18 Illumination by Coal Gas. {Jury, 
nature of the gas obtained from the same coal varies considerably, 
according to the conditions under which it is obtainable. 312 Ib. 
of common Cannel coal produce at the minimum from 350 to 360 
cubie feet of carbureted hydrogen ; but the same quantity of the 
-best Newcastle coal, that is to say, such as coke readily, and send 
out brilliant streams of flame, which undergo a kind of semifusion 
when laid on the fire, produce upon an average 300 cubic feet of 
this gaseous fluid, besides a large portion of sulphureted hydrogen, 
carbonic acid, and carbonic oxide. 
Half a cubic foot of this gas, when fresh prepared, that is to 
say, holding in solution or suspension a portion of the essential oi! 
which is generated during the production of the gas, is equal in 
~ illuminating power to from 170 to 180 grs. of tallow, which is the 
quantity of this material consumed in one hour by a well snuffed 
tallow candle six to the pound. Now 1 Ib. avoirdupois is equal to 
7000 grs., and consequently 1 tb. of candles of six to the pound, 
burning one at a time in succession, would last 4°°° = 40 hours, 
To produce the same light, we must burn one half of a cubic foot 
of coal gas per hour; therefore one half multiplied by 40 hours is 
equal to 20 cubic feet of gas in 40 hours, and consequently equal to 
1 lb. of candles, six to the pound, provided they were burnt one 
after another. 
Further, 112 Ib. of Cannel coal produce at the minimum 350 
cubic feet of gas, and are equal to 350 divided by 20, which last is 
equivalent to 1 lb. of tallow, making therefore 112 Ib. of coat 
equal to 35° = 171 1b. of tallow ; and 112 Ib. of coal divided by 
17+ of tallow gives six and four-tenths of coal equal to 1 lb. of 
tallow. 
With regard to Newcastle coals, it may be stated that one chal- 
dron of Wall’s End coal produces in this large way upwards of 
11,000 cubic feet of crude gas, which when purified diminish to 
nearly 10,000 cubic feet. But the quantity and quality of the gas, 
as stated already, is much influenced by circumstances attending the 
formation of it. If the tar and oil produced during the evolution of 
the gas in its nascent state be made to come in contact with the 
sides of the red-hot iron retorts; or, better, if it be made to pass 
through an iron cylinder or other vessel heated red-hot, a large 
portion of it becomes decomposed into carbureted hydrogen and 
olefiant gas; and thus a much greater quantity of gas is produced , 
than would be obtained without such precautions. Hf the coal be 
distilled with a very low red heat, scarcely observable by day-light, 
the gas produced gives but a feeble light: if this distillatory vessel 
be of a dull redness, the light produced by the burning gas is more 
brilliant: if a bright, or cherry-red, heat be employed, the gas 
produced burns with a brilliant white flame: and if the heat be 
increased so far that the retort is almost white het, and conse- 
quently in danger of melting, the gas given out has little illumi- 
nating power, and burns with a elear bluish flame: and if this coal 
abounds. in pyrites, a large portion of sulphureted hydrogen gas is 
