[Illustration:  Fig. 33—­Part of a trunk of Sigillaria, showing the thin outer carbonised bark, with leaf-scars, and the seal-like impressions where the bark is removed.]

The last great danger to which we have here to make reference, is the explosive action of a quantity of coal-dust in a dry condition.  It is only now commencing to be fully recognised that this is really a most dangerous explosive.  As we have seen, large quantities of coal are formed almost exclusively of lepidodendron spores, and such coal is productive of a great quantity of dust.  Explosions which are always more or less attributable to the effects of coal-dust are generally considered, in the official statistics, to have been caused by fire-damp.  The Act regulating mines in Great Britain is scarcely up to date in this respect.  There is a regulation which provides for the watering of all dry and dusty places within twenty yards from the spot where a shot is fired, but the enforcement of this regulation in each and every pit necessarily devolves on the managers, many of whom in the absence of an inspector leave the requirement a dead letter.  Every improvement which results in the better ventilation of a coal-mine tends to leave the dust in a more dangerous condition.  The air, as it descends the shaft and permeates the workings, becomes more and more heated, and licks up every particle of moisture it can touch.  Thorough ventilation results in more greatly freeing a mine of the dangerous fire-damp, but the remedy brings about another disease, viz., the drying-up of all moisture.  The dust is thus left in a dangerously inflammable condition, acting like a train of gunpowder, to be started, it may be, by the slightest breath of an explosion.  There is apparently little doubt that the presence of coal-dust in a dry state in a mine appreciably increases the liability of explosion in that mine.

So far as Great Britain is concerned, a Royal Commission was appointed by Lord Rosebery’s Government to inquire into and investigate the facts referring to coal-dust.  Generally speaking, the conclusion arrived at was that fine coal-dust was inflammable under certain conditions.  There was considerable difference of opinion as to what these conditions were.  Some were of opinion that coal-dust and air alone were of an explosive nature, whilst others thought that alone they were not, but that the addition of a small quantity of fire-damp rendered the mixture explosive.  An important conclusion was come to, that, with the combustion of coal-dust alone, there was little or no concussion, and that the flame was not of an explosive character.

Coal-dust was, however, admittedly dangerous, especially if in a dry condition.  The effects of an explosion of gas might be considerably extended by its presence, and there seems every reason to believe that, with a suitable admixture of air and a very small proportion of gas, it forms a dangerous explosive.  Legislation in the direction of the report of the Commission is urgently needed.