193 Prof. J. Le Conte on the Influence of 



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 vapour is present in large quantities, 

 there can be no doubt of its controlling agency on combustion. 

 This is illustrated by the successful application of the plan pro- 

 posed by M. Dujardin of Lille, in 1837, for extinguishing fires 

 occurring in steam-ships, by permitting the steam from the boil- 

 ers to escape into the apartment in which the combustion origi- 

 nates*. But experiments are still wanting for determining its 

 influence on the rate of burning, when existing in the small 

 quantities in which it is usually associated with the atmosphere f. 

 The experimental researches of Mr. David Waldie, in relation to 

 the mixture of various gases with air, led him to the general 

 law, that " of incombustible gases which remain undecomposed, 

 the power of preventing combustion is in the order of their den- 

 sity;" and tliat "this effect of density in cooling the flame 

 depends on the excessive diffusion of the flame in the denser 

 gas J." Under ordinary circumstances, the density of the aqueous 

 vapour existing in the air is comparatively small, so that, accord- 

 ing to Mr. Waldie's law, its influence on combustion ought not 

 to be very striking. It is very desirable that this point should 

 be submitted to a more rigorous experimental investigation. 



Having discussed the pi'obable influence of the three external 

 conditions on the rate of combustion, we are in a measure pre- 

 pared to investigate their adequacy to explain the variations in 

 the rapidity of burning, as indicated by the experiments which I 

 have brought forward. In none of them have we the observa- 

 tions necessary for ascertaining the hygrometric condition of the 

 atmosphere ; this must thei-efore be thrown out of considera- 

 tion. In Dr. M'Keever's experiments the barometric indications 

 are not given, neither is it known how many of them were per- 

 formed on any one day ; in my experiments, as well as in those 

 of Mr. Mitchell, we are furnished with the data requisite for 

 estimating the combined influence of pressure and temperature. 

 Assuming, with Sir H. Davy, that the rapidity of combustion is 

 in the direct ratio of the density of the air, we may submit these 



* Comptes Rendus, vol. v. p. 28. Paris, 1837. Also vol. xsxv. pp. 368, 

 706. Paris, 1852. 



t The curious results obtained by Mr. J. F. Dana, and subsequently 

 Mr. Samuel Morey, in relation to increasing the brightness of the flames of 

 highly carbonaceous combustibles, by throwing a jet of steam into them, 

 are obviously inapplicable to candles. ( Vide Silliman's Journal, 1 st series, 

 vol. i. p. 401 ; vol. ii. pp. 118, 122; and vol. vii. p. 141.) 



+ Vide Phil. Mag. S. 3. vol. xiii. p. 86 et seq. August 1838. 



I 



