difficult to reconcile, is seen to be a necessary consequence of combustion, considered from the modern point of view. In all instances of combustion the weight of the products of the action is equal to the total weight of each of the two substances taking part in the chemical combination. When, for example, the metal magnesium burns in the air, the weight of the product of the combustion is equal to the weight of the metal, plus the weight of a certain amount of oxygen with which it united in the act of burning. This gain in weight during combustion may be demonstrated in a number of ways. Thus, if a small heap of finely divided iron, obtained by the reduction of

T

3456

the oxide, be counterpoised upon the pan of a balance, and then ignited, the iron will be seen to burn, and as it burns the balance will show that the smouldering mass is increasing in weight. In this case the sole product of the combustion is a solid substance, namely, iron oxide, which remains upon the pan of the balance; but the same result follows when the product of the action is gaseous. Thus, for instance, when a fragment of sulphur is burnt, although it disappears from sight, it, like the iron, combines with oxygen to form an oxide. This oxide, however, being a gas, escapes into the atmosphere. If the sulphur be burnt in such a manner that the sulphur dioxide is collected and weighed, it also will be found to be heavier than the original sulphur. In the process of burning, 1 gramme of sulphur unites with about 1 gramme of oxygen, and the product therefore weighs 2 grammes. By causing an ordinary candle to burn in the apparatus shown in Fig. 76, where the invisible products of its combustion are arrested, the increase in weight may easily be seen. The candle being essentially a compound of carbon and hydrogen, the products of its burning will be carbon dioxide and water, both of which will be absorbed by the

FIG. 75.

sodium hydroxide in the upper part of the tube. Consequently, as the candle burns away, the arrangement gradually gains in weight; the increase being the weight of the atmospheric oxygen which has combined with the carbon and the hydrogen to form the compounds carbon dioxide and water.

Heat of Combustion.-During the process of combustion, a certain amount of heat is evolved, and a certain temperature is attained-two results which are quite distinct. The temperature is measured by thermometers or pyrometers, while the amount of heat is measured in terms of the calorie, or heat unit.*

The amount of heat produced by the combustion of any substance is the same, whether it burns rapidly or slowly, provided always that the same final products are formed in each case. Thus, when I gramme of phosphorus burns in the air to form phosphorus pentoxide, it evolves 5747 calories; and when the same weight of phosphorus is burnt in oxygen, although the combustion is much more rapid and energetic, and the temperature consequently rises higher, the amount of heat evolved is precisely the same.

Again, when iron is heated in oxygen it burns with great brilliancy, and with evolution of much heat; if, however, the same weight of iron be allowed slowly to combine with oxygen, even without any manifestation of combustion, but simply by the process of spontaneous oxidation, or rusting, it is found that the amount of heat produced in forming the same oxide is absolutely the same.

So far, therefore, as the quantity of heat produced is concerned, there is no difference between active combustion and slow combustion, or (confining ourselves to the case of combinations with oxygen) between active combustion and the ordinary process of spontaneous oxidation at ordinary temperatures. In the latter case the heat is given out slowly-so slowly that it is conveyed away by conduction and radiation as fast as it is produced, and consequently the temperature of the material undergoes no perceptible change. In the case of active combustion, the action is crowded into a few minutes or seconds, and, as all the heat developed is evolved in this short space of time, the temperature of the substances rapidly rises to the point at which light is emitted. That heat is developed during the process of spontaneous oxida

The major calorie sometimes used is equal to 1000 calories. See Thermochemistry, Part I. chap. xv.

tion is readily shown. Thus, if a small heap of fragments of phosphorus be exposed to the air, it will be evident from the formation of fumes of oxide that it is undergoing oxidation. As the action proceeds, and as the heat produced by the oxidation is developed more rapidly than it is radiated away (especially from the interior portions of the heap), it will be seen that the phosphorus quickly begins to melt, and finally the temperature will rise to the point at which active combustion begins, when the mass will burst into flame.

It has been shown that many destructive fires have arisen from masses of combustible material, such as heaps of oily cotton waste, undergoing this process of spontaneous oxidation, until the heat developed within the mass has risen sufficiently high to inflame the material. To the operation of the same causes is to be referred the spontaneous firing of haystacks which have been built with damp hay, and also the spontaneous inflammation of coal in the holds of ships.

As the temperature produced by combustion is augmented by increasing the rapidity with which the chemical action takes place, it will be at once obvious why substances which burn in the air, burn with increased brilliancy and with higher temperature in pure oxygen. In the air every molecule of oxygen is surrounded by four molecules of nitrogen, therefore for every one molecule of oxygen that comes in contact with the burning substance, four molecules of this inert element strike it; and by so doing they not only prevent the contact of so much oxygen in a given interval of time, but they themselves have their temperature raised at the expense of the heat of the burning material. The number of oxygen molecules coming in contact with a substance burning in the air, in a given time, may be increased by artificially setting the air in rapid motion: hence the increased rapidity of combustion (and consequent rise of temperature) that is effected by the use of bellows, or by increasing the draught by means of chimneys and dampers.

The augmentation of temperature obtained by the substitution of pure oxygen for air is well illustrated in the case of burning hydrogen. The temperature of the flame of hydrogen burning in oxygen, known as the oxy-hydrogen flame, is extremely high, and when allowed to impinge upon a fragment of lime, it quickly raises the temperature of that substance to an intense white heat, when it emits a powerful light-the so-called oxy-hydrogen limelight.

The following results obtained by Bunsen show the temperatures reached by the combustion of hydrogen, and of carbon monoxide, in air and in oxygen

The flame of hydrogen burning in air

2024°

2844°

1997°

3003°

It will be seen that whereas the flame of hydrogen in air is hotter than that of carbon monoxide in air, when these gases burn in oxygen the temperature

FIG. 77.

of the carbon monoxide flame is higher than that of hydrogen. This is due to the partial dissociation of the water which results from the combustion of the latter. It has been shown that when a mixture of hydrogen and oxygen, in the proportion to form water, is ignited, the temperature produced by the union of a portion of the mixture rises above the point at which water dissociates; and consequently for a certain small interval of time a condition of equilibrium obtains, during which as many molecules of water are dissociated as are formed: during this state the temperature falls, when rapid combustion once more proceeds. It will be seen, therefore, that the limits to the temperature which can be reached by combustion are influenced by the points at which the products of combustion undergo dissociation.

Ignition Point.-The temperature to which a substance must be raised in order that combustion may take place is called its ignition point. Every combustible substance has its own ignition temperature. If this point be below the ordinary temperature the substance will obviously take fire when brought into the air, without the application of heat; such substances are said to be spontaneously inflammable, and must necessarily be preserved out of contact with air.

Passing from cases of spontaneous inflammability, we find a very wide range existing between the igniting points of different substances. Thus, a jet of gaseous phosphoretted hydrogen may be ignited by causing it to impinge upon a test-tube containing boiling water; carbon disulphide vapour is inflamed by a glass rod heated to 120°, while the diamond requires to be raised nearly to a white heat before combustion begins.

The difference between the temperatures of ignition of hydrogen and marsh gas may be

well seen by means of the old steel mill of the miner (Fig. 77). By causing the steel disk to revolve at a high speed, while a fragment of flint is lightly pressed against its edge, a shower of sparks is thrown. out; and on directing a jet of hydrogen upon these

FIG. 78.

sparks the gas is instantly ignited, while they may be projected into a stream of marsh gas without causing its inflammation. The same fact is also made strikingly apparent by depressing a piece of fine wire gauze upon flames of marsh gas (or coal gas) and hydrogen. In the former case the flame will not pass through the gauze, although it may be shown that marsh gas is making its way through by applying a lighted taper immediately above the wire. If the gauze be held over the issuing jet of gas the latter may be ignited by a taper upon the upper side of the gauze, but the combustion will not be communicated to the inflammable gas beneath (Fig. 78). The gauze conducts the heat away from the flame so rapidly that the temperature of the metal does not rise to the ignition point of the marsh gas on the other side, and therefore the combustion cannot be propagated through