April 18, 1913

THE COLLIERY GUARDIAN

799

liable to destruction by an explosion. There had been far
more lives saved, he said, by the stoppings giving way and
short-circuiting the foul air, so that it failed to reach men
beyond, than had ever been lost for the same reason.
Wingate Colliery furnished one of the most striking
instances. If the intention had been to provide stoppings
which would tend towards a greater volume of air reaching
the face, one could better have understood the motive; but
a stopping which could provide both tightness and com-
parative weakness would then have been required. A colliery
explosion increased in violence with its progress against
the resistance furnished by the gallery along which it was
travelling. If that resistance found some relief through the
stoppings giving way, the consequent violence should not
be so great, and its progress, therefore, would have a better
chance of coming to an end. In other words, if a gun had
holes in the side of its barrel as well as in its muzzle, would
the muzzle velocity of the projectile be as great? More-
over, how often had it happened, within the experience of
members, that the broaching of some stopping had been of
urgent and, perhaps, vital importance in order to establish
an air-current or to divert one ? In such cases, stoppings
of great thickness and permanent character would be most
objectionable.

Reversals of the Air Current.

It had been suggested lately—not, however, by anyone
with much practical knowledge of mines—that, by periodical
reversals of the air, coalmines could be systematically
cleared of quantities of dangerous coaldust. Mining men
could quote numerous examples in their own experience
where “changing the wind” did not carry away the dust,
or even raise it; and it was difficult to see how dust which
had already proved itself too heavy to remain in an air-
current could again rise into an exactly similar air-current
going in the reverse direction. Even the lighter top-dust
clung together with much mutual attraction and was not
easily disturbed.

The light-hearted suggestion to change the ventilating
pressure in various parts of mines—even if it were entirely
feasible on account of the valve-like character of the doors
—had many possibilities of danger. The reversal of air in
a mine might almost be compared to the reversal of, say,
sewage in a town, and could only be carried out in fairly
safe mines where the air-sewage was clean, or, under dire
necessity, after very careful consideration of the attendant
circumstances. Hitherto it had been an unwritten law that
the principal “ return ” airways should be so arranged that
they might cut off the foul gases of the goaves from the
clean “ intake ” air, and to reverse that condition might be
serious. In doing that, a certain co-relationship of pressures
had been established, which, even under barometrical
changes, remained comparatively constant. To put the
higher water-gauge pressure of an “intake” upon some
large area of goaf, instead of its customary “return”
pressure, or vice versti, might result in some other part of
the mine being flooded with gas.

He well remembered a very prominent instance when it
was desirable to reduce the ventilating pressure in a mine
where a fire was raging, and the fan was therefore slowed
down so that a greater pressure accumulated in the main
return, which had usually the pull of several inches of
minus water-gauge and had shared the drainage of a very
large acreage of abandoned workings with an adjoining
colliery possessing a separate fan and separate system of
ventilation. When the first-mentioned mine ceased to do
its share and failed to maintain its usual pull, the neigh-
bouring mine was invaded by a dangerous flow of firedamp,
and work therein was suspended. Had the pressure been
actually reversed, instead of reduced, the gas invasion
would have been much more copious and dangerous; the
same sort of thing was bound to take place also, not only
between mines communicating with each other, but also
between parts of a mine. It might be taken that in every
coalseam there was a certain maximum pressure of pent-up
gas which found issue with increasing ease as it neared its
exit at the artificial surface laid bare by the miner. That
decreasing pressure terminated at last in that of whatever
atmosphere might exist at the point of issue; but there
would always be an average feeder, sometimes with little or
no actual issue, when being pressed back, and sometimes
with more if the opposing pressure became less. It must
be obvious, however, that, after being dammed back by an
increasing pressure, there would be a greater flow when the
dam was removed. In such a case as that, therefore, after
the pressure of an “ intake ” air was changed to that of the
“ return,” more gas would escape for a time, exactly as in
the case of a decreased barometer. That phenomenon had
reference only to the exit of gas by fissures and cleavages in
the natural strata, but very different conditions were to be
met with in the case of the artificial spaces and goaves
made by mining. Some goaves and spaces might be
regarded as bottles into which firedamp was received and
stored. Some of the bottles would have their mouths lying
upwards, some level, and some downwards. If increase of
pressure were regarded as making a heavier air fluid, these
bottles would act as if water could, in the first case, flow
into them, displacing the contained gas, and, in the latter
case, seal up the bottle-mouth and its contents. In one, the
gas would then have exit more or less continuously under
changes, while, in the other, there would be none at one
time and great quantities at another.

The Use of the Hygrometer.

Mr. Blackett concluded by questioning the wisdom of
the provision, in section 71 of the Coal Mines Act, as to
the periodical readings of the hygrometer. The mining
engineer, he said, understood and appreciated the
barometer, which was of some use in helping officials to
account for the presence of gas when they might other-
wise be anxious, for instance, as to whether some
interference had taken place with the ventilation.
Could anyone explain, however, how it was intended
that observations of the hygrometer day by day or week
by week were going to affect the conduct of the official
making them ? The mining engineer knew that it told
him the amount of moisture in the air. He knew that
drier air made drier dust, and that moister air was less
endurable from a hygienic point. He did not know,
however, that the ordinary changes in that dryness, from
day to day, would make one day any more dangerous
than another from a practical point of view. Even if
it did, what could be done? Was an official, under
drier conditions, to be more anxious about the dust than,
under moister conditions, about the health of the
workmen? Wiser men might be able to tell the
speaker, but, meanwhile, he rested under the impression
that such provisions as these lowered the prestige of
English mining laws amongst England’s miners.

The Discussion.

Mr. John Gerrard, H.M. inspector of mines
(Worsley), wrote supporting Mr. Blackett’s views. He
felt sure that, if Sir Fredk. A. Abel had lived, he would
have made further experiments and have altered his
views. He thought Sir Frederick’s pronounced opinion,
formed upon a very slender basis, had done much to
hold back the protection which Mr. Blackett so well
advocated, and which he (the writer) was ready to
support in every possible way. Personally, he liked
shaledust. Some in his district had a nice soapy feel.

Dr. J. S. Haldane (Oxford) wrote expressing his
complete agreement with Mr. Blackett in what he said
about stonedust. Like Mr. Blackett, he found salvation
on the coaldust question after reading Messrs. W. N.
and J. B. Atkinson’s most valuable book. Thence-
forward, he had no doubt that practically every great
colliery explosion was due to coaldust alone. On the
physiological question as to whether a man would be as
well off when breathing air containing 17J per cent, of
oxygen and 82J per cent, of nitrogen in a mine as when
breathing pure air at an altitude of 5,000 ft. above sea
level, he thought that experimental investigation had
given the clearest affirmative answer.

Mr. H. W. G. Halbaum, Cardiff, wrote expressing
the opinion that there was continued need for the
principles of coaldust theory to be earnestly promulgated
since it was still found that many colliery officials had
great difficulty in grasping the question, especially so
far as the incombustible dust treatment was concerned.

Dr. John Harger, Liverpool University, wrote
saying that Mr. Blackett’s paper was of especial
interest as being a supplement to the paper of 19 years
ago. Most mines were now more dangerous than then,
owing - to the introduction of excessive ventilation,
balanced, however, to some extent, by the enormous
precautions taken, to prevent an ignition of gas, by the
introduction of better lamps and the watering of dusty
places prior to the firing of a shot. Where the roadway
of a mine was safe at present, it was nearly always due
to natural causes or to an accident in the shape of a wet
or stone-dusty road—but these preventives of explosion
were recognised more than 19 years ago. There was
not, so far as he knew, a single experiment which could
be quoted to show that stone-dusting of the roads would
prevent a dust explosion under ordinary mining condi-
tions. There should, however, be no difficulty in making
the main intake roads safe by stone-dusting. With
plenty of stonedust on the floor, so that the dust would
always be raised into the air by the traffic and subse-
quently deposited on the sides, &c., with the coaldust,
they should be safe. As one approached nearer to the
working faces, however, the problem became more
difficult. Any amount of stone-dusting could not
prevent a similar disaster to that which occurred at
Cadeby Main Colliery.

He ventured to say that Mr. Blackett was out of his
depth when he discussed the effect of the percentage
of oxygen on respiration, and, in fact, had himself
confessed his ignorance on the subject. If Mr. Blackett
could not grasp the fact that air containing 17| per
cent, of oxygen at sea-level was the same for respiration
as air containing 21 per cent, of oxygen at an altitude
of 5,000 ft., the writer was afraid that Mix Blackett
must be labelled, in his own words, as “ one of the
crowd of unenlightened experts ” in that particular
section of science. Mr. Blackett had referred to the
writer’s patent in connection with the use of inert gas.
The writer generally did patent his work, when it was

patentable, chiefly to obtain priority, for there were
many people nowadays who had no originality of their
own and liked to commandeer other people’s work. The
writer added that there was no difficulty in rendering
any quantity of inert gas free from poisonous material
and at very little cost. He had no doubt that Mr.
Blackett knew of many instances in his mining expe-
rience where a supply would have been invaluable.

He entirely agreed with Mr. Blackett in his
preference for fluedust to ground shale. Some shales
when powdered gave off a dangerous amount of heavy
hydrocarbon gas. One shaledust, in actual use in a
large mine which he had been investigating within the
last few days, gave off gas sufficient to form an explosive
mixture with the air above it. The hygrometer, as
used under the Act, was, he agreed, of no use whatever.
The ordinary barometer, also, was of little use. A
pocket hygrometer used in the working places was,
however, of great use as indicating the quantity of
moisture in the air, which was an important matter in
hot mines for the health and comfort of the men, and
was invaluable in mines subject to gob fires. The
registering barometer was of great use, too, as giving a
complete and continuous record of changes, but one or
two readings in 24 hours was of little, if any, use to a
mine manager.

Mr. J. G. Weeks said with many of the facts
mentioned by Mr. Blackett he was in full accord from
beginning to end. Probably four out of every five
clauses of the Coal Mines Act, 1911, had to be amended
in some way or other, and now the Regulations were
very much in the condition in which the Act was at
first. Some of the clauses were, he thought, deadly in
their consequences. In all his experience, he did not
know anything more dangerous than the clause to which
Mr. Blackett had alluded. He himself would take no
responsibility if that stipulation was insisted upon.
There were several other points—such as the reversal of
the air—which were endeavouring to be thrust down
their throats.

Mr. J. H. Merivale seconded the motion. He was
so thoroughly in agreement with Mr. Blackett that
really there was nothing to criticise.

Prof. Henry Louis said he was in thorough agree-
ment with the great majority of the points raised by
Mr. Blackett. Personally, he was not altogether a
believer that boiler ashes were the best material to use
for dusting the coalmine. They possessed certain
advantages, he admitted, amongst them being the fact
that particles of inorganic material were more or less
rounded by the action of the flame and, therefore, there
was less risk of dangerous lesions of the lungs of men
than when dust which had been ground and possessed
sharp angles was used. On the other hand, he did not
think that, from the chemical point of view, the boiler
ash was the best material, because he had a very great
belief in the action of dust as a heat absorbent in
destroying coaldust explosions, and heat absorption
could be very extensively brought about by using a
material which contained water of hydration, like a
shale which contained hydrated silicate of alumina. In
districts where there were large deposits of crude
gypsum, that crude gypsum, finely ground, would
probably make the very best material for quenching and
preventing colliery explosions. He called attention to the
great success which appeared to have realised in recent
Belgian experiments in what they had called “ prolonged
stemming,” prolonging the stemming of a shot-hole in a
pile of inert dust outside. He saw that these experi-
ments had been repeated in Austria, and were spoken
of as highly successful. He trusted that that promising
line of investigation would not be neglected in this
country. It was only those people who had a very
large consumption and a very dirty coal who would
have sufficient boiler ash for the purpose mentioned-
Mr. Blackett spoke of a dust explosion as a combustion
pure and simple. They knew from what had occurred
within comparatively recent times that it behoved them
to be very careful in distinguishing between inflamma-
tion and explosion, and he submitted that, in the case
of coaldust, the ignition of coaldust commenced with
inflam mation, the velocity of which proceeded with greater
and greater rapidity until they got a true explosion. He
defined an explosion as “a self-propagating ignition,
propagated with a velocity which was practically that of
sound in air.” As to the danger produced by the
thickening of oxygen in front of a dust explosion, he did
not think that anybody could doubt that, when there
was a compression of oxygen, an excess of oxygen, dust
explosion was more likely to take place than when they
had a deficiency of oxygen. What, then, were they to
think of the recent action of the Government in making
it easy to pass as safety explosives explosives which
contained an over-balance of oxygen and many of which
gave out free oxygen into the air.