December 10, 1915.

THE COLLIERY GUARDIAN

1183

pass these glands with the lymph stream. According
to Arnold’s observations, free dust particles enter the
lung along with the dust cells, and take the same course
in the lymph stream.

Arnold found that if an animal was exposed continu-
ously to a very smoky atmosphere for several weeks, the
lungs became quite black and highly charged with soot
particles, partly free and partly taken up in dust cells.
A few weeks or months later, however, if the animal
meanwhile breathed pure air, the dust had almost com-
pletely disappeared from the lungs, except from the
lymphatic glands, which remained black. It is thus
clear that the lungs normally possess a great power
of getting rid of, at any rate, some sorts of dust, even
after it has passed the bronchi and got into the alveoli
and actual lung framework. It can be readily under-
stood, however, that this power depends, to a large
extent, on the lungs being normal and uninjured. In
injured or diseased lungs, on the other hand, dust tends
to remain and gradually accumulate, to judge from
human observation.

Effects of Flint and other Hard Dusts.

It appears from Arnold’s and Beattie’s experiments
that comparatively little real injury to the lungs resulted
from inhaling the soft soot particles of smoke, or the
particles of shale dust or coal dust. With emery dust,
and with dust from the disintegration of the red sand-
stone used for building purposes round Heidelberg,
serious injury to the lungs was prominent, and Beattie
found the same for flint dust, emery (alumina), car-
borundum (carbon silicide), granite dust, and other hard
materials., In scattered nodules the walls of the alveoli
became thickened and infiltrated with cells, thus show-
ing signs of chronic inflammation; and later an excess
of fibrous tissue developed in these nodules, so that the
consistency of the lungs became firmer. Similar nodules
and fibrotic changes developed round the bronchi and
blood vessels and on the pleural surface. No tubercular
infection was present in these nodules. The bronchi
themselves became thickened; the ciliated epithelial cells
showed signs of proliferation and conversion into
“ goblet ” cells. The lymphatic glands also showed
signs of real injury. The amount of these changes
depended on the amount and duration of the dust inhala-
tion, as well as on the nature of the dust and the resist-
ance of the animal. Dogs, for instance, were far more
resistant than rabbits or guinea-pigs. Even with the
comparatively harmless soot dust or coal dust, scattered
nodules of chronic inflammation were produced if
sufficient dust was inhaled.

One may conclude with considerable probability from
all the experiments that while inhalation in sufficient
amount of any kind of dust is prejudicial to the lungs,
they are capable of getting rid of lesser amounts with-
out real injury resulting, but that the amount which
can be got rid of without harm depends on the nature
of the dust, and is far less for such dust as emery, sand-
stone, or flint, than for others such as soot or coal dust.
This hypothesis seems to explain the facts of human
experience as regards dust inhalation. It is also in
accordance with the fact that Prof. Beattie has shown
that even coal dust or shale dust will, if inhaled in
sufficient amount, produce serious changes in the lungs
of animals.*

Taking together the results of experiments on animals
and the facts of human industrial experience with regard
to dust inhalation, it appears probable that the damage
to the lungs from inhaling certain kinds of dust is suffi-
cient to render the human lung extremely liable to attack
by tubercle bacilli, though often insufficient to produce
any very evident symptoms by itself. The nature and
amount of the damage, if any, due to the dust inhala-
tion alone, is evidently dependent on the nature and
amount of the dust inhaled, and the duration of the
inhalation. A good many facts seem also to indicate
that the natural resistance to ill-effects from dust inhala-
tion diminishes with age, although in older persons the
damage to the lungs shows itself in increased liability
to bronchitis rather than to phthisis.

The results of our experiments are given in an appendix
by Mr. Mavrogordato. The shale dust used was that
employed for stone dusting at Bentley Colliery, near
Doncaster. It is from a shale adjacent to the Barnsley
seam, worked at the colliery. This shale was selected
as being free from gritty material and oil. It is ground
up in a mortar mill and applied freely to all the roads
requiring it in the mine, just as at Altofts. The dust
used was passed through a 90-mesh sieve, and thoroughly
dried before use, so that a thick dusty atmosphere might
be produced.

Our experiments gave results similar to those of Prof.
Beattie. If anything, the shale dust seemed to produce
less effect than a similar exposure to coal dust. Plenty
of very fine dust could be seen in sections of the lungs
at the end of a fortnight’s exposure for two hours daily
to the dust; but no marked pathological changes could
be detected, and in animals kept after the exposure the
dust in the lungs diminished gradually, so that after six
weeks only a few dust-containing cells were visible, and
there was no change which, so far as we could judge,
could have any appreciable effect on the functional
activity of the lungs. The shale dust seemed to dis-
appear somewhat more rapidly than the coal dust.

Effects of Flue Dust.

Besides shale dust, flue dust from the furnaces of the
surface engines at collieries has recently been employed
somewhat extensively for stone dusting, and the experi-
ments at Eskmeals have shown that this material is
very effective in rendering coal dust inexplosive. In
accordance with the wishes of the committee, we have
investigated the influence of flue dust when inhaled.

Unfortunately, there is no extended human experi-
ence to go on with regard to the effects of inhaling flue
dust, and in no occupation known to me are men much

* First Report of Explosions in Mines Committee, p. 12.

exposed to this variety of dust. From such enquiries
as I have been able to make in connection with
collieries where flue dust is used for stone dusting, it
would seem that no complaints have, as a rule, been
made by men exposed to inhalation of air containing flue
dust. On the other hand, Mr. A. M. Henshaw, of
Talk-o’-th’-Hill Colliery, informed me that he received
such serious complaints from men employed on roads
stone dusted with flue dust that he had given up its use
in favour of shale dust. He has kindly written to me
as follows with regards to his experiences :—

My experience with flue dust is not altogether satis-
factory. You remember the main road you examined
in January 1913. It had been treated twice, and your
samples, taken after treatment with flue dust, showed
(Mr. Winmill’s) figures :—

Fineness.

Per cent.

Through 30 ...................... 38*5

„	30, left on 60.......... 18’0

„	60	„	90............. 8'0

„	90	„	200........... 3*0,

„ 200 ........................ 10-0

Analysis.

Per cent. ash.

Through 30 ...................... 63*0

,,	60	.................... 65 4

„	90...................... 61*4

„	200	  64*7

These were very good results from the point of view
of rendering the coal dust harmless, but as time went
on, and the deposit increased, the men began to com-
plain. After the fourth treatment, when all the
ledges and cavities were practically filled, the dust
was easily disturbed by passing tubs or men, and
carried along in •the air. The discomfort of this
became so great that some of the haulage boys who
worked on the road left, and others frequently stayed
away. The dust was extremely irritating to the eyes,
and gave a nasty gritty taste in the mouth. This was
also the experience of officials passing along the road,
and was the subject of complaint by the men when
they travelled the road in a crowd at the end of the
shift. The complaint applied to about 1,000 yds. of
the main intake haulage road, which was about 60 to
90 ft. in area, and carried about 60,000 cu. ft. of air
per minute.

We were obliged to discontinue the use of flue dust
in this road. Possibly the objection could have been
remedied by cleaning up oftener, and using less flue
dust, but this would not be likely to give such security
in practice as heaping up plenty of incombustible dust
in every ledge and crevice until big proportions were
got of 5 to 1, rather than occasional and indifferent
cleaning and questionable proportions of about 1 to 1.

In other roads in-bye we have not found flue dust
so objectionable, but, on the whole, I think finely-
ground shale dust is preferable.

Flue dust has been used at other collieries here,
but only to a small extent, and I have heard no com-
plaint. Most people are using shale dust. The light
colour of shale dust and the readiness with which it
can be seen is an advantage not to be lightly ignored.
We have tested the effects of flue dust on guinea pigs
in the same way and with the same periods of exposure
as with coal dust and shale dust. As will be seen from
the appendix by Mr. Mavrogordato, the effects of the
flue dust on the lungs of the animals were much more
marked and more lasting. Signs of proliferation of
epithelium were very marked in the lung alveoli, and
remained very evident even after six weeks. The flue
dust seemed to produce definite signs of slight broncho-
pneumonia, and cells charged with dust were more
abundantly present. Moreover, the dust remained in
the alveolar epithelium to a much more marked extent
than coal dust or shale dust. Six weeks after the
exposure the dust cells were still abundant, and the
sections of flue dust lungs had a very definitely abnormal
appearance, contrasting distinctly with the sections of
the coal dust or shale dust lungs. No definite outward
signs of illness were observed in the animals, however,
and they continued to take their food as usual.

The result of these experiments was somewhat of a
surprise, since there was no reason to suspect the pre-
sence of fragments of crystalline silica in the flue dust.
In order to throw further light, if possible, on the matter,
we tried the effect of breathing the dusty atmospheres
ourselves for short periods, through a face mask con-
nected by a short tube with the box in which the dusty
atmosphere was maintained. With the flue dust it was
noticed that discomfort was produced in the eyes, and
gritty matter was very soon noticed in the mouth. The
presence of this gritty sensation in the mouth reminded
me at once of former experience in inhaling the dust
from machine drills working in Cornish granite without
a water jet. There was no gritty sensation or definite
irritation of the eyes when shale dust or coal dust was
breathed.

On taking a little of the dust (previously passed
through a 200-mesh sieve) into the mouth, the sensation
of grittiness between the teeth was much less marked
with shale dust than with flue dust; and the particles
of shale dust seemed to yield much more to the teeth.
With quartz dust from the Transvaal the grittiness was
still more marked, and the same was the case with fine
flint dust and ganister dust. Samples of flue dust kindly
sent from different collieries in England and Scotland
were examined in this way, but always with the same
result.

On warming and digesting some of the flue dust with
about an equal quantity of distilled water, and examining
the filtrate, it was found that though the latter con-
tained some sulphate in solution, it was neutral to litmus
paper, practically tasteless, and had no irritant action
when dropped into the eye. The irritant effect of the
dust was therefore presumably due only to its mechanical
properties and not to soluble substances contained in it.

The analysis of Altofts shale dust by Dr. Mellor (Fifth

Report of Explosions in Mines Committee, p. 33) gave
as its result that about 35 per cent, of free crystalline
silica is probably present in Altofts shale dust, along
with aluminium silicate and other material. As there
was only about 52 per cent, of silica, combined and
uncombined, in this shale, and as other apparently harm-
less, or relatively harmless, varieties of stone dust
contain a much higher total of silica, and presumably
a much higher percentage of free crystalline silica, it
seems pretty evident that the dangerous effects of a dust
are not in any direct proportion to the free crystalline
silica present. Dust from stone consisting almost
entirely of crystalline silica seems, indeed, to be invari-
ably dangerous, and to be still so when mixed with, or
inhaled in conjunction with, harmless silicate dust, such
as china clay, as in the pottery industry. Dust from
true granite is also dangerous, although the percentage
of crystalline silica in the granite is not very high, much
felspar and mica being present in the stone.

Action of Silica.

Taking the facts at present available together, it
seems probable that crystalline silica is only dangerous
when its particles are broken up into fine, sharp-edged
dust particles. Grains of silica embedded in shale and
many other stones do not appear to be sharp edged.
They have presumably been rounded by the action of
water, like grains of sea sand, and may apparently be
also coated with colloid silica, as Dr. Mellor has pointed
out. When shale is disintegrated in the ordinary ways,
these particles, since they are surrounded by soft
material, and are quite small, will not be broken up,
and consequently no sharp-edged siliceous dust will be
formed. On the other hand, when the crystalline silica
is present in larger masses, as in granite, or the stone is
a mass of crystalline silica, as in flint, ganister, compact
sandstone, etc., the silica will be broken up in the process
of mechanical disintegration, and fine, sharp-edged dust
particles will be formed. Such dust will therefore be
dangerous.

Fused silicate in the form of dust is less hard than
quartz, and usually more liable to solution or disintegra-
tion when it enters the lungs. For this reason it is
probably less dangerous than quartz, but if the particles
are hard and irregular in shape they must produce con-
siderable irritation for a time. Flue dust consists,
presumably, of fused silicate to a large extent. It is
more or less rough and gritty when introduced into the
mouth or eyes, and probably its gritty character explains
its effects on the lungs of guinea pigs. Nevertheless,
there is no clear evidence that, if inhaled in small
quantities over long periods, it would produce the serious
liability to phthisis produced by the inhalation of fine
quartz dust. It seems, on the whole, more probable
that it might not produce worse effects than an increased
tendency to bronchitis, such as seems to be produced to
a greater or less extent by certain other kinds of the less
dangerous dusts.

Although flue dust is a suitable and very convenient
material for rendering coal dust inexplosive, and
although it does not seem probable that very serious
effects on health would, in practice, arise from its use for
stone dusting in mines, I cannot, in view of the evidence
mentioned above, recommend its use. If it were used
wholesale in coal mines it might introduce a danger to
health which, even though small as compared with the
effects of the more dangerous forms of dust, would cost
far more lives or cases of disablement than would be
saved by abolishing coal dust explosions. At present,
we do not know how great or how small the danger
arising from flue dust inhalation would, in practice, be,
but it does not seem to me justifiable to experiment in
this direction on fellow men. As shale dust and other
harmless materials are available for stone dusting, and
there seems to be no doubt as to their safety, there is,
in any case, no necessity for using a substance as to the
effects of which on men serious doubts exist.

Characteristics of Harmless Dusts.

It may be of use to endeavour to give some indication
of the characters to which shale or other material used
for stone dusting ought probably to conform, in order to
ensure its harmlessness when breathed.

It ought evidently to be capable of being easily dis-
integrated into a fine powder, otherwise it would be
unsuitable for its purpose in stopping coal dust explo-
sions, or would require very elaborate treatment to make
it suitable. Hard stone of any kind is thus unsuitable,
apart from any danger arising from its inhalation; but
in any case a hard stone should not be used, as the
dust from it would very probably be dangerous to
breathe. A soft and easily disintegrated stone contain-
ing coarse particles of sand or quartz seems also
unsuitable, as the coarse particles might easily be broken
up to some extent in the process of grinding up the
material, and might thus yield fine, sharp-edged particles
which would cause trouble when inhaled. If the stone
were very finely ground, this risk would be increased.
Some of the rock adjacent to coal seams contains so
much sand that, although it is quite soft and easily
disintegrated, the sand can readily be recognised by the
roughness which it imparts; and the stone itself may be
classified as sandstone. When this rock is pounded up,
the clay of which it mostly consists can be washed
away, leaving the sand, which is easily recognised by
its feel, colour, and appearance under the microscope,
particularly on rotating the field with crossed Nicols.
On similarly attempting to separate sand from a small
quantity of such shale as is used at Altofts or Bentley for
stone dusting, no residue of sand is obtained by ordinary
rough methods, such particles as are presumably present
being too small to separate out. The shale itself is
also without roughness on its surface, and it yields quite
easily to the teeth.

It seems desirable that any shale used for stone
dusting should be as free from grit as that just described.
Whether the presence of oily matter in a shale is a
serious objction to its use for stone dusting is still
doubtful. Provided that a mixture of shale dust and