67 



the operator, and we do not possess any theory of ,their perfor- 

 mance.(c) 



To estimate completely the circumstances on which it depends 

 is, in the present slate of our knowledge, impossible ; we are not 

 accurately acquainted with the capacities of bodies for heat, their 

 conducting powers are almost unknown, and we have no means 

 of deciding whether they vary with the temperature, and ac- 

 cording to what law. But though a rigorous investigation can- 

 not be attained, we may arrive at limits within which the real 

 values are contained: and as the process of combustion includes 

 in it the consideration of fuel in a state of ignition, and of the 

 vessel in which it is consumed, we will enquire into them se- 

 parately. Combustion is well known ; if a fragment of charcoal 

 be placed in atmospheric air it suffers no change, but if it be 

 heated to redness, it rises to a higher temperature, a portion of 

 it disappears, while light is evolved, and the air M'hich lias sup- 

 ported the process, and which, when cooled, is found not to have 

 changed in bulk, now contains no oxygen, it has been channred 

 hito carbonic acid. It is found, that the consumption of 1 part 

 of charcoal in common air affords 9.3 parts of nitjogen by 

 weight, and 3.6 of carbonic acid, and as this mixture cannot 

 support combustion, it must be apparent, that the continuance of the 

 process requires its removal and a fresh supply from the atmosphere. 

 We are much in the dark as to many of the circumstances of com- 

 bustion, I am not acquainted with any plausible reason for the ne- 

 cessity of a red heat to its commencement, it cannot be said that by 



l2 



(e) Ih Rees's Cyclopedia, a theory of the action of chimneys is given, which is almost 

 completely erroneous. 



