69 



round the central portions radiate back to them as much caloric 

 as they receive, and for the same reason the conducting power of 

 the materials of tlie furnace cannot much diminish the effect, 

 the centre of the fire is surrounded by one of the worst conduc- 

 tors with which we are acquainted, ignited charcoal ; and ex- 

 perience shews that in furnaces of a certain size, this reasoning 

 may be depended on. I have produced as high a heat as ever was 

 measured, 169 of Wedgewood in a furnace whose walls were 

 only two inches thick, and another equally well constructed, but 

 more massive, did not give a higher result. The loss from 

 heating the gases is much more serious, their capacity is equal to 

 that of charcoal, and their weight much greater, so that most of 

 the caloric produced is absorbed by them. To ascertain the 

 amount of the loss, let us suppose a quantity c of charcoal at 

 the temperature d to undergo combustion, the caloric evolved is 

 divided between the air which has burned it and the unconsumed 

 charcoal, now we know how much is required to raise a determi- 

 nate quantity of this air 9 degrees, and therefore how much goes 

 to heat the remainder of the fuel and the crucible. It is clear 

 that as 6 is greater more of the caloric will be required to heat 

 the gases, and of course less will remain to raise the temperature 

 of the furnace, so that there is a certain value of it at which all 

 would be employed in bringing them to the heat of the coals, 

 and this would be a major limit, for if 6 were greater than this, 

 the air which had burned a quantity of fuel after receiving all 

 the heat proceeding from it would still be less hot than the sur- 

 rounding bodies, and therefore would cool them. In practice the 

 maximum temperature is a little below this on account of the 

 loss of heat by the walls of the furnace, as I have already stated* 



