Page:The American Cyclopædia (1879) Volume V.djvu/140

 136 COMBUSTION for the phenomena which attend it. The de- velopment of these depends upon the rapidity with which it goes on, and this distinguishes it from the other cases of oxidation, which are sometimes called slow combustion, as when metals rust, spirituous liquors turn to acetic acid, and the blood by respiration is oxidized. The views which prevailed respecting the na- ture of combustion before the discovery of oxygen gas by Priestley in 1774, and the de- velopment of its properties by Lavoisier in the succeeding year, were necessarily incorrect. With the ancients, fire was an element that de- voured other bodies and converted them into itself. Dr. Hooke in 1665, and Mayow soon after, advanced the opinion that there existed in the air a "nitrous spirit," which dissolved bodies susceptible to its influence when their temperature was suitably raised, and that the light and heat were the sensible effects of the rapid motions taking place. This theory, though close to the truth, was for a long time lost sight of in the general acceptance of the cele- brated phlogistic theory, which was advanced soon afterward by Becher, professor at Mentz, and ably sustained by Stahl, professor at Halle. They considered that in combustion a certain element, which Stahl named phlogiston (from Gr. 9/loytt>, to burn), left the burning body, and the product was this body deprived of its phlogiston, by regaining which it was restored to its original character ; as sulphur or phos- phorus when consumed became sulphuric or phosphoric acid, and on regaining their phlo- giston were again sulphur or phosphorus. It was known that the earthy "calx," into which some of the metals were transformed by fire, gained rather than lost weight; but this was explained by attributing to phlogiston a prin- ciple of levity. Notwithstanding the defects of the theory, it was still an important step in the progress of chemical science, serving first to group correctly together the phenomena of combustion, acidification, and respiration. Its nomenclature was incorporated with the sci- ence, and when Priestley made his great dis- covery of the new kinds of air, he gave to nitrogen, which he supposed to be a combina- tion of air with the phlogiston of the combus- tible, the name of phlogisticated air, and to oxygen or pure air that of dephlogisticated air. Lavoisier, by subjecting the products of com- bustion to the test of weighing, showed that the combustible gained weight by the process, and he proved, on restoring it to its former condi- tion (as in the case of a metallic oxide), that the substance taken up and given out again was the pure air of Priestley, to which he gave the name of oxygen, from its acidifying prop- erty (b&s and yevvdu). Thus was established the antiphlogistic theory, that in every case of combustion oxygen combines with the burning body. Dr. Black's theory of latent heat was adopted to account for the production of light and heat ; the latter being evolved or rendered sensible when substances without change of form pass from a rarer into a denser state, also when a gas becomes liquid or solid, or a liquid solidifies. The oxygen of the air was supposed to contain heat and light in a latent state, which were evolved with its change into a more condensed form, and the products of combustion were supposed to have less com- bined or specific heat than the original sub- stances. But this application failed in the case of combustion of solid bodies by explosion, the gaseous compounds expanding in some instances to 2,000 times their original bulk, and-yet pro- ducing intense heat instead of cold, as the theory would require ; and the specific heat of the new compounds, in this as in the combus- tion of charcoal, it was shown by Dulong and Petit, was often quite equal to, and sometimes exceeded, that of the combining bodies, and this, moreover, bore no relation to that evolved in combustion. Davy considered that the burn- ing body and the supporter of combustion were in opposite electrical conditions, and that the heat and light were evolved in the discharge of these electricities ; which view was also held by Berzelius, though unsustained by any posi- tive proof. Despretz ascertained the number of pounds of water which the burning of 1 Ib. of different combustibles would heat from the temperature of 32 to 212 F. The following are some of his results : T lb. of Lbs. of water. Pure charcoal 78 Charcoal from wood 75 Baked wood 36 Bituminous coal 60 Wood containing 20 per cent, of water 27 Turf. 25 to 30 Alcohol 67-5 Olive oil, wax, &c 90 to 95 Ether 80 Hydrogen 236 4 Carbon and hydrogen are the two common elements, which by uniting with oxygen pro- duce combustion. They are furnished in a variety of forms suitable to this application, the source of all which is traced to vegetable growth ; and this ever continues to gather up the products of combustion, and, separating them by decomposition, places them again in condition to renew the process. In combina- tion they assume a volatile form, and float up- ward with the air rarefied by the heat, thus allowing the admission of fresh supplies of oxygen to constantly reach the ignited body. Though the combustible bodies are enveloped in the atmospheric air, and are ever disposed to unite with its oxygen, the process cannot commence until the temperature of the com- bustible has been raised to a certain point, when it is said to catch fire ; the process thus begins, and afterward evolves the heat neces- sary for its continuance. The condition of the air as to temperature, density, and the pres- ence of aqueous vapor, variously affects the process of combustion. Increase of density adds to the quantity of oxygen in a given vol- ume, and consequently may be expected to in- crease the rate of combustion. The effect of