DISTILLATION OF STUMPWOOD. 48 
show the same tendency to follow the weight a cord and field classi- 
fication of the wood. This tendency is shown also by the yield 
of heavy crude oil and of pitch. The acid liquor and charcoal, how- 
ever, are not subject to any such general deductions, although the 
highest yields of acid liquor are generally given by the green woods, 
followed by the richer stumpwood. In all probability this is due 
to the fact that acetic acid is one of the decomposition products of 
rosin. 
An experienced person can classify stumps in the field into several 
grades from which the average yields of valuable products differ to 
such an extent as to necessitate a proper selection of the material 
before collection. 
COMMERCIAL DISTILLATION PROCESSES. 
There are four general processes for the recovery of products from 
resinous wood. Two of these are destructive distillation processes 
and two are nondestructive extraction processes. They are: (a) The 
common or ordinary destructive distillation process; (>) the con- 
trolled temperature destructive process; (c) the steam distillation or 
extraction process; and (d@) the solvent extraction process. Of these 
the ordinary destructive distillation process is the only one which 
seems to be well adapted to the stump-disposal project in the North- 
west. 
ORDINARY DISTILLATION PROCESS. 
The wood-distilling oven now in general use for the destructive 
distillation of wood is an outgrowth of the old charcoal heap. By- 
product charcoal kilns, round iron retorts, and rectangular iron or 
concrete ovens are in use, the rectangular oven being preferred in 
the best practice. Experience with these different forms has taught 
that there is a mean temperature which gives the most satisfactory 
yields. This temperature is necessarily more difficult to maintain 
in direct-heated retorts, the smaller of which have the further dis- 
advantage that the charcoal must be removed by hand, necessitating 
a loss in time required for cooling as well as a fuel loss in reheating 
the retort for the next charge. 
The uneconomical working of the round retort has led to the de- 
velopment of the rectangular oven. Such ovens are of steel or con- 
crete construction and are heated either directly by fires under them, 
in the case of the steel ovens, or by means of internal-heating flues 
in the concrete ovens. The second method is said to be better 
adapted to softwood distillation. The height and width of the ovens 
are uniform, being in general 8 feet 4 inches and 6 feet 3 inches, re- 
spectively, and the length ranges from 26 to 54 feet or more, accord- 
