10 BULLETIN 102, PART 1, UNITED STATES NATIONAL MUSEUM. 
The volatile third of bituminous coal as it distills off on heating* 
consists essentially of the familiar household-commodity coal gas, 
commonly supplied to municipalities. Thus, quantitatively, there 
are two major products from coal, and as a matter of fact the coal- 
product industries of to-day are a development from two distinct 
industrial sources, the one producing coke for metallurgical use, 
the other providing a municipal gas supply. A large measure of this 
original distinctness has persisted even to the present day in America, 
despite the remarkable fact that the only essential difference is that 
the product in the one case constitutes to a great extent the waste or 
at least the inefficiency in the other, and the reverse. The funda- 
mental aspects of each separately, followed by the combination of the 
two into a mutually complementary whole, provides the best ap- 
proach to the present coal-product industry whereby, as has been 
indicated, the opportunity for a smokeless fuel supplemental to the 
waning anthracite resources is to stand or fall. 
The relationship which coke bears to coal parallels that of char- 
coal to wood and the manufacturing procedure has developed along 
similar lines. If bituminous coal is ignited and allowed to burn with 
sufficiently limited air supply, the burning will be confined to the 
escaping volatile portion; and, in turn, the heat of this gaseous 
combustion will maintain a temperature in the coal mass sufficient 
to cause complete distillation of the volatile ingredients, yielding 
coke as the product. The earliest application of this principle to 
coke manufacture was in the form of open-heap roasting, illustrated 
in plate 6, but this procedure quickly gave way to a practice of 
burning within an oven so constructed that the incoming air, in- 
stead of passing through the coal bed, meets and mingles with the 
distilling gas in the dome-shaped upper part of the oven, where com- 
bustion is accordingly confined. The inclosed oven with its dome- 
shaped roof is known as the beehive coke oven. In it the only 
product saved is coke, the volatile matters providing the fuel 
for their own distillation. If instead of some form of direct 
smothered combustion such as that provided in the beehive oven, 
however, the coal be heated in a sealed inclosure — a retort, in other 
words— -the distillation will proceed as in the open, and the entrapped 
gases and vapors may be piped off from the retort to suitable recep- 
tacles for further treatment. This is the principle upon which muni- 
cipal gas works supplying coal gas operate. Just as the battery of 
beehive coke ovens is to-day’s expression of the solid fuel line of coal- 
product development as a distinct issue, municipal gas manufacture, 
the other distinct issue, finds its typical expression in the arrangement 
of retorts illustrated under the name of gas bench on plate 8. 
Contrasting the operations of the two, the beehive oven is open 
to the air whereas the bench retort is sealed off from air; the bee- 
