1238

THE COLLIERY GUARDIAN

December 17, 1915.

Surveying.

(Two questions to be done from this section.)

5.	Draw an equilateral triangle ABC, of 3| in. side.
Take this triangle as drawn on a colliery plan, of which
the scale is 1 in. to 100 ft., and that the relative levels of
the seam at A, B, and C are 50, 120, and 100 respectively.
Show on your drawing the direction of level course and the
direction of full dip. (20)

6.	You have reason to suspect that the plumb lines by
which the direction of a roadway under your charge is being
driven have been moved. The roadway contains materials
that affect the compass needle, and that cannot be removed.
State shortly how you would check the plumb lines if you
had only a compass without a fast needle attachment. (20)

7.	Draw to a scale of | in. to 1 ft. -a cross section of <a
brick-lined drift. The width between the side walls is 10 ft.
and the height in the centre of the arch is 8 ft. The arch
is semi-circular, and the brickwork is 14 in. thick. (20)

8.	Calculate the cubic contents of the brickwork in one
lineal yard of the drift in question 7. (20)

Subject No. «6.—General Management and Mining
Legislation.

(Five questions only to be answered. No. 1 is compulsory.)

1.	Compulsory Question.—Write a report, such as you
would submit to your manager, describing the most serious
accident with which you have been connected. (20)

2.	Explain what is meant by systematic timbering, and
state the regulations relating to this matter. (20)

3.	What are the requirements of the Coal Mines Act and
G-eneral Regulations concerning the necessary ambulance and
rescue appliances to be provided at and in a mine, at which a
rescue brigade is maintained? (20)

4.	Describe in general terms the colliery at which you are
now employed; state the number and thickness of the seams
now being worked, and explain the system of working and
the organisation of the staff in each seam. (20)

5.	What are the requirements of the Coal Mines Act and
General Regulations with regard to electric cables placed in
shafts and haulage roads? (20)

6.	What are the duties of coal getters at the face under the
Coal Mines Act and Regulations? (20)

THE CASES IN COAL *

By N. H. Darton.

(Continued from page 1146 J

Methane in Coal.

Methane undoubtedly is a product of the decomposi-
tion or molecular rearrangement of organic matter
during its transformation into coal, putrefactive bacilli
being the cause of the change. It is reasonable to
believe that the process may have lasted for some time,
but all the evidence now available appears to indicate
that it is no longer in progress in the coal. Possibly,
however, the diminution of underground pressure in the
coal deposits by erosion and.mining may be a factor in
the disassociation of some as yet unrecognised inter-
mediate compounds with emanation of more or less
additional methane.

It appears highly probable that different conditions of
bacteriologic action in the original vegetable accumula-
tions have had much to do with the cause of variations in
the amount of methane in coal. The first and most
important change in the transformation of organic
matter into coal is putrefaction, sustained by certain
bacilli; this process and others causing changes in the
carbon compounds progress until the material reaches
the stage of peat. Elaborate studies of these processes
made by various observers, notably by Renault, have
aided greatly in determining the organic and chemical
conditions involved in the change of vegetable matter
to coal. Renault discovered abundant remains of many
micro-organisms in coal and coaly substances. He and
others have found .that the putrefaction in vegetable
accumulations is caused and sustained by a large variety
of organisms, depending somewhat on the nature of the
materials, which comprise many kinds of plants, on
numerous local conditions of temperature, moisture,
etc., and on the rate of accumulation. Thus their
chemical products may vary considerably. The precise
chemical reactions are not well known, but in general
there is elimination of oxygen and hydrogen, with con-
sequent evolution of more or less carbon dioxide and
methane. In some stages there are developed by-
products that arrest putrefaction, so that it proceeds
irregularly in the mass, and with varying proportions
of resultant gases. The degree of decomposition at the
time the coal deposit is buried may be an important
factor, and the nature of the covering material, whether
sand or clay, and its rate of deposition probably have
great influence on the amount of gas retained. Doubt-
less these two features, the variations in kind and
degree of putrefaction, and the cover conditions
imprisoning the gas in the coal, have most to do with
variations in the amount of gas in the coal as now found.
Naturally, they varied greatly from place to place and
from time to time in basins where vegetable deposits
were accumulating and being buried, so that the great
variation in volume of methane now found in the coal
was largely caused early in the history of the beds. Of
course, there has also been subsequent escape of gas
through porous overlying strata, especially as uplift has
brought up the coal measures, and they have been more
or less bared by erosion. As a result of erosion, the
escape of gas has progressed without relation to the
original content, and the result gives further complexity
to the problem.

* From Bulletin 72 of the United States Department of
the Interior, Bureau of Mines.

Relations to Class of Coal and Metamorphism.

It does not seem possible to reconcile the methane
conditions in coal with generally held theories ascribing
the cause of the differences between varieties such as
anthracite, bituminous coal, and lignite, to fractional
distillation after the coal had been buried in the coal
measures. It is assumed in such theories that more or
less of the volatile matter was baked out by under-
ground heat or “ metamorphism,” and escaped through
porous rocks or joints. The baking of coal seen where
igneous intrusions have occurred is cited as an illus-
tration of the phenomenon. The nature of the result-
ing product is supposed to depend upon the stage to
which the process advanced, graphite and anthracite
being greatly baked, and most lignites having lost little
of their original volatile components. Anthracite differs
from bituminous coal and lignite in composition, mainly
in that it contains only about 2 per cent, of oxygen
instead of 15 to 20 per cent, or more. In part of the
anthracite basin of Pennsylvania it has been found that
the amount of methane is far greater than in most bitu-
minous coal fields.

As anthracite in America and Europe contains highly
volatile methane, which must be believed to have been
developed during the original formation of the coal, it
is impossible to conceive of the distillation of the heavier
hydrocarbons of the so-called “ volatile matter ” of the
coal, and the retention of the great volume of methane.
It implies that vast volumes of “ volatile matter ” have
escaped from coal through strata which are sufficiently
impermeable to prevent the escape of methane under
high pressure. As shown above, it is most improbable
that the methane developed after the metamorphism.
Renault, Bertrand, and others have held that although
metamorphic agencies may have acted to some extent,
the differences in coals are mainly due to difference in
character of vegetable accumulations, conditions of
deposition, and the nature and degree of biochemical
processes that have operated. Doubtless dynamic con-
ditions have caused molecular re-arrangement of some
of the original components of coal, perhaps with more
or less additional methane as by-product, but the extent
and the chemistry of the process have not been
elucidated.

Condition of Methane in Coal.

The condition under which methane is included in
the coal in such large volume is still an unsolved problem,
but the most general opinion is that the gas is enclosed
in minute “ pores ” or crevices of various sizes, and
manifestly under high pressure, as indicated by the
large volume found. It has been suggested that the
methane in coal may be in liquid form; but, with a
pressure of 7351b. to the sq. in., the temperature of
condensation of methane is —114 degs. Fahr. The word
“ occluded,” which has been applied in various senses,
is misleading. Chamberlin states that even if gases are
dissolved in coal, they are also held in the minute pores
and cavities, as well as in the larger crevices, where
their presence usually is evident. Analyses of gases
given off by samples of coal kept in bottles for a long
time show that methane is not forming in the coal, at
least under laboratory conditions, for the gas emanation
diminishes steadily.

As the volume of gas spontaneously liberated from coal
at ordinary temperatures is usually considerably more
than the volume of coal, and vastly more than that of
the interstices or pores in even the most porous coal, its
physical condition in the coal is problematical. When
coal is mined and broken up, the gas escapes rapidly
from the pores and crevices, but under ordinary condi-
tions mined coal is in compact lumps, and much time
is required for the complete liberation of the gas, especi-
ally when the tension or gas pressure has diminished.

Chamberlin found that, during the crushing of the coal
in vacuum, only about one-quarter as much methane
was obtained from the sample as was obtained after it
had been bottled for six months in a vacuum. It would
seem that crushing of the coal would so shatter all
particles, that most of the gas could escape at once,
but clearly a large amount is held so tightly that much
time is required for its liberation.

Relation of Methane to Age and Depth of Coal.

There is no relation whatever between the occurrence
of methane and the age of the coal, for all coals from
carboniferous to late tertiary, as well as peat and other
vegetal accumulations in marshes, give off methane. In
some districts the younger coals may be more gaseous
than the older ones, or vice versd, the difference being
due to local conditions.

In general there is more methane in deeply buried
coal beds than in beds near the surface, but seemingly
the principal factor influencing this condition is the
tightness of the cover. There is no evidence that the
original content of gas in coal is in any way related to
depth, but, on the other hand, the proportion varies from
bed to bed, and in different parts of the same bed,
without relation to the present position of the surface.
Many deep workings are free from large quantities of
gas, notably the Monceau-Bayemont, Sacr4 Madame,
Marchienne, and Poirier mines in Belgium. In investi-
gating mines in various parts of the United States, the
author found that it is not always the deepest bed, or
the deepest-lying part of the same bed, that gives off the
most methane. Stainier is of opinion that, in wide
basins of coal accumulation, the chemical changes that
produce methane have increased in activity toward the
centres of the basins, but these centres are not always at
the bottom of the present synclines or structural basins.
Moreover, conditions in some of the earliest deposits may
not have been such as to produce the most gas. Stainier
found that some of the coal areas in Belgium and
France are remnants of a wide basin in which the
proportion of volatile matter increases toward the centre.
On the original edges the beds are anthracite, even
graphitic, and are thinner. After studying those beds
over a wide area, he found that they were much more
gaseous toward the centre of deposition. Where this

part of the field is in the deepest part of the structural
basin, the more gaseous coal lies at a great depth; but
in some districts, on the other hand, the original centres
of deposition have been uplifted into anticlines, and
either lie near the surface on the crests of these uplifts,
or have been entirely removed by erosion.

Relation of Methane to Structural Features.

Those who have studied the methane problem are
generally agreed that structural disturbances alone do
not affect the proportion of gas in coal. There are, of
course, certain obvious relations when other conditions
are equal; the more profound synclines carry the coal
so deep that the factor of depth with increased thickness
of cover becomes important; workings on anticlines may
serve as reservoirs for gas; and zones of crushing con-
tain coal in such condition that it readily gives off a
large volume of gas. Many faults serve as channels by
which gas may enter workings from lower beds, and
shattered measures along faults may carry larger volumes
of gas. On the other hand, in many places the faults
are outlets to the surface, which drain off the gas near
the plane of movement. A fault may also bring a rela-
tively impervious mass of rock against gaseous beds in
such a way as to prevent the escape of gas. In many
of the districts where the gas content or pressure is
greatest in mines the beds show no signs of structural
disturbances or crushing.

In some Prussian mines it was found that in anticlines
from which the crests had been removed by erosion,
much of the gas had escaped, whereas in the anticlines
in which the coal did not reach the surface, a large
volume of gas remained. This was well shown at Wurm,
where coal in the limbs of outcropping anticlines was
free from methane to the deepest workings, whereas in
the closed anticlines there was so much gas that it had
to be drained off by advance drifts before mining could
progress.

In some places more gas is encountered in horizontal
beds than in those that are inclined, because the
strata are more shattered, and the gas drains off through
the joints and crevices.

Relation of Methane to Character of Cover.

As no rocks are impermeable to gas, a certain pro-
portion of the original gas content in all coal has come
to the surface and drained into the atmosphere, and
this process is continually in operation in all coal areas.
Time, the porosity and thickness of the overlying
deposits, and the volume and pressure of the gas, are
the important factors in the rate and extent of the
drainage. The total amount of gas emanation by this
avenue, however, may be only a small proportion of
all that is contained in the coal where the bed lies deep
and where the cover is fine grained and compact. That
tightness of cover is an important feature is shown by the
fact that in the Westphalian coal basin 90 per cent, of
the gas explosions were in the area in which there is a
covering of chalk marl, which is almost impervious. In
the Obernkirchen district the main bed is especially
gaseous only where the roof of porous sandstone is
covered by Wealden clay, and in those mines in Natal
where there is a thick covering of diorite, much gas
remains in the coal. Doubtless beds containing water
impede the escape of gas more than the dry beds, for
moisture must greatly diminish the porosity of the
strata.

Diffusion of Methane.

Where methane is given off gradually from the pores
and crevices in coal, generally there is perfect diffusion
of the gas in the mine air, especially if the ventilation
is active. However, where the gas comes from crevices,
especially those near the roof, the methane, being
lighter than air, rises to the roof, and, if the air is still,
remains there in a somewhat pure state. Carbon dioxide,
on the other hand, being much heavier than air, tends
to accumulate in low places. Of course, the tendency
of either gas to separate by gravitation is of no import
if there is a brisk air current, for diffusion is rapid when
the air is in motion. Experiments show that three or
four hours are required for the complete diffusion of a
body of methane lying on still air in a mine, and that
after the gas has been absorbed by the air it cannot
separate again.

The Prussian Commission made tests at Neunkirchen
on the diffusion of methane. The gas was introduced
into compartments by a top inlet, and in volumes corre-
sponding to 3, 4, and 5 per cent, of the size of the com-
partment. The diffusion progressed slowly, but was
complete in about four hours; in three hours it
approached completeness. The composition did not
change afterwards.

Gases in Coal Samples.

The many determinations of the volume and nature
of gases in coal samples afford important evidence as to
the conditions under which gas is given off in mines,
and the volume of gas to be expected from coal in
mining. Some of these tests have been made by
heating the samples, and others by collecting gas given
off at ordinary temperatures in varying periods of time.
The results have shown great diversity in the volume of
gas and in its components, not only in coals of the
same class, but in samples from different places in the
same bed.

In the earlier determination of gases liberated from
English coal samples, each sample was heated to 212
degs. Fahr., a condition that gives a result not com-
parable with the emanation of gas at ordinary tempera-
tures in coal mines. In making these tests, the coal in
one large piece was placed in a vacuum pump; when
the air had been exhausted the temperature was raised
to 212 degs. Fahr. It was found that the rate of eman-
ation of the gas depended on the hardness of the coal,
and the quantity of gas it contained.

Bedson and McConnell investigated gases contained
in coal and coal dust from English mines. Material
from a working face in the Ryhope mine gave particu-