Science and Chemistry of Putting Out Fire
At a school exhibition not long ago a visiting member of the School Board, from whom no hard questions were anticipated because he was not thought to be himself much of a scholar, although he had an abundance of shrewd common sense, proposed a poser which sent all the prize scholars to their seats and gave the professor himself no small trouble to find a satisfactoryanswer. It was a question that is particularly interesting to everybody just now on account of the great Equitable disaster in New York. This was it:
“How does water put out fire?”
The pupil, on exhibition had all been studying chemistry and had passed very creditable examinations on what they had learned in their books, but unfortunately the books had failed to inform them on this extremely important and practical point.
One said: “Water puts out the fire by drowning it.”
“How does it drown it?” asked the questiotier.
The pupil couldn’t tell, and everybody laughed. Another thought he knew how it was : “It makes it wet.” he said, “and fire can’t burn when it’s wet.”
“How can anything make fire wet?” demanded the relentless questioner; and again the pupil had to shake his head in despair and sit down amid the universal merriment. And so the question went the entire round, and not one w-as found who could tell how water puts out fire, livery one knew the fact that it does put it out, but nobody could tell how or why. Then, as if to redouble his triumph, the questioner put another poser, which proved even worse than the first:
“Well,” he said, “we all know that water puts out fire, although it seems you don’t know how it does it. Now, tell me, please, why does a little water make the fire burn fiercer and a great deal of water put it out?”
Nobody could answer that, either, although one bright-witted young lady suggested that it might be for the same reason that a glass of water does a person good, while a pondful might drown one. She had hit the nail very fairly on the head, as we shall see a little later. Hut reall, it is not so easy to explain how water puts out fire. In order to understand how water acts when thrown upon a fire by a fire engine or from a bucket we must first know what fire is and what keeps it burning. Fire is aform of chemical combustion in which flames make their appearance. There tnay lie combustion, or burning, without flame, although not without heat. When a body becomes heated front any cause and wastes away, turning into something else (as, for instance, into smoke and ashes), it is said to undergo combustion. Now, in the process of combustion, or burning, there must always be at least two things concerned. In the rst place, there must be tbe combustible, or tbe body which burns, and in the next place there must be some supporter of combustion, in tbe midst of which tbe combustion takes place. When wood burns in tbe open air the wood is the combustible and tbe air is the supporter of the combustion. It is easy to prove that the wood could not continue to burn if it were not surrounded by air, for the air may be removed, leaving the burning wood in a vacuum, or perfectly empty space, whereupon tbe flame will disappear and the flame will die out. Now, how does the air support the combustion of the wood? The answer is that it supplies something which is essential to tbe act of combustion, and that something is the gas called oxygen. There cannot be combustion without oxygen. There cannot be a combustion without oxygen, because in the chemical change that goes on in burning bodies the elements of those bodies unite with oxygen to form the substance which chemists call oxides. If there is no oxygen to combine with there will be no chemical change, and. therefore, no combustion. The heat and light that accompany the burning of wood or gas are produced by the combustion or oxidation of the elements composing the wood or gas. Heat may serve to start combustion, but after the burning begins the heat given off is the result of and not the cause of tbe combustion. We must get a clear understanding of the nature of fire before we try to see bow water puts it out, and so we will
go a little further in the explanation of combustion and tbe part that the oxygen plays in it. Anybody who has ever watched a big building burning must have noticed how. without regard to the efforts of the firemen, the (lames w ill alternately die down and then burst out again with tremendous fury and light up all the surroundings almost with the brightness of day. Whenever such a sudden outburst occurs we may be sure that more oxygen has, in some manner, been supplied to reinforce the combustion. It may be that some new fuel has been reached by the flames, or it may be that a sudden draught, or flow of wind, has blown fresh air upon the embers and thus revived them. The air consists principally of two gases, oxygen and nitrogen, which are simply mixed together, and not chemically combined, so that when it (the air) reaches tbe Hame the oxygen immediately begins to lake part in tbe process of combustion and spreads and intensifies tbe fire. That is the reason why a strong wind is so dangerous during a big fire. It constantly brings fresh supplies of air to the flames and the latter seize upon the oxygen and grow ever brighter and hotter. On the other hand, if a dead calm prevails during a fire, the flames are much more easily controlled, because after a time all the oxygen in the air immediately in contact with the fire is burned out, so to speak, and the oxides formed by the combustion, such as carbon dioxide, being themselves unlnirnable and hanging in thickening clouds about the fire tend to smother it and put it out. A most striking and beautiful proof of the necessit yof a supply of oxygen to keep a fire going, and of the power of oxygen, if enough of it be present, to make a fire get hotter and hotter, can be shown by an experiment. Everybody knows that iron will not burn like wood with a flame and a brilliant light, although in a big fire like the Equitable disaster it will melt and twist and even be partially consumed. But iron can easily be made to burn, giving off a light far more intense than that of a gas flame, if it he placed in an atmosphere of pure oxygen. This can be done in what chemists call a belljar filled with oxygen. In such a jar an iron wire that has been made red hot by plunging it into a fire will blaze and fly off in sparks like a brilliant piece of fireworks. On the contrary, if a piece of wood or of coal is burned under a bell-jar containing ordinary air, and if the jar is closed so that no fresh air can enter it, after a little while the fire will go out of itself, because when all the oxygen has been exhausted from the air in the jar nothing will be left except the nitrogen, the other gas that goes to make up air—and nitrogen, instead of being, like oxygen, a supporter of combustion, tends to entinguish combustion and will not itself burn at any ordinary temperature. Besides, the oxides formed by the combustion, as already explained, tend to put out the fire. And now we are prepared to see how water acts in putting out fire. First, we may take the last of the posers that the school board examiner put to the pupils: “Why does a little water sometimes make the fire burn brighter?” The reason is because the elements of water themselves are combustible. The reason is because the elements of water themselves are combustible. Water consists of oxygen and hydrogen. The first, as we have just seen, is the great supporter of combustion, while hydrogen, instead of resisting combustion, like nitrogen, will also burn if it is heated up to a few hundred degrees. So, if a sprinkler of water . is thrown on a very hot fire, consisting of a great bodys of flames, the heat is sufficient almost instantly to turn the water into steam and to dissociate the oxygen and hydrogen, and both of them immediately burn, tbe oxygen especially adding to the intensity of the fire, just as happens when a draught of wind brings more atmospheric oxygen into the flames. Thus a little water is worse than useless when thrown upon a conflagration. But now turn tothe other question, how comes, it then, that a great deal of water does not also reinforce the combustion bysupplying oxygen all ready to unite in the burning? Tbe answer is that in this case, when there is an abundance of water potired in from huge pipes, and falling in a deluge upon the flames, the heat of the latter is insufficient in quantity to rapidly turn the water into steam and then dissociate the oxygen and the hy-drogen. And a
very remarkable property of water now comes into play. Water has a very high specific heat. That means simply that it takes an extraordinary amount of heat to change water from the cold liquid to the hot. vaporous form which we call steam. The heat of the fire is swiftly devoured. so to speak, by the water falling upon it and absorbing it. If you are near a burning building when the water from the firemen’s hose pipes reaches it. pouring in through the windows or the broken roof, or walls, you will perhaps hear a tremendous hissing and see clouds of steam rising. The formation of that steam has suddenly drawn an immense quantity of heat from the combustion going on in the building, and so has tended powerfully to cool off the combustibles and to subdue the rage of the flames. Because of the great specific heat capacit yof water and the consequent absorption of heat from the fire to form the steam, there is not heat enough left over to release the oxygen from the hydrogen, or to set the two in their still combined state burning, anti thus the constant streams of water pouring in and the clouds of steam arising serve as a blanket, not itself combustible. which shuts off the air from the flames, and thus prevents them form obtaining oxygen from the surrounding atmosphere as fuel as they did before.—New York American.