134 A TREATISE ON GAS-LIGHT.

may be performed in a common boiler ; butas it is extremely liable to boil over, thegreatest precaution is necessary in conductingthe evaporation. A boiler constructed on thefollowing plan is very convenient for the con-version of coal-tar into pitch. The contriv-ance consists in adding a spout, or rim, to thecommon boiler, into which the tar spreadsitself as it rises, and by this means becomescooled, and the boiling over is cheeked.

Kettle for boiling Tar.

looolb. of coal-tar produce, upon an average,from 400 to 48olb. of pitch. A subsequentfusion, with a gentle heat, converts the coal-pitch into a substance possessing all the cha-racters of asphaltum.

Ammoniacal Fluid .—The properties of theammonial liquor, which accompanies the tar,