1826.] Rev, Mr. Emmett on Combustion. 429 



is that combination which is attended with the evolution of 

 light and heat, may we not properly consider every similar 

 act of combination in the same light? If we dOj the scale is 

 unbroken; heat is excited by every such combination; when it 

 becomes so intense as to be accompanied with light, it is an 

 example of combustion : the light and heat are emitted by both 

 or all the substances jointly ; no oiie in particular is the sup- 

 porter. Considering the subject in this light, we have a number 

 of substances placed in regular order, whether galvanic, that of 

 real chemical attraction, that of visible chemical attraction^ 

 which makes no corrections for the effects of cohesion, temper- 

 ature, elasticity, solubility, 8cc. or any other, and the combusti- 

 bility of substances combined, or the ratio of the quantity of 

 heat excited during their combination, will be reciprocally as 

 their distance from the extremes of the scale. If it could be 

 proved that the light and heat arise and are derived from one 

 body only, that would properly be termed a supporter ; but since 

 this is not the case, neither is a supporter, neither is an inflam- 

 mable (except popular language be used) ; but the union of the 

 two produces both light and heat. If the term combustion be 

 applied to all cases where a temperature not less than that ot 

 ignition is excited, ultimately some force or forces will be deve- 

 loped, whose operation may be seen in every case where conden- 

 sation takes place, and to which it is to be ascribed. 



If we examine the nature of the phenomenon, the rationality 

 of what I have stated will be more apparent. The law seems to 

 be universal, that condensation or reduction of volume excites? 

 heat.* If then the density of a compound exceed the mean 

 density of its component parts, heat must be excited. Let m, n, 

 be the weights of two substances ; a and b their specific gravi- 

 ties : the volume of the compound = — — - — ^ ; its spec. grav. 



weight \ m -t n . ah 



volume 



I = — - — '- — :t if then its specific gravity exceed 



^ "^ "'^ , heat is evolved during; the process of combination, 



mb + na^ . 



because condensation is accompanied by the evolution of heat. 

 If the condensation be such that the temperature of ignition, at 

 least, be produced, it is a case of true combustion. Why then 

 arbitrarily limit it to the cases where the presence of one of the 

 received " supporters " is requisite? It is surely more scientific 

 to recognize one operation from the production of the most 

 intense cold to the excitation of the most powerful heat, appli- 

 cable to every operation and combination of " undecompounded 



♦ The end of a rod of soft iron may be made red-hot by hammering ; if a gas be 

 suddenly compressed, heat is evolved ; stamping heats metals, &c. 



+ This excellent formula is taken from the Retrospect, and does not materially differ 

 from Newton's. 



