October 26, 1893] 



NATURE 



629 



In the case of charcoal burnt at a higher temperature, it may 

 be taken that the cause of heating is to a great extent physical, 

 whilst in the low-burnt charcoal it becomes chemical as well as 

 physical, and it is this chemical action which is the most dan- 

 gerous, and acts in most cases of spontaneous combustion. 



The spontaneous ignition of coal has been the cause of an 

 enormous number of serious accidents, and the earliest theory as 

 to its cause was that it was due to the heat given out during the 

 oxidation of the pyrites or "coal brasses," which are com- 

 pounds of sulphur and iron.and are present in varying quantities 

 in nearly all coal. This idea has held its ground nearly up to 

 the present time, in spite of the researches of Dr. Richters, 

 who twenty years ago showed the explanation was an erroneous 

 one, and even earlier, in 1864, Dr. Percy pointed out that the 

 cause of spontaneous ignition was probably the oxidation of the 

 coal, and that the pyrites had but little to do with it. Pyrites is 

 found in coal in several different forms, sometimes as a dark 

 powder closely resembling coal itself, and in larger quantities in 

 thin golden-looking layers in the cleavage of the coal, whilst 

 sometimes again it is found in masses and veins of considerable 

 size ; these masses, however, are very heavy and are carefully 

 picked out from the coal.and utilised in various manufactures. The 

 yellow pyrites,and even the dark varieties, when in the crystalline 

 form, remain practically unaltered, even after long exposure to 

 moist air, but the amorphous and finely divided portions will 

 oxidise and effloresce with great rapidity, and it is during this 

 oxidation that the heat is supposed to be generated. 



Some coals that are very liable to spontaneous ignition only 

 contain 08 per cent, of pyrites, and if we imagine this to be 

 concentrated in one spot instead of being spread over the whole 

 mass, and to be oxidised in a»few hours, the temperature would 

 rise only a few degrees, and under ordinary circumstances this 

 rise in temperature would be practically inappreciable. 



The oxidisation of masses of pyrites under certain conditions 

 gives rise lo the formation of ferrous sulphate and sulphur 

 dioxide, with liberation of sulphur, and one might easily imagine 

 that this free sulphur, which has an igniting point of 250° C, 

 would play an important part in the action by lowering the point 

 of ignition. This, however, could only happen with large 

 masses of pyrites undergoing oxidation, and with the small 

 amount of pyrites present in coal, supposing air were present 

 in sufficient quantity to oxidise it, the sulphur formed would be 

 converted into sulphur dioxide at temperatures as low as 60° C. 

 This oxidation of sulphur at low temperatures is an action not 

 generally known, but in my experiments I have found it takes 

 place with considerable rapidity. The only way in which 

 pyrites can assist the spontaneous ignition of coal is that when 

 it oxidises, it swells and splits up the coal, thus exposing fresh 

 surfaces to the action of the atmospheric oxygen. 



I have carefully determined the igniting points of several kinds 

 of coal, and find that 



Cannel coal ignites at 698° F. = 370° C. 

 Hartlepool coal ,, ,, 766°iF. = 4o8°C. 

 Lignite coal ,, ,, 842° F. = 450° C. 



Welsh steam coal ,, ,, 870° F. = 477° C. 



So that it is impossible for the small trace of pyrites scattered 

 through a large mass of coal, and slowly undergoing oxidation, 

 to raise the temperature to the necessary degree. 



When coal is heating, a distinctive and penetrating odour is 

 evolved, which is the same as that noticed when wood is 

 scorched, and the gases produced consist of nitrogen, water 

 vapour, carbon dioxide, carbon monoxide, hydrocarbons of the 

 paraffin series, and sulphuretted hydrogen, the presence of the 

 latter gas showing beyond doubt that oxidation of the sulphur 

 has nothing to do with the action. 



Ever since coal has been generally adopted as a fuel, it has 

 been recognised that great care was necessary in the storing and 

 shipment of masses exceeding locx) tons, and if the coal has been 

 stored wet or in a broken state, firing or heating of the mass has 

 frequently taken place. Much inconvenience and loss has been 

 caused by this on shore, but the real danger has occurred during 

 shipment, and owing to this many a vessel has been lo^t 

 with all hands, without any record of the calamity reaching 

 shore. 



Owing to the greater facility for treating the coal when it 

 becomes heated on shore in coal stores and gas works, absolute 

 i,^nition only rarely takes place, and it is mainly from evidence 

 obtained in the case of coal cargoes that we learn most as to 

 the causes which lead to it. 



NO. 1252, VOL. 48] 



Coal is a substance of purely vegetable origin, formed out of 

 contact with air, by long exposure lo heat and pressure, from 

 the woody fibre and resinous constituents of a monster 

 vegetation which flourished long before the earth was inhabited 

 by man. Coal therefore may be looked upon as a form of char- 

 coal, which having been formed at a temperature lower than 

 that of the charcoal-burner's heap, and under great pressure, is 

 very dense, and still contains a quantity of these constituents 

 which, in the ordinary burning, are driven off as wood naphtha, 

 tar, &c., and these bodies consist of compounds containing 

 essentially carbon and hydrogen, together with a little oxygtn 

 and nitrogen, and form the volatile matter and hydrocarbons of 

 the coal. Coal also contains, besides these, certain mineral 

 bodies, which were present in the fibre and sap of the original 

 wood, and these form the ash which is left behind on the coal 

 being burnt. These mineral substances consist almost entirely 

 of gypsum or sulphate of lime, silica, and alumina, together with 

 some oxide of iron, which gives the colour to the reddish-brown 

 ash of many coals, and which has been formed by the decom- 

 position of the pyrites in the original coal. 



The mineral constituents of coal are the only ones, with the 

 exception of the pyrites, that play no part in the phenomena 

 attending the heating and spontaneous ignition of coal, and we 

 need therefore only regard the actions which take place when 

 the carbon, hydrocarbons, and pyrites in freshly-won coal come 

 in contact with air apd moisture. 



Certain kinds of coal exhibit the same power of absorbing 

 gases which charcoal has, although to a less degree. The 

 absorptive power of new coal due to this surface attraction varies, 

 but the least absorbent will take up one and a-quarter times its 

 own volume of oxygen, whilst in some coal more than three 

 times their volume of the gas is absorbed, which gives rise to an 

 increase in temperature, and tends to increase the rate of the 

 action which is going on, but is rarely sufficient to bring about 

 spontaneous ignition, as only about one-lhird the amount of 

 oxygen being absorbed by coal that is taken up by charcoal, and 

 the action being much slower, tends to prevent the temperature 

 reaching the high ignition point of the coal. 



All coal contains a certain proportion of hydrogen, with which 

 some of the carbon is combined, together with the nitrogen and 

 oxygen, forming the volatile matter in the coal. The amount 

 of this volatile matter varies greatly, anthracite containing the 

 smallest quantity, and cannel and shale the largest. When the 

 carbon of the coal absorbs oxygen, the compressed gas becomes 

 chemically very active, and soon commences to combine with 

 the carbon and hydrogen of the bituminous portions, converting 

 them into carbon dioxide and water vapour. As the temperature 

 rises so this chemical activity increases, so that the heat generated 

 by the absorption of the oxygen causes it to rapidly enter into 

 chemical combination. This kind of chemical combination — 

 oxidation — is always accompanied by heat, and this further rise 

 of temperature helps the rapidity of oxidation, so that the 

 temperature rises steadily ; and this taking place in a large mass 

 of coal, which from physical causes is an admirable non- 

 conductor, will often cause such heating of the mass that if 

 sufficient air can pass into the heap in order to continue the 

 action the igniting point of the coal will be reached. 



It has been suggested that very bituminous coal, such as cannel 

 and shale, are liable to spontaneous ignition from the fact that 

 heavy oils would exude from them on a rise of temperature, and 

 that these, by oxidising, might produce rapid heating. Experi- 

 ment, however, shows that this is not the case, and that the 

 heavy mineral oils have a decided effect in retarding heating. 



We can now trace the actions which culminate in ignition. 

 As soon as the coal is brought to bank, absorption of oxygen 

 commences, but except under rare conditions the coal does not 

 heat to any great extent, as the exposed surface is comparatively 

 small, and the largeness of the masses allows of the air having 

 free access to all parts, so keeping down the temperature. After 

 the coal has been screened and the large pieces ol pyrites picked 

 out, it is put in trucks. Here it begins to get broken up, owing 

 to the many joltings and shuntings, and so offers a larger surface 

 to the action of the air. When it has arrived at the ship, it is 

 further broken up by being shot down the tips or shoots, and 

 more harm is done at this than at any other period, for I he coal 

 is broken by reason of the distance it has to fall, and it has to 

 bear the impact of every succeeding load failing upon it, and 

 it rapidly becomes slack, so that the under part of the shipload 

 is a dense mass of small coal, which soon rises in temper.nture 

 by reason of the large surface exposed to the air and the con- 



