THE suggestion in my former letter on this subject (vol. xxxiv. p. 595) that “keeping the ventilating air-current saturated with aqueous vapour” might prove the most effective way of rendering the dust in coal-mines innocuous, has, I am glad to see, been since shown to be practicable, in a South Wales colliery. Since the above date, I have considerably extended my research, with results that confirm the conviction therein expressed that many of the most disastrous colliery explosions during the last seven years in this northern district have been practically dust explosions, and therefore preventable; that the rough method of watering the floors only, or floors and sides, of the mines is delusive, since it leaves the most dangerous dust undisturbed, the upper and flocculent dust; and last, that probably the reasons why dust in dry pits does not explode more frequently are now within grasp. To this latter conclusion, with your permission, I will now briefly address myself. That every firing of a shot that is accompanied by flame in a dry and dusty pit does not produce an explosion is well known; that sometimes such firing of a shot does is unhappily also well known. That the local presence of gas, even in small amount, is sometimes the reason of this is universally acknowledged. That the amount and condition of the dust present (even in the practical absence of gas) is at other times the reason is now believed by many. Setting aside the amount of dust, which every one will allow must be an essential factor, and also the varying energy which the shot, blown out or not, develops, let us look at the other conditions. The temperature and hygroscopic state of the air-current is one most important factor, and consequently the concomitant temperature and hygroscopic state of the dust traversed by such current. Beyond this, the degree of fineness and the constituents of the dust will have much to say in the matter. The finer the particles the more readily will they ignite, and the more completely will they place their substance under the influences present. Thus ordinary screen coal-dust will not ignite when a common match is lighted and applied to it, but it will when finely pounded in a mortar. Now the dust resting on the baulks and upper portions generally of the ways will invariably so light and burn when dry, although the constituents vary greatly in different pits and in different seams of the same pit.