The Mechanical Handling and Storing of Material

652 THE MECHANICAL HANDLING OF MATERIAL been found, even with the most active coals, that the rise of temperature would exceed from 3° to 5° C. in twenty-four hours—that is, in the first stage of heating. But after a certain degree is attained, the action is much more rapid; and where air is available it is destructive. Various theories have been put forward to account for the spontaneous combustion of coal; and they may be summed up under two heads, viz., (1) the decomposition of iron pyrites, and (2) the oxidisation of the coal. In the case of coal mines, there is a third theory—that of friction. However this friction may operate in coal mines, with regard to the cases under consideration friction as a cause of heating may be dismissed. It is very generally admitted that in the presence of moisture pyrites undergo oxidisation and disintegration, furnishing additional fresh-cut surfaces of coal to absorb oxygen; the chemical action develops heat and the further oxidation of the coal. Every one knows that coal varies considerably in chemical composition and also in physical structure. Given a number of fine coal particles containing rich hydrocarbons in the presence of moisture and atmospheric oxygen, and we have all the elements required. Heating always takes place in the small coal; but cases have been known where large coal, and also nuts, lying against small, or covered by small, have developed spontaneous combustion. It would seem in some cases that the interstices, or air spaces, formed between the large and nut coal provided the necessary atmospheric oxygen which, acting upon the hydrocarbon particles of the small coal, sets up chemical action and the heat produced develops a certain peculiar chemical change in the coal. One of the first indications of heating is the steam, or vapour, which usually escapes from the stack by the line of least resistance, which may be far removed from the scat of the cause. As the heat develops, certain gases are evolved of the acetylene series, which give a distinctive and peculiar odour, exactly the same as that found in coal mines. The odour is characteristic, and exclusive to the gases evolved from spontaneous combustion. No one could mistake this odour, which can be detected even at a considerable distance. Mr Irving states that he suspects that there is sortre kind of chemical action induced when coals from different collieries are mixed together, different to some extent from simple oxidation. Friction cannot apply to the hold of a ship or coal stored on land. Oxidisation is certainly one cause, if not the chief cause, of spontaneous combustion. Coals stacked not more than 6 or 7 ft. deep never showed any tendency to heating. In this case the heat of oxidisation is carried off in the atmosphere, without raising the coal to the temperature necessary to induce active or destructive chemical change, and little or no damage is done to the coal. Attempts have also been made to ventilate by running air shafts through the coal stack; but the latter state of that coal has been worse than the first, because the air, instead of keeping the coal cool, supplied the necessary oxygen, and greatly facilitated the destruction of the coal.1 Several suggestions or recommendations have been made to prevent the heating of, or to cool the already heated, coal in ships. The application of steam pipes is compulsory in vessels carrying coal under the American flag, and Professor Lewes some time ago suggested the distribution of cylinders of compressed carbonic acid gas, secured with fusible plugs, throughout the coal cargo. Coal should, of course, in all cases be received as dry as possible, and care taken so that no damp cöal is covered up with dry. 1 This may depend upon the size of the air shafts.