724 GEOLOGY OF OHIO. | 
the ash to a greater degree than is the case with Youghiogheny gas coke. 
The fourth and last general point to be considered in determining 
the value of any particular coal for gas making, is its impurities. The 
term impurity is here used in the sense that is usually applied in the 
gas-works of the United States, and particularly in the smaller works, 
referring to the sulphur compounds, sulphuretted hydrogen and 
bisulphide of carbon. 
An analysis for total quantity will not always determine the ques- 
tion whether a coal contains a too large per cent. of sulphur to be 
profitably used in gas making. ‘T'wo coals containing equal per centages 
of sulphur may produce gas—the one easily purified, and the other 
foul and hard to purify. . But the question of purification is one of 
much less importance now in determining the value of coals than was 
the case a few years ago. 
The general adaptation of oxide of iron, in its various forms, has so 
reduced the cost, that the expense of removing the sulphuretted bydro- 
gen has become almost nominal. 
But lime is still used for the removal of bisulphide of carbon and 
carbonic acid. 
Ammonia, in its various forms, has not here been treated or con- 
sidered as an impurity, as the cost of removal in small works is insig- 
nificant, while in the large works the ammonical liquor is often a source 
of revenue. 
Both carbonic acid and carbonic oxide are impurities and are pro- 
duced to a greater extent by the use of some coals than by others. Yet 
we have not considered either of these impurities in making suggestions 
for the determination of the value of gas coals, as the manner of manipu- 
lation often has as much to do with, and is as much at fault in the 
production of these impurities, as is the character of the coal. 
Coal having no definite composition, and coals of almost identical 
composition as ascertained by ultimate analysis giving totally different 
results when distilled in a gas retort, renders it impossible to lay down 
any positive rule for judging a good gas coal by its analysis. But it is 
safe perhaps to say that the gasmaker should choose the coal that con- 
tains the largest per cent. of hydrogen and the smallest per cent. of 
oxygen. While it is well known that the combustion of hydrogen at 
the ordinary pressure of gas consumption, results simply in producing 
a blue flame without light, and it is equally certain that the light of 
