WHY WATER PUTS OUT FIRE.
I have often been puzzled, writes E. B. James in The Tradesman, to answer for myself why water extinguishes fire. A great many people say it is because the water and its steam so envelop the burning material as to exclude the oxygen, and thus the fire must stop. This seems to be an inefficient if not an entirely erroneous reason. My reason is this: We know that nothing will burn (i. c., unite with the evolution of heat and light) unless and until it has been raised to a given temperature. Thus : Sodium burns at ordinary temperature—about sixty degrees, if dry—the gas of ordinary kerosene at about 170 degrees or less, and soon. Why do we tip a piece of stick with sulphur and then with phosphorus to make matches? Because, while wood must have quite a high temperature, phosphorus will burn at a comparatively low temperature—so low that the heat developed by slight friction will ignite it. The phosphorus makes heat enough to ignite the sulphur, but not enough to start the wood. But I will ask another question : Wood ashes are white. Why is the remaining end of the match and often most of it black in color ? Is it not because carbon being black, and the wood being composed of carbon and compound substances, which are very inflammable, the latter burn, but do not generate enough heat to oxidize the former. This seems to me the reason for the carbon remaining. Thus premising and showing that carbon, which forms the bulk of wood (as well as of paper, cotton, etc.) requires to be very hot before It will unite with the oxygen, we may consider the water for a moment.
Perhaps the easiest way to make my point clear is to take a common illustration. Everyone knows that if one puts a kettle of water on a heated stove he must wait a long time for it to become boiling hot, or a temperature of 212 degrees F. Now, if he should put the same amount of quicksilver on by the side of the water, the latter would reach 212 degrees far sooner. Why is this? It is because the water takes up a large amount of heat which is there in the water, but which the thermometer docs not index. Now, if after reaching 212 degrees, he sets them off the mercury will reach the temperature of the surrounding air much more rapidly. Now, suppose we wish to boil all the water away, to evaporate it—in that case remember that in passing from water to steam the temperature is not raised even a degree, nor any fraction of one—could he not make it all evaporate instantly by raising the temperature a little? We all know he could not ; that the fire has a degree of heat several hundred degrees higher, but the kettle must remains long time, while the water very gradually goes off in steam. Why so? Because the water, in becoming steam, must take into itself a large amount of heat not shown by the themometer and as the heat can be added only gradually, the formation of steam is slow. Now, in the large amount of heat which water can take up, and the fact that ordinary inflammables must be raised to a high temperature in order to burn, we have the cause of water putting out fire. Put a burning match into a very small drop «f water and it is extinguished, because of the very large amount of heat taken from the match in reducing the water to steam which renders the temperature of the match too far below 212 degrees, or at least that far, if there is water enough, and so the carbon and its compounds forming the wood will no longer unite with the oxygen of the air. For the same reason a hot iron thrust into the water is cooled, and water sprinkled on the floor cools the air, the heat of evaporation in the latter case coming from the air itself, thus cooling it. Now, if we could find a fluid very plentiful, which requires more heat than water to make it boil, evidently we could put large fires out much more readily.