202 ANNUAL OF SCIENTIFIC DISCOVERY. 



that the " fuses of shells " burn more slowly at elevated stations, where the 

 atmosphere is rarefied. At altitudes of 3,000, 6,500 and 7,300 feet, the rates 

 of burning were found to be respectively 10, 20 and 27 per cent, slower than 

 at the artillery depot. These facts seem to render it certain that any in- 

 crease in the density of the air must accelerate combustion, while rarefaction 

 must have an opposite effect. 



It has long been a matter of common observation, that ordinary wood- 

 fi.es burn more freely when the barometer is high ; but, Mr. Marcus Bull 

 and others maintain, that this result is not owing to the augmented den- 

 sity of the air, but to the greater dryness of the atmosphere. The facts 

 brought forward in this paper, are strongly opposed to this explanation ; 

 for, there are not the slightest grounds for supposing, that there was less than 

 the ordinary amount of aqueous vapor present in the condensing cylinders 

 of M. Triger ; or more than the usual quantity mixed with the air at the ele- 

 vated stations in India. On the contrary, physical considerations lead us 

 to precisely opposite conclusions. 



2. In relation to the influence of the temperature of the air on the process 

 of combustion, Professor Le Conte admitted that our information was very 

 meagre. He showed that the experiments of Davy and others do not touch 

 the question, inasmuch as they do not refer to the effect of temperature on 

 the rate of burning. He contended, however, that so far as increase of tem- 

 perature influences the density of the air, it is sufficiently evident, cateris 

 partbus, that its effect must be equivalent to a diminution of barometric pres- 

 sure, and consequently, must tend to retard combustion. In like manner, as- 

 suming the temperature of the flame to be constant, it was shown that the 

 draught created by it must be diminished in a warm atmosphere, and there- 

 by tend to retard the rate of burning in hot weather. On the other hand, an 

 augmentation of temperature might tend to accelerate the combustion by fa- 

 voring the liquefaction of the wax, and facilitating the oxidation of the 

 combustible matter. The last two effects were considered comparatively 

 insignificant ; and hence the conclusion was drawn, that the principal in- 

 fluence which temperature exercises on the rate of combustion is connected 

 with its effects on the density of the air ; and that, consequently, an increase 

 of temperature should, cccteris paribus, retard the process. 



3. In relation to the influence of the hygrometric condition of the air on 

 combustion. 



Sir Humphry Davy found that " a very large quantity " of steam was re- 

 quired to prevent sulphur from burning ; that an explosive mixture of oxy- 

 gen and hydrogen, when mixed with five times its volume of steam, still ex- 

 ploded by the electric spark ; and that a mixture of air and carburetted 

 hydrogen gas, required "a t/ii'd of steam to prevent its explosion, whereas 

 one fifth of azote produced the effect." Under any point of view, it is ob- 

 vious, that the presence of aqueous vapor can only tend to retard the process 

 of combustion. First, because it diminishes the amount of oxygen in a given 

 volume of air, and secondly, because an admixture of any inactive gas tends 

 to extinguish the burning body, as is abundantly proved by the experiments 

 of Sir H. Davy and others. When vapor is present in large quantities, 



