22 



COMBUSTION. 



Bij J. MONCKMAN, D.Sc, Fehnumj 9th, 1886. 



Ignition and combustion have very nearly tlie same meaning. 

 Tlie former, however, is used to signify the state of being red 

 hot, or hotter than that, without any chemical change taking 

 place in the composition of the body. The latter always means 

 producing heat, and generally light, by changing the constitution 

 of the substance. This change is generally produced by the 

 elements of the combustible body uniting with some other 

 element or elements, commonly called the supporter of com- 

 bustion. 



All our ordinary combustibles contain carbon, most of them 

 hydrogen also, and the gas supporting combustion is the oxygen 

 of the air which unites with these elements, forming new com- 

 pounds. Carbon (C) forms carbonic di-oxide, or carbonic acid as 

 it is commonly called (CO2), or carbonic oxide (CO), and the 

 hydrogen forms water (HoO). 



If the supply of oxygen is sufficient, one atom of C, weighing 

 12, unites with two" atoms of 0, weighing 16 each, forming 44 

 of CO2. When the oxygen is deficient, one atom of carbon 

 unites with one of oxygen, producing carbonic oxide. This gas 

 is combustible, and if not burnt, there is a certain loss of heat. 

 This is not the only cause of loss when the supply of air is too 

 small. The heat, acting on the coal, drives off some of the 

 volatile gases, or rather expels hydrogen combined with carbon, 

 in the form of gas, which causes a greater loss than the other. 



This may be presented in the following way. A unit of heat 

 is defined as that quantity which will raise lib. of water from 

 0° C. to V C. 121bs. of carbon fully burnt will raise about 

 97,0001bs. of water 1" C, or produce 97,000 units of heat. When, 

 however, they are burnt to CO only instead of to CO.,, 29,000 

 units of heat are evolved, sliowing a loss of 68,000 units, or more 

 than iwo-thirds the heat is lost. When compounds of carbon 

 and hydrogen are produced the loss is very great, since every 

 pound of hydrogen should produce 34,000 units, or more than 

 fovrr times as much as one pound of carbon. 



On the other hand, if too much air be passed through the fire, 

 the nitrogen of the air, which takes no part in the chemical 

 changes, absorbs heat to raise itself to the temperature of the 

 furnace. Part of this is restored, but a considerable quantity is 

 carried up the chimney. When, owing to small size of boiler or 

 other causes, the fires are urged too much and the temperature 

 raised above 2471° C, the carbon forms carbonic oxide only (CO). 

 This may be seen in our smelting furnaces, where formerly 

 the combustible gases given off were allowed to burn at the top 



