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three rows, one above the other, the ends of the retorts being 
supported in a brick wall which also extends around the end of 
the rows and over the top, and thus entirely encloses the retorts. 
Fire, from furnaces below, is carried by flues into this enclosure, 
so that the retorts are entirely enveloped in flame and can be 
heated to a very high temperature. 
The retorts are partly filled with coal, after which they are 
sealed, so that no air can get into them. They are then heated 
to a temperature of about 2,100 degrees, Fahrenheit. Under 
this intense heat almost all the hydrocarbons of the coal pass off, 
leaving behind only the ''fixed" carbon, which comes out of the 
retort as coke. Many of the lighter compounds distilled off by 
the heat will now remain in the form of gas when they are cooled 
to ordinary temperatures, and it is some of these which make the 
gas finally used for lighting and fuel. But as it comes from the 
retorts, the gas is like a thick, yellowish-green smoke, and could 
not be used at all for such purposes. This gas escapes from the 
retorts into a series of large and costly machines where the light- 
ing gas is cleansed from its impurities, and the different by- 
products are separated from each other. First are great "con- 
densers," in which the gases are cooled. The cooling condenses 
the heavier compounds into thick liquids, which are then left be- 
hind. 
One of the substances later removed from the gas is ammonia, 
and from such gas works comes the ammonia water which is used 
in every household. Another important product of the gas mak- 
ing process is the coke which is left in the retort. Every year 
an increasing number of people use this coke for burning in 
kitchen stoves, and even in furnaces, instead of coal, for it makes 
a very hot fire and burns without any smoke. 
The heavy, strong smelling, black liquid which is collected in 
the cooling of the gas is what we know as coal tar. This is an 
exceedingly complex mixture of substances. From it are ob- 
tained not only creosote oil, but most of the dyes which are used 
now-a-days, perfumes, and even flavoring extracts. 
Gas, coal tar and coke are also made in what is known as the 
by-product coke oven, which is adapted to different objects but is 
operated on the same principle. Its coal tar is equally as good 
as the gas-works tar for making creosote oil. In recent years a 
great amount of gas has been made in the United States by an- 
other process, and is known as water gas. This process also pro- 
duces a tar. which looks much like coal tar and is often difficult 
to tell from it. But this tar is really derived from petroleum, 
and does not make a good oil for preserving wood from decay. 
Wood creosote, with which so many people are familiar, is like- 
wise obtained from a wood tar which is produced by distilling 
wood. But like water gas tar creosote, wood creosote is not so 
good for wood preservation as is the coal tar creosote. When 
