In these experiments the cannon, formed out of a solid steel ingot, was fired at the bottom of a small vertical boiler, with the upper end cut away. The dust was riddled down from the top immediately before firing each shot.

The value of these experiments is detracted from in consequence of the mouth of the cannon being 3 feet below the line of sight, so that it is quite possible there may have been flame in some instances below this level. Roburite shows the best record.

In the next series of experiments the cannon was arranged so that its mouth was visible, and carbonite was included in the list of explosives :—

SERIES NO. 1.-WITHOUT TAMPING OR DUST IN SUSPENSION.

SERIES No. 2.-COAL-DUST TAMPING AND DUST IN SUSPENSION.

SERIES No. 3.-CLAY TAMPING AND DUST IN SUSPENSION.

Roburite and carbonite have the best record in this series, but it will be noticed that when the explosives are fired without tamping they are none of them flameless. It should also be noted that the gases resulting from the explosion of some of the explosives under examination are inflammable, whilst in others they appear to be non-inflammable. How far this circumstance may affect their security is not easy to determine.

None of these explosives, therefore, present absolute security; for it seems clear that with the best of them feeders of fire-damp in the drill hole in contact with the body of the charge might become ignited and lead to danger, but some are much more promising than others and far surpass ordinary gunpowder, which is still so universally used for underground operations.

It is a difficult position, but it is quite clear that those explosives which, although tamped with clay, give off flame on blowing out must be discarded altogether, and we then have to decide whether any and what precautions are necessary in the use of those of the safer character.

It will be well to state fairly the conditions under which blasting has to be done. May we assume that in many mines 10 per cent. of the shots are fired in the presence of gas, either in the shape of small blowers or small accumulations, and in some instances in a body of air fouled to the extent of a quarter or half an inch of cap. Exception may be taken to this, but the writer makes the statement as a fair conclusion drawn from an extensive experience. To this danger must be added the presence of dry dust, which as has already been pointed out, it is almost impossible to avoid, though it may be very considerably mitigated by the application of water. Have we any explosive which ensures absolute safety under the above conditions? Those who adopt a high standard of safety will answer this question with a negative. The writer then submits, finally, that the dangers of dust and gas combined cannot be altogether neutralized by any known process of damping, although they may be greatly diminished-that we possess no explosive absolutely safe under the ordinary conditions of fiery mines-and that the best precautionary measure still remains, as it has hitherto been, to confine blasting operations to such times as the workmen are out of the mines.

Whether or no the coal-dust theory may be accepted in its entirety, that is in the absence of all fire-damp, few of us will be prepared to deny the serious danger of dust in conjunction with partial fouling of the atmosphere of mines by the constant emission of fire-damp, which goes on from day to day, and which being unseen is seldom sufficiently appreciated. And really this is the most important aspect of the question. It is immaterial whether we prove coal-dust to be explosive per se, so long as it is acknowledged to be so with imperceptible quantities of fire-damp, because this condition is normal over large areas in most mines.

Those who are of opinion that colliery explosions are due to fire-damp alone, might possibly strengthen their position by undertaking a series of experiments to prove whether large effects can be produced by the explosion of comparatively small quantities of fire-damp mixtures per se. This is another phase of the question which it would be well worth while to try to elucidate. Old men who worked in shallow damp mines many years ago tell us that considerable bodies of gas were purposely ignited to save the trouble of bratticing, and that the explosions resulting were quite local and harmless.

The PRESIDENT said that in the first explosion he had to do with he found charred coal-dust sticking to the wagons and props, and they were able to trace the source of the explosion. There were many methods of watering the dust, but it could only obviate the danger where the explosion was caused by the dust taking fire. The firing of the dust would very often explain the explosions which took place almost simultaneously in different parts of the mine, although the ventilation might be quite separate, and the places widely apart.

Mr. N. R. GRIFFITH (Wrexham) said he had listened with great interest to Mr. Hall's able paper, as he also did to a paper on the same subject which that gentle. man read in the same room fifteen years ago relating to an explosion which took place at a colliery with which he (Mr. Griffith) was well acquainted and which explosion appeared to have been one of coal-dust ignited by a blown-out shot of gunpowder. He rather differed with the author on one point in the paper he had just read. Mr. Hall told them that none of the explosives he mentioned was absolutely flameless under all circumstances, and he, therefore, thought it possible if gas was given off by a blower at the back end of the hole that any of them might possibly ignite such a feeder of gas. Mr. Hail stated that roburite had never been seen to give flame when fired in a hole which had been properly stemmed, and yet, even when that explosive was used, he thought it possible to ignite gas at the back end of the hole. He (Mr. Griffith) did not think so, if the explosive used was one which, when fired in a tamped hole, gave no flame outside the hole, because if there was no air in the hole the feeder of gas would not ignite in the hole, and with a feeder of gas at the back of the hole there would be no air in the hole, as the gas would drive the air out of the hole. He considered that the feeder of gas could not be ignited in the absence of oxygen; and that probably before the explosion got to the mouth of the hole the explosive would be flameless and could not ignite gas.

Dr. BEDSON said of course they all knew that if they introduced a flame into a combustible gas they would not get any ignition except where it came in contact with air; but, at the same time, they must remember that the feeders of gas referred to were not essentially and necessarily marsh gas, they contained other gases. They might have air; oxygen and nitrogen might be associated with marsh gas in such a feeder, That was the conclusion he would arrive at from the examination of samples of fire-damp collected in mines. Of course, the composition of firedamp varied considerably under different circumstances. There was one matter referred to in Mr. Hall's paper, and that was with regard to the difference in the composition of the two classes of coal-dust. It was well known that coal itself is a very easily oxidizable material, and undergoes chemical change on exposure to the air, and the new coal-dust and the old coal-dust referred to would naturally have very different compositions. The oxygen of the air, acting upon certain resinous substances in the coal, would remove hydrogen in the form of water, and a portion of the carbon in the form of carbon-dioxide, and a certain proportion would be

oxidized. The rate of this oxidation was increased by a rise in temperature, as was shown in some experiments he had made some years ago* on coal-dust from Ryhope Colliery in connexion with the explosion of an air-receiver at that colliery. He then found that by exposing coal-dust to air at a temperature of 140 degrees centigrade was sufficient to bring about so rapid an oxidation of the dust, causing it to take fire and burn. He would like to be able to answer the question-What was it that made one coal-dust more inflammable than another? He had been engaged for some years in investigating this subject, and had, a few years ago, read a short notice on the subject of coal-dust.† He believed that no ultimate analysis would give any clue, although approximate analysis might, as to why one variety of coal-dust was inflammable and another was not. The only clue they could get was by careful examination of the gases enclosed in the coal-dust; that was the method he adopted in the experiments referred to, and in his paper he had described the results obtained by extracting the gases from the coal-dust under reduced pressure and at high temperatures varying from 50 to 100 degrees centigrade. Those experiments were being continued, and he hoped before long to have material for another paper which he would be able to lay before this Institution. He had obtained from Ryhope Colliery a variety of coal-dust which Mr. W. F. Hall said, from his experience at the screens, was inflammable. From this sample he obtained a volume of gases containing hydro-carbons which were not marsh gas, but evidently mixtures of marsh gas and other higher hydro-carbons. If they could prove the existence of such hydro-carbons, he thought they would be able to form some idea of the source of the inflammability of some coal-dusts. By examining coal-dust in this way they might be able to distinguish one variety from another.

Mr. W. N. ATKINSON said it would add to the value of Mr. Hall's experiments if that gentleman could give them a little fuller information as to the conditions under which the experiments were made. The quantity of coal-dust was given in hundredweights; was any means taken to ascertain whether the whole of the dust was in suspension in the shaft at the time the shot was fired? In the case of continued experiments it was necessary that the dust which remained from the previous experiments should be taken into consideration. In the case of one experiment it was stated that the cannon was fired twenty minutes after the dust had been thrown down the shaft; did that mean that the dust was simply dropped down the shaft and allowed to settle? and was there any dust in suspension or deposited in the shaft itself in that experiment and in the other experiments? This appeared to have a direct bearing on one of the most important points in the coal-dust question, that is the distance traversed by the flames after the supply of dust ceased. There was another point respecting the proportion of dust used. He (Mr. Atkinson) had worked out the number of cubic feet of air per pound of dust in some of these experiments on

* Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. xxxvii., page 205. Transactions of the North of England Institute of Mining and Mechanical Engineers, Vol. xxxvii., page 245.