Coal Gas.
—Coal gas is derived from the solid hydrocarbons of coal transformed into the more convenient, gaseous form of fuel by means of distillation. Coal gas was first made by distilling coal from an iron pot over a fire and to some extent this is still the principle of the present practice. The gas as it comes from the retort is subjected to a refining process of washing and scrubbing to remove the undesirable properties when it is stored in a large gasometer for distribution through pipes to its places of use. Coal gas is now used largely for fuel as well as for lighting. Unless the heating value of gas is regulated by law in any community and determinations of its quality are made regularly by some competent official, the amount of heat contained in coal gas is entirely at the option of the manufacturer and manager’s conscience. The value as given in the table on [page 252] is the number of B.t.u. coal gas should contain. The heating value of any gas is determined by burning the gas in a calorimeter made expressly for the measurement of the heat of combustion for each foot of the gas consumed.