TEMPERATURE OF IGNITION

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make such measurements it became evident that the greatest heatproduction is not necessarily associated with the most powerful affinities, and therefore the calorific effect is not a suitable measure of the combining power. Further, it has been most clearly shown that the amount of heat produced depends upon the changes of state in each of the reacting substances, and not upon the reciprocal action of two bodies, and therefore not on their mutual affinity.

Since the early conception of the origin of the heat of combination must be abandoned, there only remains the hypothesis that this heat has its origin entirely or in part in the motion of the atoms, which they lose when combination takes place, and which must be imparted to them when this union is destroyed. It certainly may be assumed that this heat, at any rate in part, is the product of the forces of affinity; but such an assumption is at the present time quite useless, and only complicates the problem unnecessarily.

Inasmuch as the hope that the heat produced or used up in chemical changes might be utilised as a measure of affinity has not been realised, the investigations of these calorific actions declined in interest, but are now again becoming important, as the numerous results of observations in this field are studied and investigated free from and unprejudiced by preconceived notions, with the object of learning something of the changes in state which accompany chemical action.

§ 95. Propagation of Chemical Change. Temperature of Ignition. Explosion.—Whether a chemical change produced at any given point in a body or a mixture will spread throughout its mass depends as a rule not only upon the cause of the change, but also upon the heat produced by the action. For instance, supposing a mixture of a combustible gas and oxygen be heated at any given point by an electric spark, or any other means, to such a degree that the combustion begins, it does not necessarily follow that the burning will spread throughout the whole of the mixture. Whether it does so depends upon the amount of heat produced by the combustion. If this suffices to raise the immediate layers of combustible material to the temperature required for its inflammation, i.e. to the 'temperature of ignition,' then these layers are burnt up, and in turn yield heat sufficient

to ignite the next stratum, and so on until the whole is consumed. Since, however, in cases of this kind a portion of the heat produced is always given out either by radiation or conduction to the surrounding bodies not concerned with the reaction, it may happen that the progress of the combustion is interrupted before the entire mass has been attacked. This will be the less liable to occur the more the heating consequent upon the reaction exceeds the temperature of ignition. In case the mixture contains non-combustible bodies, e.g. nitrogen, then, as such bodies have their temperatures raised at the expense of the heat produced by the combustion, the temperature is thereby reduced, and with a considerable admixture of such bodies the temperature may sink so low that the advance of the combustion ceases. Every combustible mixture may therefore be rendered non-inflammable by the admixture of a sufficient quantity of non-combustible material. If no such disturbing influences are to hand, and the heat of combustion be great, then the heating may rise far above the temperature of ignition. Further, if the products of the combustion are gaseous or vaporous, then a considerable sudden expansion results, which may increase until it becomes an explosion.

Something of the same kind takes place in the case of substances which can be exploded by mechanical disturbance or by percussion. This property is alone exhibited by substances in which the atoms are in a state of more or less unstable equilibrium, from which condition they can pass with production of heat, or corresponding amount of work, into a more stable state of equilibrium. Examples of this class of bodies we have in the chlorine, bromine, and iodine compounds of nitrogen, in the organic nitrates and nitro-organic compounds. When such bodies yield gaseous or vaporous products of decomposition, and produce much heat, they may also act as explosives.

The liquid chloride of nitrogen, for instance, is decomposed by very slight causes; this decomposition is expressed by the following equation:

NCI。 + NC1 ̧ = N2 + Cl2 + Cl2 + Cl2.

This action is attended by a considerable heat-production, and consequent marked expansion of its gaseous products.

Glyceryl nitrate (commonly but erroneously described as nitroglycerine) C,H,(ONO2)3, in which, or in reality, of the oxygen is combined with the nitrogen, yields gaseous and vaporous oxidation products of carbon and hydrogen, whilst the nitrogen is also set free in the gaseous state. This decomposition can be brought about by percussion or detonation as well as by heat. If the nitrate be ignited in an open space, it burns slowly and quietly, for the gaseous products pass freely away. If the nitrate be enclosed so that this free passage is prevented, or if it be ignited by a powerful blow, then the violent shock and pressure produced will immediately decompose the particles near those first struck, and thus the decomposition will spread in the form of an explosion. If the decomposition does not produce heat or do work sufficient for its extension, then the reaction ceases.

These explosives are quite analogous to gunpowder, with the single exception that in the case of gunpowder the combustible constituents, charcoal and sulphur, are only mechanically mixed with the nitre, which contains the oxygen, whilst in the former the oxygen is combined chemically with the other constituents.

§ 96. Dissociation of Gases.-One of the simplest forms of chemical change, which is in the main produced by heat, is that which H. Sainte-Claire Deville styled dissociation. Dissociation is characterised by the decomposition lasting only so long as the cause is active, the substances returning to the original state on withdrawal of the cause. Many substances are found to undergo dissociation; still it is often difficult to observe and demonstrate the dissociation. More especially is this the case when very high temperatures are needed to bring about the decomposition. In many instances the action is associated with a change in colour, and can be recognised by this; thus, for example, the colourless vapours of nitrogen peroxide, N2O4) dissociate into dark brown vapours of NO2. Dissociation is recognisable in the increase in the number of molecules resulting from it; for, as Avogadro's Law still holds, the density of the gas or vapour is also altered. In the case just mentioned, viz. N2O1 = NO2+ NO2,

2

the number of molecules, and consequently the volume, is doubled,

and therefore the density is reduced to one-half. Observations have, however, shown that this change does not occur suddenly, but takes place gradually as the temperature rises, so that the progress and extent of the dissociation may be calculated from the changes in volume and density.

The density of the compound NO, in relation to air is 1·59; therefore that of the non-dissociated compound, N2O4, is twice as great, viz. 3.18. Mixtures of these two, such as are produced by the dissociation, would have densities lying between these values; the more nearly the observed density approaches the lower value, the more advanced the dissociation. If in 100 particles, have been dissociated, and therefore 100—x are still unaltered, then we have

100 (N2O1) = (100—x) N2O1 + 2xNO2.

Consequently there are now 100+x particles instead of 100, the volume is increased in the proportion of 100: 100+x, and the density, D, decreased in the inverse proportion, viz. of 100 + 100. To determine x, the percentage of dissociation, we have the following proportion:

By this formula the percentage of dissociation can be calculated for every observed density; in this way the following valueshave been obtained:

The average increase in the dissociation for each degree centigrade rises until a maximum is reached, when the dissociation is about half completed, and then it gradually diminishes, until at 140° C. the dissociation is complete.

One might imagine that the dissociation having once begun, it must be suddenly translated through the mass so soon as the temperature required for its commencement has been reached. That this is not the case finds an explanation in the fact that in consequence of the frequent and irregular collision of the particles they do not all retain an equal velocity, and as the temperature is determined by this motion, the particles have not all the same energy. The particles having the greatest energy, i.e. those in the most rapid motion, are first dissociated, and those having the least heat-motion will be the last to dissociate. What we measure as the temperature of a gas is only the mean or average temperature of all the particles; some of the particles may have temperatures differing considerably from this. As great differences are seldom found, but smaller differences more frequently occur, dissociation will proceed most rapidly when the mean temperature is the same as the temperature of dissociation. At this temperature 50 per cent. of the entire mass is dissociated, and in the case of nitrogen peroxide this point is reached at 60° C.

When the temperature of dissociation is too high to permit of exact measurements of density, then, in order to make it evident, other means must be employed. Deville has employed many ingenious devices for this purpose. For instance, by diffusion through porous septa he separated the hydrogen from the oxygen formed by the dissociation of steam at a white heat, which gases, if not separated at this temperature, would recombine at a somewhat lower temperature. By rapid cooling carbon monoxide and carbon were separated from the dissociated carbon dioxide, and chlorine in a similar manner was obtained from hydrochloric acid gas.

Bunsen has shown from the pressure produced by the explosion of a mixture of two volumes of hydrogen and one volume of oxygen that combination ceases as soon as the temperature bas reached about 3000° C., and therefore above this temperature steam cannot exist, but is resolved into its elements. Whether