w 



J 



324 /. LeConte on the Injluence of Solar Light on Comhistion, 



owing to tlie augmented density/ of tlie air, but to the greater dry- 

 ness of the atmosphere. The facts brought for ^yard in this paper, 

 are strongly opposed to this explanation ; for, there are not the 

 slighest grounds for supposing, that there was less than the ordi- 

 nary amount of aqueous vapor present in the condensing cylin- 

 ders of M. Triger ; or more than the usual quantity mixed with 

 the air at the elevated stations in India. On the contrary, physi- 

 cal considerations lead us to precisely opposite conclusions. 



2, Temperature of the Air. 



In relation to the influence of the temperature of the air on the 

 rate of combustion, our information is still more meagre. The 

 experiments of Grotthuss and Sir H. Davy on the " Effects of 

 Earefaction by Heat on Combustion and Explosion," give contra- 

 dictory results :* but, as they relate exclusively to the influence 

 of temperature on the ignition of explosive mixtures of gases, 

 they test its effects on comhustihility ^ and are obviously inapplica- 

 ble to the question under consideration. The -well-known effects 

 of the "Hot Blast" in increasing the teniperature of furnaces 

 cannot be applied as a test of the influence of warm air on the 

 rate of combustion under ordinary circumstances. First^ because 

 the air of the ''hot blast" is noi in its natural ^i^ie oi density ; 

 and secondly^ because the augmentation of temperature observed 

 in such cases, probably arises from its greater availahility, grow- 

 ing out of the fact, that less heat is earned oif in the products of 

 combustion, rather than an absolute increase in the rapidity of 

 burning. 



In the absence of direct experimental evidence, it may be 

 admissible to apply general reasoning based upon well-known 

 physical principles. So far as an increase of temperature influ- 

 ences the density of the air, it is sufficiently evident, that its 

 effect must be equivalent to a diminution of barometric pressure ; 

 and, consequently, must tend to retard the process of combustion. 

 Assuming the temperature of the flame to be constant^ it is like- 

 wise plain, that the draught created by it, — depending, as it is 

 known, on the difference of temperature between the flame and 

 that of the surrounding air, — must be diminished in a warm at- 

 mosphere; and, therefore, also tend to retard the rate of combus- 

 tion, uteris paribus, during hot seasons. But inasmuch as the 

 variations in the velocity of the draught, are proportional to the 

 square roots of these differences of temperature, it is obvious, that 



»sph 



must 



ture of the flame to be 1500 



For example, supposing the tempera- 

 iOO° Fahr., then the fluctuation of the 



draught between the temperatures of 80° and 60^ Fahr., would 



I 



Phil. Trans., for 1811, p. 53. 



