f 



i 



tflitence of 



321 



These 



if solar light exercised the decided influence on the process of 

 combustion which Dr. M'Keever^s experiments seem to indicate, 

 we should expect the effects to be much more marked and con- 

 spicuous, when the light was increased in intensity from eir/ht to 



fi 



The fact that the rays of 



the sun traversed the glass lens before they fell on the flame, can ' 

 scarcely be urged as a possible explanation of the discrepancy ; 

 for Dr. M'Keever obtained analogous results when he employed 



hyiterns. (Vide experiment No. 5.) 



The obvious 



^ 



dm 



entdays^ (as exhibited in my experiments,) illustrates in a most 

 striking manner, the decided influence exercised on the process, 

 by comparatively slight alterations in the external conditions. 

 IHs fact should inspire us with wholesome caution, and check 

 the spirit of rash generalization. Throwing out of consideration 

 the possible fluctuations in the rate of burning arising from want 

 of homogeneity in the combustible materials, and imperfections 

 m the mechanical arrangements by which they are consumed, 

 there are three external conditions which may be supposed to exer- 

 cise more or less influence on the rapidity of the process. These 

 are, first^ barometric pressure; second^ temperature of the air; 

 and ihird^ amount of aqueous vapor present. I propose to con- 

 sider each of these separately. 



1. Barometric Pressure. 



From d priori considerations, we should be led to expect, tliat 

 an increase of barometric pressure, tbroiigh tlie consequent con- 

 densation of tlie air, would, ceteris paribus, tend to augment the 

 rapidity of combustion, by furnishing the burning matter with 

 a greater amount of oxygen in a given volume. Unfortunately, 

 direct experiments are wanting to test this in as satisfactory a 

 manner as we should desire. The older experiments in the 

 Boylean vacuum, inasmuch as they relate to the degree of ^ rare- 

 faction at which combustion ceased, do not give us information in 



arious 



same 



haustion. The ,, 



Grotthuss, as well as to the admirable "Eesearches" of Sir Hum- 

 phry Davy on the " Effects of Rarefaction, by partly removing 

 the Pressure of the atmosphere, upon Flame and Explosion."* 

 The experiments of the latter show, that rarefaction produces 

 striking alterations in the size and character of the flame, but do 

 Dot touch the question of the relative rate of burning under dif- 

 ferent pressures : — they test the comparative combustibility of dif- 



* Vide Davy's " Researches on Flame." Phil. Trans, for 1817, p. 45 et seq. Also, 

 Works of Sir H. Davy, edited by Dr. John Davy. Vol. 6, p. 31 et seq. Lond. 1840. 



SECOND SERIES, VOL. XXIV, NO. T2. SEPT., 1837. 



41 



