COMBUSTION. 79 



appears always to be connected with a different action 

 of the substances ; therefore, in combustion, and every 

 other case of change of temperature, the action of the 

 substances employed, and not the transfer of any one 

 substance, is that to which it is safest to refer. It has 

 sometimes been said that the production of sensible 

 heat is always attended by condensation, or a diminu- 

 tion in the bulk of substances, and that the con- 

 densation is the cause of the heat. That they do 

 often accompany each other is true, for the rapid 

 condensation of air by a syringe will ignite a piece of 

 German tinder, placed over a small aperture at the 

 bottom. But it is not the mere condensation that pro- 

 duces the heat, or, at all events, the ignition, in this 

 case ; for air forced into a close vessel containing a 

 piece of the same match, will not set fire to it, though 

 the condensation be ten times greater than in the 

 former case. It is the violent action of the air in 

 escaping by the small aperture that produces the effect. 

 Expansion often accompanies ignition ; as in the very 

 familiar case of the firing of gunpowder ; and it must 

 not be said that the heat there is occasioned by the 

 condensation of the atmosphere, arising from the con- 

 version of part of the powder into gas, because that 

 follows, instead of accompanying the ignition. We 

 are not so well acquainted with the rationale of light- 

 ning, but probably that, too, is accompanied by ex- 

 pansion. These remarks are necessary in order to 

 caution us against that dogmatical imputation of 

 causes by which the history of nature has been so 

 much clouded and confused ; and but for which the 

 laws of the year, as dependent upon atmospheric and 



