730 GEOLOGY OF OHIO. 
the Hocking coal yields, leaving out the cannel of No. 6, 3.75 to 4.20 
feet of 13 to 15 candle gas per pound of coal carbonized. 
The difference in results obtained in practice now and by the 
laboratory tests made twelve years ago, seems too great. However, this 
difference is partially accounted for by the fact that gas-works are now 
making a much greater yield of gas than was obtained from the same 
kinds of coal used ten or twelve years ago, when it was conceded by 
Prof. Newberry that in practice, gas-works were able to get 4 feet of 
gas from Pittsburgh coal. It often occurs that the samples selected for 
analysis by parties interested in having a good report therefrom will be 
coal from the part of the seam containing the least volatile combustible 
matter. Again, the temperature of the crucibles of the laboratory is 
usually below that of a good working retort in gas-works, and the result 
is, that a larger per cent. of tar and oils are produced with a less quantity, 
but better quality of gas. But the greater difference in the results ob- 
tained doubtless occurs in the length of time the vapors—permanent and 
condensable—are in contact with the highly heated surface of the retorts. 
In the laboratory the vapor rapidly escapes and probably retains, on its 
exit from the crucible, the same chemical composition it possessed when 
bursting from the lump of coal, largely condensable at atmospheric 
temperature, and the permanent gases composed of the richer grades of 
hydro-carbons, while in gas-works practice, the vapor is required to 
travel slowly a distance of two to ten feet, and to be brought in contact 
with the hot surface of the retorts and of the highly heated coal and 
coke. In this manner the condensable vapors of the laboratory are 
decomposed in the retort and converted into peemanent gas, and the 
heavier hydro-carbons are broken up into hydrogen and solid carbon 
or soot, the volume ot gas being thereby augmented, but the illum- 
inating power of an equal volume of the gas being largely reduced, 
though the actual candle feet or total light obtained from a ton of coal 
will be greater with high than low heats. 
The tables given below show the result of tests made by one of the 
larger gas-works of this State. The tests were first made on a small 
scale, 150 to 200 pounds, and afterwards generally repeated and the 
results verified on a larger scale, using 76 to 100 tons. 
The engineer making these tests, for the sake of convenience in 
comparison, assumed that the gas weighed 26 feet to the pound, ora 
specific gravity of about 500, air veing 1,000. The “loss” reported is 
