COMBUSTION. 197 COMBUSTION. the combustion of a candle, etc. In its more scientitic usage, however, the term may designate any possible ease of direct combination with oxygen, whether rapid or slow, whether accom- panied by light or not. By analogy, the term is also sometimes applied to the rapid union of substances with 'supporters of combustion' other than oxygen, such as chlorine gas, in which a candle may burn almost as well as in air. The light and heat of combustion are utilized for purposes of every-day life, the combustible material employed, i.e. the illuminant or the fuel, being usuallj- some product containing car- bon. Thus ordinar}' ilhuninating gas contains a number of gaseous chemical compounds of car- bon. Coal and wood are mixtures of carbon com- pounds, the former containing even a consider- able amount of free carbon. Hydrogen, too, is one of the chemical eonstituent-s of most fuels and illuminants. When combustion takes place, the carbon and hydrogen, combining with oxy- gen, give, respectively, carbonic-acid gas (carbon dioxide) and water vajior. These are, therefore, the chief products of ordinary combustion. The heat produced by the combustion of a given amount of material is .independent of the rate at which the combustion takes place, but depends entirely upon the composition and chemical nature of the material burned. Every combustible chemical compound has, therefore, its own definite heat of combustion; that is to say, the number of heat units (say, gram-calo- ries) produced by the combustion of one chemi- cal equivalent (gram-molecule) of a compound, depends upon nothing but the nature of the com- pound. The following are the heats of combus- tion of a few well-known compounds of carbon: ordinary alcohol, 340,000 gram-calories; acetic acid (the sour principle of vinegar), 210,000; ethyl-acetic ester, 544,000; eane-sugar, 1,35.5,000; cellulose, 680,000; urea, 152.000. The combus- tion of a chemical compound may be conceived as taking place in two consecutive steps: first, the compound is decomposed, i.e. every one of its molecules is broken up into its constituent atoms — a process usually involving not evolu- tion, but absorittion of heat; secondly, every single atom capable of so doing combines with oxygen (0) an atom of carbon (C), thus yield- ing a molecule of carbonic acid (CO.), and two atoms of hydrogen (H) yielding a molecule of water ( H,0 ) . This second step of the process is accompanied by the evolution of a quantity of heat depending upon the number of carbon and hydrogen atoms in a molecule of the com- bustible compound. But. owing to the absorption taking place during the first part of the process, a portion only of the heat produced during the second part actually appears in the form of sensible heat, and it is that portion which is called the heat of combustion. An exact knowl- edge of the heats of combustion of various sub- stances is of great importance for theoretical as well as for immediate practical purpo.ses. Its practical importance in comparing, for instance, different kinds of fuel, is self-evident and re- quires no explanation. Its theoretical impor- tance is mainly in the fact that with the aid of it the exact amount of heat evolved or absorbed during various chemical transforma- tions can be readily calculated. .According to the first law of thermodynamics, the amount of heat evolved or absorbed during any transforma- tion whatever, is independent of the manner in which the transformation tjikes place. For ex- ample, the amount of heat produced by burning one equivalent weight of ordinary alcohol and one equivalent weight of acetic acid, is the same whether we burn them directly or first cause them to combine into ethyl-acetic ester, and then burn the latter. In llie second case, the heat absorbed during the formation of the ester must, of course, be combined with that evolved during its combustion. But this suggests a simple way of obtaining the heat of formation of the ester by merely carrying out two combustions. The total heat of combustion of free alcoliol and acetic acid is 340,000 -f 210,000 = 550,000 gram- calories (see above) ; that of ethyl-acetic ester is 554,000 gram-calories. The excess of 4000 gram- calories must therefore rej)resent the amount absorbed <luring the combination of alcohol and the acid into ethyl-acetic ester. In a similar manner, the heat of any chemical reaction may be determined, if the heats of combustion of the reacting substances and the heats of combustion of the products of the reaction are known. In many cases this is the only certain way of determining with some precision the heat of reactions, as direct measurement during a reaction would often involve very great experimental dilliculties, while the direct measurement of the heat of com- bustion is a comparatively simple matter. The heat of combustion is usually determined by chemists in the following manner: A known amount of the combustible substance is inclosed in an air-tight steel vessel filled with compressed oxygen and lined on the inside with platinum; the vessel is immersed in a calorimeter (see Caloeimetry), and the substance is ignited with the aid of an iron wire heated by means of an electric current; the observer measures the rise of temperature in the calorimeter, and from this calculates the amount of heat produced. The importance of knowing the heat of chemical reactions is dis- cussed in the article Thermochemistry (q.v.). Now, while the heat of combustion depends onl.v on the chemical nature of the material burned, the rise of temperature caused by it depends to a very great extent on the manner in which the combu,stion takes place. If other gases, such as the nitrogen of the air, are present without themselves adding to the amount of heat produced, part of that amount goes to heat such gases, and as a result, the temperature is considerably lower than if the same substance were burned in precisely the amount of oxygen gas required for its combustion. The rapidity with which a combustion takes place is another factor on which the temperature depends; for heat may be gradually dissipated by conduction even while it is being produced, and so the actual amount remaining at any moment during a slovp process of combustion may be very small. Thus, when phosphorus is exposed to the air at ordinary temperatures, a slow process of o.xidatipn (com- bustion) takes place, very little heat being given out at any given moment. If ignited in the air, phosphorus burns vividly, giving out much heat and light for a short time. Finally, if ignited in an atmosphere of pure oxygen, it enters into most vivid combustion, evolving, for a very short time, a most intense heat and a brilliant light. An analogous phenomenon may be obsen'ed when coal is burned in a furnace. So long as the door of the furnace is open and there is but little
