6^8 



NA TURE 



[October 26, 189^ 



Many substances are capable of undergoing all three rates of 

 combustion. For instance, it can reidily be proved that when 

 organic substances containing hydrogen undergo decay, some 

 of the hydrogen present unites with the oxygen of the air to form 

 water, and the heat generated by the combination is spread over 

 so long a period that at no one moment of time is it perceptible 

 to the sense. If, however, hydrogen gas be confined under 

 pressure in a gas-holder, and allowed to escape through a jet 

 into the air, on being ignited it burns with an intensely hot 

 flame, the heat energy of which can be converted, by suitable 

 contrivances, into other forms of energy, such as mechanical 

 force. In this case as much hydrogen is converted into water 

 in the course of a minute as would have been formed in some 

 years by the process of slow combustion, and the increase in 

 calorific intensity obtained is solely due to the increased rate of 

 combustion, the total thermal value of the hydrogen being the 

 same, whether it is burnt by a slow process taking years, or a 

 rapid one in a minute. If now the same volume of hydrogen be 

 mixed with sufficient air to supply it with the oxygen required to 

 convert it into water, and if a light be applied to the mixture, 

 the hydrogen being side by side with the oxygen necessary for 

 its conversion into water, combustion takes place with enormous 

 rapidity, and the intense heat generated expands the vapour 

 formed to such an extent that an explosion results. 



We have now seen that during the decay or slow oxidation of 

 combustible bodies, heat is generated, and that it is only 

 necessary for this heat to reach a certain point, i.e. the point of 

 ignition, for the little noticeable slow combustion to become 

 ordinary combustion with its manifestation of flame and in- 

 candescence, and it is this action to which the term spontaneous 

 combustion has been given. 



When the combustible substance has a great affinity for 

 oxygen, and at the same time a low point of ignition, spon- 

 taneous combustion will take place with great ease. Indeed, in 

 some cases, such as that of phosphorus, we are obliged to pre- 

 vent the access of air to the body if we wish to prevent ignition 

 taking place, and we also find that the finer the state of divi- 

 sion of the substance, the more readily will its spontaneous 

 ignition take place, not because dividing the body up in any 

 way lowers the point of ignition, but because the increase in 

 the size of the surface exposed to the oxidising action of the air 

 is so much increased, that the heat is generated with greater 

 rapidity than it can be dissipated. If we take a piece of phos 

 phorus, and expose it to the action of the air, it almost directly 

 commences to give off white fumes, and if the weather is warm, 

 it will in the course of a short space of time even ignite ; in 

 cold weather, however, it may be left until it has nearly all 

 undergone slow oxidation without ignition. If, however, we 

 dissolve it in the liquid called bisulphide of carbon, and pour 

 some of this solution upon a piece of blotting-paper or linen, the 

 carbon bisulphide, being highly volatile, will all evaporate, and 

 leave the phosphorus in such a fine state of division that it will 

 at once spontaneously ignite. 



In practically all of the cases of spontaneous ignition which 

 come under our notice, we have the heat evolved during the 

 slow combustion kept in by the presence of a mass of non- 

 conducting material, and this heat being unable to escape 

 gradually grows higher and higher, the chemical combination 

 becoming more and more rapid as the temperature increases, 

 until we reach the point at which ignition of the mass takes 

 place. 



Sometimes, also, the increase in temperature necessary to 

 bring about spontaneous ignition is partly due to physical 

 actions. If a gas be suddenly compressed heat is always evolved, 

 a fact prettily shown by the so-called fire syringe, in which the 

 heat evolved by the compression of air is sufficient to ignite a 

 piece of German tinder. 



Certain bodies have the power of absorbing many times their 

 own volume of gases, and in doing this they not only give rise 

 to a certain increase in temperature, due to the compression of 

 the absorbed gas upon their surfaces or in their pores, but they 

 also increase the chemical activity of the gas so compressed. 



Carbon is one of those substances which possess to an extra- 

 ordinary degree the power of attracting and condensing gases 

 upon their surface, this power varying with the state of division 

 of the particular form of carbon used. The charcoal obtained 

 from dense forms of wood, such as box, exhibits this property 

 to a high degree, one cubic inch of such charcoal absorbing — 

 according to Saussure — 



NO. 1252 VOL. 48] 



Ammonia gas 

 Sulphuretted hydrogen 

 Carbon monoxide 

 Ethylene — olefiant gas 

 Oxygen 

 Nitrogen 



This absorption is very rapid at first, but gradually decreases, 

 and is, moreover, influenced very much by temperature. It is at 

 first purely mechanical, and itself causes a rise of temperature, 

 which in the case of charcoal formed in closed retorts, as in pre- 

 paring alder, willow, and dogwood charcoal for powder making, 

 would produce spontaneous ignition if it were not placed in sealed 

 cooling vessels for some days before exposure to air. The rate of 

 absorption varies with the amount of surface exposed, and is, 

 therefore, able to take part in this condensing action, so that 

 when charcoal is finely powdered, the exposed surface being 

 much greater, absorption becomes more rapid, and rise of 

 temperature at once takes place. If, after it has been made 

 charcoal, it is kept for a day out of contact with air, and is 

 then ground down into a powder, it will frequently fire after 

 exposure to the air for thirty-six hours, whilst a heap of charcoal 

 powder of one hundred bushels or more will always ignite. It 

 is for this reason that in making the charcoal for powder it is 

 always kept, after burning, for three or four days in air-tight 

 cylinders before picking over, and ten days to a fortnight before 

 it is ground. 



There are several very interesting points with regard to the 

 spontaneous combustion of charcoal, which call for more atten- 

 tion than has yet been devoted to it. It is self-evident that the 

 more porous a body is, the greater amount of exposed surface 

 will be available for the condensation of gases, and the great 

 power that charcoal has of absorption is undoubtedly due to its 

 great porosity. Now the temperature at which wood can be 

 carbonised varies very considerably, and wood will begin to 

 char ; that is to say, will begin to be converted into charcoal at 

 temperatures very little above that of boiling water, and in the 

 manufacture of some of the newer kinds of gunpowder the 

 charcoal is formed by heating with superheated steam. 



Charcoal formed at this low temperature, however, still con- 

 tains large quantities of hydrogen and hydrocarbons, and is not 

 nearly so porous as charcoal made at a high temperature ; and 

 although the diminution in porosity reduces the quantity 

 of oxygen absorbed, yet another cause which tends still more 

 to dangerous rise of temperature comes into play. 



When a substance condenses oxygen upon its surface 

 from the atmosphere, the gas is in a very chemically active 

 condition, and will bring about chemical combination with 

 considerable rapidity. For instance, if a piece of platinum foil 

 be heated to redness, so as to drive off all gases from its surface, 

 and be then allowed to cool until it ceases to be visibly red, and 

 is held in a stream of mixed air and coal gas, or air and hydro- 

 gen, it again becomes red-hot, owing to the chemical combination 

 of these substances upon its surface ; that is to say, it has been 

 able to condense these gases together and set up combustion. 



If now charcoal be burnt at a high temperature, the carbon is 

 in a dense condition, and resists to a considerable extent the 

 setting-up of chemical action by the oxygen condensed and 

 absorbed in its pores, but if it has been formed at a low tempera- 

 ture, this condensed oxygen will rapidly act upon the hydro- 

 carbons and hydrogen still remaining in the mass, and will 

 raise in this way the temperature to a dangerous point ; and 

 it is more than probable that very many unexplained fires have 

 been brought about by beams and woodwork becoming charred 

 in contact with flues and heating pipes. 



It has been experimentally determined that when wood has 

 been charred at 500° it will take fire spontaneously when the 

 temperature is raised in the presence of air to 680°, and that when 

 wood has been carbonised at 260° a temperature of 340° only is 

 required for its spontaneous ignition. 



If a beam is in contact during the winter months with a healed 

 flue, or even steam-pipes, it becomes carbonised upon its surface, 

 and during the summer, when the flue or pipe is probably not at 

 work, it absorbs air and moisture, and during the next winter it 

 again becomes heated and further carbonised, whilst the mois- 

 ture and air are diiven out, leaving the pores in a condition 

 eminently adapted for the absorption of more air as soon as 

 the temperature is allowed to fall, and in many cases sufficient 

 heat is gener.ited to cause the charred mass to smoulder and, 

 when air is freely admitted to it, to burst into flame. 



