THE GAS COALS OF OHIO. 743 
of the oxide generally changes during revivification from black to red 
or brown. After exposure for twenty-four or thirty-six hours, or until 
the material has assumed its natural color, the oxide is again ready for 
use, and this process may be repeated over and over until the deposited 
sulphur will equal 40 to 50 per cent. of the bulk. No unpleasant smells 
are given off in manipulating the iron process, no sulphuretted hydrogen 
escapes when the material is exposed. There are several salts of 
ammonia and some of iron formed during the process, not here men- 
tioned, they being of no particular interest to either the public or the 
eas manufacturer. 
There are numerous inventions in use for the manufacture of gas 
from materials other than coal, as well as for using coal, only in part, 
the most prominent being that for the manufacture of “water-gas ;” 
carbonic oxide and hydrogen being generated by the decomposition of 
steam, and this gas enriched by the use of the lighter oils obtained in 
refining petroleum. 
But, as these works are not users of gas coal, and as it is not believed 
that any of these processes will materially affect the gas coal industry, 
they will not be further discussed here. 
Some authors, in discussing the question of inferior gas made at or 
near the end of the charge, or when coal is nearly or quite coked, 
attribute the large per cent. of carbonic oxide that is then passed off 
with the gas, to vegetable (mineral) charcoal found in some coals. Others 
attribute it to leaky retorts with undue exhaustion, thereby drawing in the 
carbonic oxide from the furnace, the carbonic oxide resulting from the 
imperfect combustion in the furnace. 
Both these positions seem untenable to the writer. With reference 
to the first, the vegetable charcoal cannot of itself give off carbonic 
oxide, and owing to the fact that all the water of the coal has passed 
off during the earlier stages of the distillation, there is less oxygen 
present to combine with the charcoal at the end of the charge than at 
the beginning. And if oxygen is present, vegetable charcoal is not 
necessary to the formation of carbonic oxide when an excess of carbon 
in form of coke is present. With reference to the second proposition, 
over-exhaustion does not occur any more at the end of a charge, 
particularly in large works, where fresh charges are being constantly 
introduced, than occurs at the beginning of the charge. 
‘If, however, over-exhaustion did ocenr, and the furnace gases were 
drawn into the retort, it does not follow that because carbonic oxide is 
