THE GAS COALS OF OHIO. 741 
1800° to 2100° Fahr. The time the coal remains in the retort differs 
somewhat with the ideas of different engineers, and different character 
of coals. But itis the American practice usually to leave coal in the 
retorts four hours. ; 
The volatile matter is driven off about as follows: 
During first hour, 35 per cent. 
iy second hour, 30 per cent. 
& third Go ea | ie 
sf Towra IU ae 
Most of the water is driven off during the first half-hour, and most 
of the tar during the first two hours. Whether the compounds of which 
the tar is composed are nearly all driven off during 1st and 2nd hour, or 
whether they continue to be evolved to near the end of the charge, but by 
reason of the higher temperature of the surrounding mass they are decom- 
posed, and the carbon of the tar deposited as coke, soot and graphitic car- 
bon, is a question that perhaps cannot be easily determined. At any rate 
the volatile matter passing out of the retort is composed of tar (in itself 
containing almost innumerable compounds), ammoniacal liquor of about 
A oz. strength, olefiant gas, light carburetted hydrogen, free hydrogen 
and in smaller volumes carbonic oxide, carbonic acid, sulphuretted 
hydrogen, oxygen and nitrogen. After the gas has left the retort, there 
are perhaps few chemical changes occur in passing through washers and 
scrubbers, until the purifiers are reached. Some depositions occur from 
change of temperature and change of velocity in the gas while passing 
through condensers, washers and scrubbers, and some chemical chang. 
oceur by the contact of freeammonia with sulphuretted hydrogen au 
earbonie acid, the result being the formation of the sulphate and car- 
bonate of ammonia; both salts being soluble in water, are washed out in 
the washer or scrubber. Much stress is laid on the union of the two 
impurities named, by many gas engineers, more perhaps than the facts 
of the case warrant. ‘There are usually four purifying boxes in a gas- 
works ; three are in constant use, and one off for cleaning or for changing 
material. 
It is the custom to pass gas through the three boxes at work until 
a test of the gas shows impurities passing the second box, when change 
is then made and the foulest or first box is thrown out of use. The 
second box becoming the first, the third the second, and the newly 
cleaned box becoming the third. If lime is used as the purifying agent, 
