April 25, 1913.

THE COLLIERY GUARDIAN.

851

To gain some estimate of the extent of the mixture
effected with different velocities of air-current, readings
were made of the caps formed in a safety lamp placed
(i) on the floor of the gallery, and (ii) 5 in. above the
floor. In the first case, the air inlet of the lamp was
Sin. above the floor; in the second case, 10in. above
the floor. In each trial the gas was adjusted until a

distinct and steady cap was formed in the lamp on the
floor ; the lamp was then raised to the higher position
without altering the gas or air current.

(a)	Velocity of Current 130 ft. per Minute.

(i.)	(ii.)

Position of air- 5 m. from floor ... 10 in. from floor,
inlet of lamp.

Observation ...... A	cap j in. high A cap was formed

formed.	reaching to the top

of the lamp but not
spreading down-
wards.

(b)	Velocity of Current 200 ft. per Minute.

(i.)	(ii.)

Position of air- 5 in. from floor ... 10 in. from floor,
inlet of lamp.

Observation ...... A	cap | in. high A cap was formed

formed.	varying from 2 to

4 inches. It did
not reach to the
top of the lamp.

(c)	Velocity of Current 800 ft. per Minute.

(i.)	(ii.)

Position of air- 5 in. from floor ... 10 in. from floor,
inlet of lamp.

Observation ...... A	cap | in. high A cap | in. high

formed.	formed.

Similarly when, with the velocity of the current at
800 ft. per minute, the gas was adjusted to give caps of
% and 1 inch respectively when the lamp was on the
floor, no alteration could be observed with the lamp was
raised 5 in.

(d)	Velocity of Current 200 ft. per Minute.

(i.)	(ii.)

Position of air- 5in. from floor ... 10 in. from floor,
inlet of lamp.

Observation ...... A	steady 1 in. cap A cap formed, ran up

formed.	to the top of the

lamp and came
down the gauze,
where it continued
to burn, extin-
guishing the lamp
flame.

In our apparatus there is evidently more perfect
mixture of gas and air the higher the velocity of the

current.	!

To sum up this series of experiments, we do not
profess to have elucidated all the differences that appear
between Abel’s work and our own; but on re-reading |
Abel’s account of his observations in the light of these
experiments we see how closely he describes many of
the appearances we ourselves have witnessed. It will
be understood that our chief difference is not with
regard to the observations themselves, but to the
interpretation to be placed upon them.

(ii.) Experiments in an Iron Tube, 1 ft. in diameter.

A tube of mild steel, of 1 ft. internal diameter and
50 ft. long, was attached, through a chamber and a short
fan-drift, to a Sirocco fan driven by an electric motor
(see fig. 2). The speed of the air-current could be
varied by varying the speed of the motor or by (iiameter< it was closed at the bottom end by a card of I
asbestos, through the centre of which a hole 1 in. in'
' diameter was cut in order to admit the nozzle of a I
| Meeker burner. The tube acted as a chimney, drawing i
! a supply of air, in addition to that which passed through a dense cloud. The essential parts of the time-pressure
the Meeker burner, through the joints at the asbestos curves obtained are reproduced in fig. 3, from which it
card.	| will be seen that not only was the rate of development

The flame of a Bunsen burner was caused to project of pressure greatly reduced when the dust was present,
across the upper end of the tube, and the gas supply but the maximum pressure attained was less by about
of the Meeker burner adjusted so as to give a mixture 10 lb. per square inch,
of gas and air in nearly explosive proportions.

I When magnesia was dropped into the tube from the
orifice of the intake, which was of top without being allowed to pass through the Bunsen

ArranfenfCTit for
drqpptnf dcifl

Direction of air current

-jo-

/tin drift

~/nCa.ic«

Pomt of ignition

restricting the

Fig. 2.

conical shape. The gas (ordinary lighting gas) for flame, no effect could be observed, although the
making the explosive mixtures was admitted on the * magnesia remained in suspension in the tube for some
intake side of the fan through a 3 in. main.	, time, and a few particles were carried upwards by the

A continuous sample of the mixture passing during ascending current through the lighted Bunsen flame,

an experiment was obtained at a point half-way along around which a large blue aureole played.

the tube and analysed. A measurement of the velocity I When, however, the dust was dropped through the

-of the stream of gas and air was made beforehand by
means of an anemometer.

The means of ignition employed was the spark from
an induction coil using an electrolytic interrupter in
circuit with the primary. Using current at 110 volts, a
“flaming” spark 4in. long was obtained. The spark
was passed along a diameter of the 1 ft. tube at a point
1 ft. from its junction with the chamber.

The following experiments were made with this
apparatus:—

(a) A mixture of coal gas and air containing 5*2 per
cent, of coal gas was passed through the tube at a
velocity of 450 ft. per minute.

As soon as the tube was well and uniformly filled
with the mixture, and with the current still flowing, the
: spark was passed. No explosion took place, but a cap

12 in. long was formed following the direction of the
gas stream. (Observation of the cap was made through
small holes bored in the side of the * tube for that
purpose.)

Eighty grammes of calcined magnesia were then
allowed to enter, the spark being maintained. No
explosion took place, nor could any lengthening of the

cap be observed; it was merely rendered more visible
by the incandescent particles which it contained.

This experiment was repeated several times with the
same result.

(b)	A mixture of coal gas and air containing 7’4 per
cent, of coal gas was passed through the tube at a
velocity of 315 ft. per minute.

On passing the spark, a flame travelled slowly with
the air current, and for a short distance towards the
fan. The mixture continued to burn (at the point of
union between the 1 ft. tube and the fan drift) after the
spark had been cut off, showing that an explosive
mixture had been obtained in which the normal speed
of propagation of flame was equal to the velocity of the
air current.

Whilst the mixture was still burning (the current of
gas and air being continuous) calcined magnesia was
allowed to enter as in the previous experiments. No
explosion took place, and no alteration beyond an
increased luminosity could be observed in the flame.

(c)	A mixture of coal gas and air containing 9 3 per
cent, of coal gas was passed through the tube at a
velocity of 315 ft. per minute.

(i)	Gas and Air alone.—On passing the spark an
explosion took place. The speed of travel of the flame
over the last 25 ft. of the tube, as recorded on a
chronograph, was 54*3 ft. per second.

(ii)	Gas and Air with Magnesia in Suspension.—On
passing the spark an explosion took place. The speed
of travel of the flame, measured as before, was 30*3 ft.
per second.

| The presence of magnesia seemed, therefore, to have
a retarding effect on the rate of travel of the flame.

(<Z) A mixture of coal gas and air containing a higher
percentage of gas than in the last experiment (no
analysis was made), was passed through the tube at a
velocity of 125 ft. per minute.

(i)	Gas and Air Alone.—Speed of travel of flame
= 124*2 ft. per second.

(ii)	Gas and Air with Magnesia in Suspension.—
Speed of travel of flame = 84*3 ft. per second.

(iii)	Gas and Air Alone.—Speed of travel of flame =

121 5 ft. per second.

This experiment shows that the presence of magnesia
in suspension makes the progress of the flame less rapid,
and thus confirms the previous experiment.

(iii.) Experiments in a Vertical Glass Tube.

1 The apparatus employed was practically a replica of
that used by Mallard and Le Chatelier in their “ con-
firmation ” of Abel’s experiments, to which reference
has already been made.	I

The glass tube was 5 ft. long and of 2 J in. internal |

Bunsen flame, and thereby became red-hot, each heated
particle as it passed down the tube was surrounded by a
small blue aureole which persisted during the descent
of the particle through the mixture. By heating a
single small particle of stone or magnesia dust and
throwing it into the tube, a single “ aureole ” could be
caused to form and pass downwards, the rest of the
mixture in the tube remaining unburnt.

The effect of dropping heated particles through the
mixture was, therefore, no different from what would be
produced were a lamp flame to be moved rapidly through
a non-explosive mixture similar to that in the tube—a
“cap” would be formed round the flame and would
travel with it as it moved.

In later experiments a small fan was fixed across the
tube at a point 18 in. from the bottom. This fan could

be revolved at a high speed on a horizontal axis. At a
point 18 in. from the upper end of the tube, terminals
for the passage of an induction coil spark were fixed.

The following experiments were then made:—

A “ lower limit ” mixture of gas and air was obtained
which, when the spark was passed, ignited, the flame
travelling slowly in both directions—with and against

the gas stream.

When the fan was set running and the spark passed,
the flame travelled very rapidly in both directions.

When the fan was set running and magnesia thrown
into the tube, the magnesia was held in suspension, a
spiral whirlwind, laden with the dust, extending to within
18 in. of the top of the tube. On passing the spark no
ignition took place.

This pair of experiments was repeated four times in
succession with the same results. They are directly
opposed to Abel’s experiments, inasmuch as they show
that a mixture which could be ignited when it held no
dust in suspension became inexplosive when laden with
magnesia dust.

(iv.) Experiments in a Spherical Explosion Vessel.

A spherical explosion vessel of about 4 litres capacity
(designed in the first place for work on the lower limits
of explosion of gaseous mixtures), was employed.

The exploding vessel was fitted with a recording
pressure-gauge and electrodes for igniting the mixtures
in the centre of the sphere. A small fan was fixed near
the bottom of the vessel and could be rotated at a high
speed so as to agitate the mixture and maintain dust in
suspension.

The following experiments appear to afford conclusive
evidence of the retarding effect that magnesia dust has
on the rate df propagation of flame through an explosive
mixture.

(a.) A mixture of ethane and air containing 3*33 per
cent, of ethane was employed. In the first experiment
no dust was present, but the fan was revolved at a
speed of 6,000 revolutions per minute. In the second,
5 grammes of freshly calcined magnesia were present;
the fan was revolved at a speed of 6,000 revolutions
per minute as before, this speed sufficing to maintain
practically the whole of the dust in suspension, forming

Time — seconds. (Time 0 = time of first appearance of
pressure.)

Fig. 3.—Explosions of Ethane and Air.

Ethane = 3’33 ]5er cent.

20

GO

02

l.) No du«t in «o<ipenclon

06

70

Fan* running nt a npMd of 6.000 I
per min in oacb case.

| (b.) A pair of experiments was now made to see
whether finely divided platinum would have any effect
on the development of the explosions. A mixture of

ethane and air containing 4*05 per cent, of ethane was
used. The results are shown in fig. 4, from which it
will be seen that any accelerating effect that the presence
of the finely divided platinum may have had on the rate

of development of the explosion was but slight.

(c) Finally, four experiments were made with the

same mixture of ethane and air (containing 4*15 per
cent, of ethane), in one of which no dust was present, in
one finely divided platinum, whilst in the remaining two
different quantities of calcined magnesia were main-
tained in suspension. The results are shown in fig. 5.

The conclusion that we have drawn from the above
experiments is that the presence of suspended incom-
bustible dust in weak gaseous mixtures does not render
them more explosive than before. On the contrary, we
are of opinion that as heat-absorbing material they
render such mixtures less explosive.

We examined this matter as carefully as we could to
aid in dispelling any apprehensions that may exist as
to possible increase of danger from gas explosions
through the introduction of inert dusts into mines.

Finally, we may remark that the introduction of