Return to Physics of the Ether

158. The Physical Conditions which determine Combustion. — In proceeding to consider briefly the physical conditions upon which the development of the phenomena of combustion, &c, depends, we may note, first, the fact previously referred to, that heat (i. e. an elevation of temperature above the normal) is in general unfavourable to a stable state of chemical union; or heat, as a general rule, tends to separate chemically combined molecules, and indeed at a certain elevation of temperature (temperature of dissociation) chemical combination is entirely prevented. Heat, therefore, can by no means be said to be in itself the essential condition to develop combustion.

The condition required to produce combustion may be simply stated to be, that the molecules concerned should be brought into a certain degree of proximity; to effect which result a certain amount of force is in general required.

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Now, in most cases the most convenient way of bringing the molecules concerned into the requisite proximity is by producing a forcible molecular disturbance by some cause for which heat (i e. matter in intense molecular vibration) is well adapted; so that this may serve to explain why in most cases in practice heat is applied in the first instance to develop combustion, and after combustion has once been developed, the continuous molecular disturbance thus kept up, by continually urging additional molecules into the requisite proximity, has the effect of maintaining combustion, so that combustion becomes, as it were, a self-acting process.

159. Thus, for example, when a jet of gas is turned on, the molecules of air and gas are interchanging motion among each other in the free translatory motion characteristic of the gaseous state; but this normal rate of translatory motion is not sufficient to carry the molecules over the outer neutral point in their mutual interchange of motion. By the application of a flame (vibrating matter), vibratory motion is imparted to the molecules of air and gas, and this vibratory motion being converted into translatory motion, combustion accordingly sets in as soon as the molecules have by increase of their translatory motion acquired a sufficient momentum to carry them over the outer neutral point in their mutual interchange of motion; and after combustion has once set in, the flame, by continually developing translatory motion in the adjacent molecules of air and gas, thereby serves the purpose of giving that small initial impulse which is requisite to bring the ether into action in chemical combination. The special action of heat in developing and maintaining combustion would therefore simply consist in giving such a degree of translatory motion to the molecules concerned that they are carried over the neutral point in their mutual interchange of motion.

By the combustion of a solid body, the same considerations would hold as in the case of a gas, in so far as it is known that in the case of a solid body in the process of combustion, the molecules of the solid are in great part carried up in the form of vapour before combustion ensues; although the inference is necessary that combustion might go on at the surface of a solid, provided the translatory motion of the impinging air molecules was augmented to a sufficient degree by the presence of the flame.

160. If by any means two molecules could be brought up to the outer neutral point (i.e. that point where the ether in the process of combination first comes into action) slowly, so that the vibrating energy generated in reaching this point had time to dissipate itself, so that, therefore, the two molecules arrived at the outer neutral point still at normal temperature, then this physical condition would be eminently favourable to the energetic combination of the molecules, the ether coming into action with

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augmented energy, owing to the absence of a previous abnormal increase in the vibrating energy of the molecules.

We do not mean to assume that the quantity of heat generated at the combination of molecules at normal temperatures is greater than the quantity generated at the combination of the molecules after having been previously heated, for the process of combination in the case of heated molecules is necessarily spread over a longer period of time, so that the total quantity of heat generated may well be appreciably the same as in the case of molecules at normal temperature whose combination takes place with greater energy and lasts a less time. In the case of molecules at normal temperature, the velocity of translation generated by the ether is necessarily greater, or the ether comes into action with greater energy, the molecules, therefore, being in the first instance driven in nearer to each other than in the case of molecules previously heated; but in both cases, i. e. whether the molecules have been previously heated or not, the final positions of equilibrium taken up by the molecules are the same as regards relative distance, when the temperature, after combination, has fallen to the normal, the relative distance of the molecules being determined solely by vibrating energy.

The physical means of bringing molecules thus into the requi- site proximity for combination (combustion) to ensue, without previously heating the molecules, would evidently be the application of an intense but gradually applied pressure to the molecules concerned.

161. However, we may observe that in that case, where either one or both of the reagents employed are a gas, certain considerations would indicate that it would be impossible to produce combustion without initial heat, and this would therefore apply to the general cases of combustion, where the atmosphere is one of the reagents. This would follow from the consideration that the molecules of a gas are in free translatory motion, and are there- fore quite beyond control, so that the application of a pressure would be of no avail in this case; for evidently the only possible means of causing a gaseous molecule in free translatory motion to approach nearer to the combustible from which it rebounds, is by augmenting the translatory motion of the molecule. The degree of proximity into which a gaseous molecule approaches to the surface molecules of the combustible by whose vibrations set up in the ether the approach of the gaseous molecule is at first checked, will clearly depend on the velocity of translatory motion of the molecule, and it therefore follows that since the molecule is beyond control, the only possible means to make it approach into sufficient proximity for combustion to ensue is by augmenting the velocity of approach of the molecule, whereby its momentum may be sufficient to carry it over the outer neutral point. Since, however, it is a physical impossibility to change the rate of trans-

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latory motion of a gaseous molecule without heating it, or an impulse cannot be given to the molecule without increasing its vibratory motion, it follows, therefore, that combustion cannot be developed, when a gas is one of the reagents employed, without initial heat. Thus, it would probably be impossible to explode a mixture of oxygen and hydrogen gases by a gradually applied pressure, however intense, at least so long as the gaseous state was retained; for the increase of pressure (slowly applied) would only have the effect of bringing a greater number of molecules into the unit volume of space, or of reducing the mean length of path of the molecules; but so long as the temperature, and therefore the rate of translatory motion of the molecules, remained constant, the molecules in their mutual exchange of motion would not approach into greater proximity than if the pressure had not been applied; so that the inference follows that so long as the gaseous state is retained, no amount of pressure would develop combustion. This consideration applied to the atmosphere (matter in the gaseous state), the principal reagent in combustion, would indicate the existence of a special condition for safety.

On the other hand, in that case where both the reagents are in the solid state, and therefore the molecules are completely under control, having stable positions of equilibrium, then there is no reason to doubt that combustion might be produced by the application of a pressure sufficiently intense to bring the molecules into the requisite proximity. Thus, probably percussion powder might be exploded by a gradually applied but intense pressure, and the inference appears warranted that the explosion in such a case would be exceptionally violent, since all the molecules of the mass would be brought at about the same time into proper proximity, resulting in the simultaneous combination of the entire mass, the ether also coming into action with augmented energy, due to the favourable temperature of the molecules, or the absence of a previous abnormal increase of their vibrating energy.

162. Regarding the immunity of gases from combustion by pressure, or the impossibility of combustion being developed by pressure alone when one or both the reagents are a gas, there are certain possible exceptional conditions which we may here notice as possibly having a practical interest. It is a known fact that the molecules of gases adhere to the surface of solids in the form of a thin film, the gaseous molecules in this condition being fixed as if they were the molecules of a solid body. Hence, in this case, if the substance were combustible, the deduction is war- ranted that a combination of the molecules might be effected by a gradual and sufficiently intense pressure applied to the molecules of the film, the gaseous molecules in this case being quite under control, or they may be readily pressed against the com-

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bustible to which they adhere. Porous bodies, which expose a large surface to the atmosphere, stored in considerable quantities so as to be under pressure, would be the physical conditions most favourable to produce the above result. We cannot avoid the conclusion that the phenomena popularly termed phenomena of " spontaneous combustion " are simply physical examples of combustion by pressure under these conditions. To select a well- known example, if we take the case of a stack of hay; then this material is full of pores, which are penetrated by the air, a film of air adhering to the interior of each pore, the whole forming a very extensive area: the molecules composing the film having lost their free translatory motion resemble the molecules of a solid body. The only condition required, therefore, would be a sufficient pressure to urge a few of these molecules of air into such proximity with the combustible material (carbon molecules, &c.) of the stack, that the neutral point is reached when the ether coming into action, the stack might thus be set on fire or charred in places, as in accordance with observation, assuming that the weight of the superincumbent material was sufficient to satisfy the condition for pressure. In an analogous manner, the development of combustion in coal, when stored in considerable quantities so as to be under pressure (and possibly partly in a finely-divided state), may afford another example of combustion due to the above physical causes. The occurrence of a like effect in the case of stored oiled cotton and other substances distinguished for their porosity, may be cited as possibly constituting other examples of the development of combustion under the above special physical conditions.