The complex nature of coal gas is shown by the following table, which represents an average sample:

Of these, the hydro-carbons, carbon monoxide,CH4 and hydrogen are combustible.

Coals always contain more or less sulphur, which is a great trouble to the gas manufacturer. It frequently happens that some of it gets into the gas. If such gas escapes, the sulphur compounds unite with the silverware, giving is a coating of dark sulphide of silver. If silver tarnishes quickly, it is an indication of a leak of gas or sewer gas. It is estimated that a ton of coal should yield 10,000 feet of gas, 1,400 lbs. of coke (35 bushels), 12 gallons of tar, 4 lbs. of ammonia.

More than six hundred products are obtained from the coal tar. The nature and uses of these products would form an interesting topic for futher study.

The composition of water gas is somewhat as follows:

Notice that this gas contains less methane and hydrogen (which are combustible), and their place is taken by carbon monoxide, which, although combustible, is very poisonous. There is some carbon monoxide in ordinary illuminating gas but not nearly so much. The water gas has a strong odor from the hydro-carbons (crude gasoline) added to make it luminous, but comparatively little of it in the air is likely to produce very injurious effects upon living things, plants and animals alike. It is the most poisonous substance that comes into the house. It is estimated that about fourteen per cent of the gas manufactured escapes into the earth through leaky gas mains. In passing through the soil the odorous part of water gas may be strained out, so that it becomes odorless. Whole families have been poisoned from deodorized water gas leaking into the house by way of the cellar. This emphasizes the importance of having a perfectly tight cellar, with cemented walls and floor, and the importance of ventilating the cellar, for the cellar air finds its way to the rooms above. Natural gas contains practically no carbon monoxide.