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ignition cohesion is diminished, for charcoal becomes denser by heat, 

 and by a priori arguments, it should even seem that as bodies burn 

 with difficulty in rarified air, and the heat of the burning body must 

 expand it, therefore ignition might be adverse to combustion. 

 We are also ignorant whether charcoal burns faster at a higher 

 temperature, and little can be affirmed about the most important 

 products, the light and heat for which we excite it. Probably 

 the light comes exclusively from the combustible, it is different 

 in different bodies, and in the case of hydrogen is almost nothing, 

 though the heat produced is intense, and the quantity of oxygen 

 condensed enormous ; the experiments of Saissy indeed would if 

 accurate lead to a contrary conclusion, he found that by com- 

 pressing in a glass syringe any gas containing oxygene (among 

 which he reckons chlorine) which was a supporter of combustion, 

 a flash of light was seen to pervade the barrel, but I suspect 

 that it was occasioned by the combustion of some of the grease 

 employed in lubricating the piston. The caloric is mostly de- 

 rived from the supporter, but the combustible itself affords some, 

 for much heat is produced -by the action of certain inflamma- 

 ble bodies on each other, as sulphur and iron. After this brief 

 notice we may proceed to ascertain if possible the maximum of 

 heat which the combustion of charcoal can produce ; if no ca- 

 loric were lost the temperature would uicrease without limit, 

 but this is not the case, some of it is dissipated by radiation, 

 some transmitted through the walls of the furnace, and much 

 wasted in heating the gases which have supported the com- 

 bustion and are thirteen times as lieavy as the fuel con- 

 sumed. The influence of radiation is trifling, in fact it does 

 iK)t affect the interior parts of a flnnace, as the coals which sur- 



