740 GEOLOGY OF OHIO. 
bottom of the retorts the gas is mixed with air, heated either by the 
waste heat of the escape flues, or by having been kept in contact with 
the hot walls of the furnace and benches. Only sufficient air is 
admitted, as near as can be ascertained, to give the carbonic oxide gas 
another equivalent of oxygen burning it to carbonic acid, producing an 
intense heat and almost doubling-the generative capacity of the bench 
of retorts. With the old style of furnaces, a retort of the size here 
described, will generate from good coal 5,000 to 7,000 feet of gas in 
twenty-four hours. With the generative furnaces the same retort will 
make from 9,000 to 12,000 feet of gas in the same number of hours. 
In the retort house the gas manufacturer has three enemies to con- 
tend with, viz., stopped stand-pipes, carbon in retorts, and clinkers in 
the furnace. | 
The introduction of the generative furnace will rid him of the 
last mentioned trouble, but in doing so doubles the difficulty with the 
first, and adds to his tribulations by the second. It is generally believed 
that high heats are almost entirely the cause of the stoppage in stand- 
pipes, and the difficulty is created by the rapid generation of the volatile 
matter, both gaseous and liquid, and the rush of the vapor becomes too 
great for the capacity of the stand-pipe. We give it as our opinion that 
less tar passes through the ascension pipe in a given time in a retort 
heated to 2150° Fahr. than from one heated up to only 1500°, or 
1800 Fahr.; that it is not tar that chokes up the stand-pipes, but free 
carbon; that the coal to be used must be experimented with to ascertain 
the best temperature at which to work that particular coal ;*and that it 
is possible not only to make rich gas at high temperatures, but that, up 
to a point at which the compounds composing coal tar are decomposed, 
but below the point at which free carbon is deposited the quality is im- 
proved. However, something more efficient than the stoker’s eye must 
be made use of to determine the degree of heat carried in the bench, 
before gas men will be able to determine the proper degree of heat in the 
distillation of any particular gas coal. | 
The manufacture of gas for illumination being strictly a chemical 
manipulation, it may be of interest to briefly describe some of the 
chemical changes that occur in the apparatus of a gas plant from the 
time the coal is introduced into the retort until the purified gas is stored 
in the holder. 
The coal is charged into the retorts, when those vessels are heated 
up to a temperature of not less than 1500° Fahr. and usually from 
