From time to time the whole community is shocked and saddened by great mining disasters involving the loss of many lives. The chief causes of those disastrous occurrences are Explosions, Outbreaks of underground fires, and Inundations of water. While the foregoing are sometimes looked on as being the greatest of the dangers the miners have to face, they are by no means so, but public attention is focused on them because they invariably claim a large number of victims at one time. Falls of roof and sides cause a far larger number of deaths annually, but such accidents seldom involve more than one or two deaths at a time, and public attention is not drawn to this class of accident in the same way.

Mine Explosions. — Explosions may be classed as (1) those which are caused by the ignition of firedamp, but which in a dusty mine usually end as a dust explosion; and (2) those which are caused by the ignition of coal dust by itself. As already pointed out in the section on mine gases, firedamp becomes explosive when the air contains about 6 per cent, of the gas, or thereby. The maximum explosive point is reached when 10 per cent, of the gas is present. Gas explosions, while fairly common, seldom extend over more than a small section of the mine, and the number of victims is comparatively small. Unfortunately, however, most mines that give off large quantities of firedamp are usually deep mines where the workings are dry and dusty. In such cases an explosion of gas raises the fine dust and the explosion passes throughout the mine, carrying death and devastation in its track. Thus the greatest danger of firedamp is due to the ease with which, if ignited, it raises and propagates a dust explosion. With firedamp alone the resultant gases formed by an explosion consist of carbon dioxide and steam; the nitrogen does not take part in the reaction. Thus

The products of combustion are irrespirable, and persons who may escape the violence of the explosion are often killed by the effects of this afterdamp, as it is called. Sometimes a gas explosion may occur, and some of the men may escape through the air being restored before the afterdamp has had time to do its deadly work.

When the explosion is due to coal dust, or where coal dust is present and takes part in the explosion along with firedamp, the afterdamp always contains large quantities of deadly carbon monoxide, which almost invariably claims as its victims any who may have escaped the violence of the explosion.

Coal dust, when in a finely divided state, is an extremely dangerous substance in the mine. Such dust is easily stirred up, and when a cloud of it is suspended in the air current, it may under certain conditions become ignited and cause an explosion. For many years the question of whether coal dust was explosive unless firedamp was also present was much disputed. Numerous investigations and experiments have now shown that coal dust, when acted on by a flame of sufficient volume and intensity, will cause an explosion even if firedamp is totally absent. Naturally if the air in which the dust is suspended carries a small percentage of firedamp it makes ignition and propagation easier, but its presence is by no means necessary to produce an explosion. When fine coal dust is raised by a local explosion of firedamp, or by a blown-out shot, and brought into contact with the flame therefrom, the fine particles of dust are heated to such a temperature that the contained gases are driven off. These gases feed the flame, which now rushes through the workings in a series of explosive waves, until it has travelled throughout such portion of the mine as can supply it with the necessary dust to carry it on. The presence of wet or dustless zones may, if sufficiently extensive, arrest the explosion and prevent its extension to other parts of the mine. Dust explosions travel against the air, and investigation shows that they usually develop their greatest violence along main intakes and haulage roads. These are the places where the finest dust is deposited, and this, with the fresh air available, furnishes the necessary fuel to carry on the explosive wave. While the explosion seldom reaches the workings owing to the lack of sufficiently fine and dry dust there, the afterdamp does, and the large number of deaths that take place in coal dust explosions are due to its effects.

Prevention of Dust Explosions. — Some years ago an investigation, bearing on the danger of coal dust in mines was carried out at Altofts on behalf of the Mining Association of Great Britain. These experiments have since been repeated and verified on behalf of the Home Office at the Government testing station at Eskmeals. The Committee on Explosions in Mines believe that 'as a result of these experiments they have established, as far as experiments can, that

(i) The maintenance throughout the roads of such a proportion of incombustible dust in a fine state of division as would make a mixture yielding on incineration at least 50 per cent, of ash, or

(ii) The maintenance of at least 30 per cent, of water in a state of intimate mixture with the dust throughout the roads, or

(iii) A combination of the two previous methods, i.e. the treatment of the roads first with incombustible dust and then water, would prove effective in very greatly minimising, if not preventing, explosions of coal dust.'

In other words, experiment has shown that fine stone dust, if mixed in sufficient quantity with the coal dust, will minimise or prevent a dust explosion, or that water, if sprinkled over the dust in sufficient quantity, will effect the same purpose. If desired, both dust and water may be applied.

One important point to notice is that the incombustible dust used should be obtained from argillaceous, and not from siliceous rocks, as dust from the latter source has an injurious effect on persons breathing it. It will be readily understood that where fine dust exists in a mine the workmen must inhale at least a small portion of the finer particles when breathing the air in which it is suspended.

Spontaneous Combustion. — Certain coals if left in a finely divided state under pressure in the waste, and so placed that the passing air current is limited in quantity, develop heat, and ultimately catch fire. This occurrence is called spontaneous combustion. The actual cause or causes of spontaneous ignition of coal are somewhat obscure, but it will serve our purpose to indicate how such occurrences are best dealt with. The presence of 'heating coal, is always indicated at an early stage by a peculiar tarry smell which is easily recognised. Once this smell is found the heating develops rapidly, and unless immediate steps are taken to deal with the situation, the mass soon breaks out into flame.

The gases which are given off from a waste or gob fire consist chiefly of various inflammable hydrocarbons, and carbon monoxide, with traces of sulphuretted hydrogen. Owing to the large proportion of carbon monoxide, and to the highly inflammable character of these composite gases, they must be led away and diluted with large quantities of air. Needless to say, whether the mine in which a fire occurs is in ordinary circumstances worked with safety lamps, or with naked lights, only safety lamps should be used while dealing with the fire.

Usually the best method of dealing with the early stage of an outbreak is to locate it as early as possible, and fill it out, removing the heated material from the mine to the spoil heap on the surface. If the fire breaks out and gets a hold, it cannot be dealt with in the above manner. It is then usually sealed off or drowned out if circumstances admit of the latter remedy being applied.

Where a section of mine workings has to be sealed off, the building of the stoppings should begin at the return end of the section to be closed, and the work continued gradually to the intake end. The intake should be the last opening to be closed. Once the fire has been extinguished, great care should be exercised in the removal of the stoppings as carbon monoxide is almost certain to be liberated, and because of the deadly nature of this gas no risks should be taken.

In some seams which are known to be liable to spontaneous combustion, the workings are laid out in sections 50 arranged that should an outbreak of fire take place it can be sealed off with as little interference with the remaining sections as possible. On the whole, however, prevention is better than cure, and where spontaneous ignition is common the wastes should be packed with stone or sand, and as much old timber withdrawn from them as circumstances will allow. Where pillars of coal have to be left they should be well packed round with stone or built in with brickwork if necessary.

A system of packing the waste, which involves the carrying of fine rock, shale, or sand from the surface into the workings suspended in a stream of water passing through a line of pipes is now used. This is an efficient method of packing, and can be carried out at moderate cost. Its use in seams liable to spontaneous combustion will probably be found the best method of dealing with this difficult problem.

Irruptions of Water. — In the ordinary course of working coal the old workings have to be abandoned when exhausted. When operations are stopped these workings usually fill up with water.

Should neighbouring royalties be worked at a later period, it may be necessary to approach or communicate with these old workings. Where such operations are necessary, the greatest precautions must be taken to prevent driving through on the water, and thus running the risk of what might prove a serious accident. Driving into old workings may happen through want of knowledge of their presence, through want of sufficient care, and through defective plans. To prevent such accidents the Mines Act provides that where any working has approached within forty yards of a place containing, or likely to contain, water or other liquid, the working shall not exceed eight feet in width, and that boreholes both front and flank shall be carried not less than five yards in advance of the working.

These boreholes, if properly placed, will tap the water before there is any danger of the coal between the old and new workings bursting under the pressure of the water, and flooding the workings. By means of a suitable plug placed in the borehole after it taps the old workings, the water can be drawn off gradually.