6

CH, represents a higher hydrocarbon, which may be determined by calculation from the phenomenon of combustion to be either C2H, or C3Hg. The author draws, however, definite attention to the circumstance that other products of heating are obtained (for example an acid [?acetic acid]), the gases from which are mixed with the air, are not absorbed by caustic potash, and are combustible. Dr. Bedson has obtained similar products by heating his coals to 184° Cent. in vacuum (as he imagines), and has obtained a residue of air which he has recorded as occluded gas. The composition of this residual gas is as follows:

This result absolutely upsets everything known with regard to coal. Dr. Bedson does not, however, even once point out that he has discovered a new method of producing oxygen! The writer has already pointed out sufficiently how air found its way into his apparatus. How oxygen may have been obtained by heating to 184° Cent. may be explained through the air having been drawn off too quickly, so that it had not time to come into contact with the heated coal. How, however, he obtained 27 per cent. of oxygen is inexplicable.

With regard to Table II., it may be noted that the results refer only to the conditions there given. If a larger quantity of coal is heated in a tube of the above size (70 cubic centimetres), the oxygen contained therein does not suffice for complete oxidation, and some marsh-gas remains. In order to prove this experimentally, the author has heated in a tube of 70 cubic centimetres capacity 6, 4, 2, 1 and gramme respectively of coal No. XI., and

has examined the residual air, with the results recorded in Table III.

With reference to Table III., it may be noted that the content of carbonic acid is remarkably small, less than might have been expected from the quantity of marsh-gas present, the reason being the above-mentioned, that when coal is heated in the presence of oxygen other products than carbonic acid are formed. In the first four analyses, the writer was unable to find these products in the gaseous form (the residual gas was pure marshgas), but he was able to prove the invariable formation of some strong acid soluble in water (acetic acid ?).

In Tables II. and III., the absence of oxygen is especially indicated, in order to accentuate the fact that it is impossible to find free oxygen in a closed vessel in which coal has been heated. Volumetrically this absorption of oxygen is very important. One part of coal by volume in the form of powder suffices to absorb 50 times its volume of oxygen between 160° and 200° Cent., and to render 250 times its volume of air incapable of explosion. In these results, however, time also plays a part, so that further insistence upon them would be unfruitful.

How then may a coal-dust explosion be explained? As long as we blast with explosives, whose temperature of detonation is above 650° Cent., and which show flame, we have in a blown-out shot a source of heat (not only a source of warmth) which can generate abundant quantities of gas from coal, and at the same time produce a flash, which is capable of firing the explosive gaseous mixture thus formed. One part of gas-coal gives about 300 times its volume of gas when all the latter is evolved. The crusts of coke, which cover everything in the pit after a coaldust explosion, may be looked upon as coal from which about

half of the gas has been set free. There is, therefore, in a coaldust explosion evidently no want of combustible gas, though there may be of air. For this reason, the wave of explosion is propagated in the opposite direction to the intake air-current. On its way, it liberates further gas from coal-dust that is thrown up; it, therefore, constantly requires fresh air, and thus continues its course to the downcast-shaft, which it also frequently destroys. In coal-dust explosions, the large volume of air which the miner employs for the ventilation of the mine is a source not of safety but of destruction. The flash of a blown-out shot can cause the ignition of an explosive mixture of gas, but even its presence may be dispensed with, for the powerful pressure which a blown-out shot produces may cause the ignition-temperature of marsh-gas to be attained. Here, however, the conversion of mechanical work into heat does not play the principal part, but the properties of oxygen, which are affected by compression, inasmuch as compressed oxygen has a far more violent oxidizing action than it has under normal pressure. It is, therefore, not a matter of indifference how a shot-hole is placed: whether the shot blows out straight along the centre of the road, whether it lies at an acute angle to the roof or the floor, or perpendicular to the solid mass of coal.

The main factor in a coal-dust explosion is, and remains, the fine state of division of the dust; the second condition is the heating of the dust; while the chemical properties of the coal only occupy the third place. All coals can produce dust-explosions, even coke, which is perfectly free from gas. Supposing that finely-divided coke were flung up by means of a blown-out shot, at a sufficiently high temperature, this would burn to carbon. monoxide in the presence of sufficient coke, and produce an explosive mixture with air.

Some coals give off gas at lower, others at higher temperatures; some are dry, others moist; some produce a very finely divided, others a coarse-grained dust, etc. For the production. of coal-dust explosions the former conditions require a lighter, the latter a heavier charge of explosive.

Prof. P. P. BEDSON read the following paper on euclosed in Coal and Coal-dust":

THE GASES ENCLOSED IN COAL AND COAL-DUST.

BY P. PHILLIPS BEDSON.

In the early part of the year 1899, a paper on the subject of the gases enclosed in coal appeared in Glückauf,* in which Dr. Broockmann gave an account of the results of his examination for enclosed gases" of coals of the Westphalian coal-field; and, at the same time, detailed the results of his examination of the Hutton-seam coal, which he had obtained from Ryhope colliery. This coal was submitted to investigation, because Dr. Broockmann had reason to doubt the results obtained by the late Mr. McConnell and the author, which were described in a paper read before this Institute in February, 1894.†

After the perusal of this paper in Glückauf, it appeared desirable to re-investigate the question rather than simply replying to the strictures of Dr. Broockmann, with the information then to hand. For this purpose, in November, 1899, the late Mr. McConnell collected samples of coal from the Hutton seam at Ryhope colliery, and the investigation, then commenced, has been continued until the present time. In the summer of 1900, Mr. McConnell was accidentally drowned, while boating off the Northumberland coast, consequently the author has not had the benefit of his assistance; and for information on the work which formed the basis of the conjoint paper already referred to, he has had to content himself with the laboratory-notes and journals left by his friend. This sad event, therefore, made the repetition of a portion of the previous enquiry all the more necessary.

Before describing the results of these recent experiments, it will be best to deal with some of the points raised by Dr. Broockmann in his criticism.

* Trans. Inst. M.E., 1902, vol. xxiv., page 18.

Ibid., 1894, vol. vii., page 27.

In the first place, Dr. Broockmann draws attention to the unsuitability of indiarubber, either in the form of tubing or stoppers, for work of this kind, pointing out that it is "as permeable to gases as a sieve is to water." Although the writer readily acknowledges the justice of these strictures on the use of indiarubber, he is unable to agree in the application made by Dr. Broockmann in the description given of the method employed by the late Mr. McConnell and himself: for, in the paper published in the Transactions of The Institution of Mining Engineers in 1894, to which Dr. Broockmann gives a reference, it is stated that the coal used in the experiments was contained in flasks sealed on to an air-pump of the Geissler type. Further, it is stated in this paper that "the apparatus used in these and all subsequent experiments was made entirely of glass, the several parts being fused together to prevent leakage." The properties of indiarubber cannot, therefore, be made responsible, as Dr. Broockmann suggests, for the fact that in one instance as much as 818 cubic centimetres of gas was obtained from 100 grammes of freshly-hewn Ryhope coal; nor are the proportion of the gases of the atmosphere found in this gas and the relatively small amount of combustible gas to be explained in the same manner.

Surely, in a case of this kind, it is not unreasonable to expect that a critic should show an acquaintance with the details of the work under criticism; but, in the paper printed in Glückauf, no mention is made of another experiment with the same coal, in which a much smaller volume of gas was obtained, and a gas which contained a relatively large proportion of combustible constituents, nor is any note made of the fact that this difference is specially emphasised. The writer is of the opinion that the large volume of gas obtained in the first instance is explained by the difficulties surrounding the removal of the air adhering to the glass and to the coal itself, difficulties increased by the close packing of the coal in the flask, whereas in the second experiment the coal was loosely filled into a tube, 1 inches in diameter. The amount of coal used in the first experiment, namely, 220 grammes, as against 90 grammes in the second, must also have contributed to the difficulty of removing completely the adherent air.

Dr. Broockmann, in his experiments, took 100 grammes of coal, contained in a vessel sealed direct on to a Sprengel pump. * Trans. Inst. M.E., 1894, vol. vii., page 32.