40 Of the Management of Fire 



and heated vapour, is not exactly known ; it is certain, 

 however, that the quantity of heat which goes off in the 

 heated elastic fluids, visible and invisible, which rise 

 from a fire, is much greater than that which all the 

 calorific rays united would be capable of producing. 

 But though the quantity of radiant heat is less than 

 that existing in the hot vapour (and which, for the sake 

 of distinction, may be called combined heat\ the former 

 is still much too considerable to be neglected. 



That the heat generated, or excited, by the calorific 

 rays which proceed from burning bodies is in fact con- 

 siderable, is evident from the heat which is felt in a 

 room warmed by a chimney fire ; for as all the heat, 

 combined with the smoke and hot vapour, goes up the 

 chimney, it is certain that the increase of heat in the 

 room, occasioned by the fire, is entirely owing to 

 the calorific rays thrown into it from the burning 

 fuel. 



The activity of these rays may be shown in various 

 ways, but in no way in a more striking manner than by 

 the following simple experiment : When the fire burns 

 bright upon the hearth, let the arm be extended in a 

 straight line towards the centre of the fire, with the 

 hand open, and all the fingers extended and pointing to 

 the fire. If the hand is not nearer the fire than the 

 distance of two or three yards, except the fire be very 

 large indeed, the heat will scarcely be perceptible ; but 

 if, without moving the arm, the wrist be bent upwards 

 so as to present the inside or flat of the hand perpen- 

 dicular to the fire, the heat will not only be very sensibly 

 felt, but if the fire be large, and if it burns clear and 

 bright, it will be found to be so intense as to be quite 

 insupportable. 



