About Combustion.
BY
W. J. ROLFE, A.M.
Combustion is only another name for burning, and burning in all ordinary cases is oxidation, or union with oxygen, one of the gases that make up our atmosphere. It is a chemical change; that is, one by which we get a new substance entirely unlike any of the substances united. Common salt, for instance, is formed by the chemical union of a yellow, bad-smelling gas and a soft silvery metal. When coal and wood are burned, the chief products of the union with oxygen are carbonic acid and water. The former is a colorless gas, and the latter is in the form of invisible vapor, and both go up the chimney and mix with the outer air. The ashes left behind are only what can not be burned or united with the oxygen. If we collect all the products of the burning, together with the ashes, we find that they weigh more than the coal or wood, the increase being exactly equal to the weight of the oxygen consumed. No kind of matter can be destroyed by any power known to us; it may unite with other matter, and take many new forms, but its weight can be neither increased nor diminished. The amount of matter in the universe is always the same.
Oxygen must be heated before it will unite with coal or wood. The air is at all times in contact with them, but they will not burn unless they are first kindled. The chemical process itself, when once started, generally produces heat enough to raise more oxygen to the proper temperature, and thus the combustion is kept up. The point to which the oxygen must be heated varies much with different substances, as is well shown in kindling a coal fire. The heat produced by rubbing a match on a rough surface suffices to make the oxygen unite with the phosphorus on the end of the match; the burning of this causes heat enough for the union of the oxygen with the sulphur, and the burning of the sulphur enough to set the wood of the match on fire. The shavings, the kindling wood, and the charcoal are in turn ignited, and the burning charcoal develops heat enough to enable the oxygen to combine with the hard coal. Each step in the operation requires more heat than the preceding step. This seems a very simple thing now, but the anthracite beds of Pennsylvania long remained useless because no one had found out how to kindle the fuel, and the discovery was at last made half by accident.
There are some forms of combustion which are very unlike ordinary burning, and yet are essentially the same, being cases of union with oxygen. The only difference is that the process goes on slowly instead of rapidly. We know that vegetable and animal substances decay when exposed to the air; and decay is a slow burning. The oxygen of the air gradually combines with the substances, converting them into carbonic acid and water, and leaving only a small remnant of matter as the ashes of the lingering combustion. The heat produced in this case is found to be precisely the same as in ordinary burning, but it is set free so gradually that it escapes our notice.
We know that green wood decays much sooner than dry wood. Indeed, if wood is kept perfectly dry, it will not decay for ages. In the dry climate of Egypt wooden mummy cases have been preserved for more than three thousand years. On the other hand, dry wood burns much quicker than green wood; it is not easy to set the latter on fire. Why this difference, if decay and burning are similar processes? The decay of the green wood is due to the fact that the presence of moisture causes certain changes in portions of the wood, which enable the oxygen to attack it at a low temperature; and the slow combustion, once started, is self-sustaining. But in ordinary burning the temperature must be raised to a certain point before the oxidation can begin, and this point can not be reached until the moisture is evaporated, which uses up a good deal of heat.
This process of decay is continually going on in our bodies; but during life the matter which is burned up is being constantly renewed from the food we eat. The body is not only decaying, as dead animal matter decays, but it is also wearing out. With every motion a part of the muscles is actually consumed, and must be replaced by fresh material. The heat of the body is likewise due to combustion, and must be kept up by proper fuel, like the fires in our stoves and furnaces. The products of all this burning are carbonic acid and water, which pass out of the body through the lungs.
The rusting of metals is a slow combustion, and scientific men have proved that, like decay, it develops heat. Iron can be easily burned in pure oxygen, with the production of intense light and heat. Zinc and some other metals can be burned in the air if heated very hot, and most metals are rapidly consumed in the flame of the oxyhydrogen blow-pipe. Indeed, every form of matter known to us can be burned, unless it has already been burned. All substances belong to one of these two classes—those that will burn, or unite with oxygen; and those that have been burned, or are products of oxidation. Water belongs to the latter class, and so do nearly all the rocks and solid matter of the earth.
Slow burning sometimes becomes rapid, and then we have what is called spontaneous combustion. When cotton or tow which has become soaked with oil is laid aside in heaps, the oxygen of the air begins to unite with it; but the heat developed causes the oxidation to go on faster and faster, until in some cases the mass bursts into a flame. The same thing sometimes takes place in moist hay, the moisture starting the process, as explained above, and the confined heat increasing until it is sufficient to set the heap on fire.