000. Hence there is a saving both of labor and wear and tear of works. The cost of works for making pure petroleum-gas is also much less than that of coal-gas works. Notwithstanding these facts, the commissioners, whose report we have