ON FUR A 7 ACES. 383 



rated, together with the mixture of CO and N, produced by the passage 

 of the air itself through the mass of fuel, flow forward into the 

 working chamber, and there burn, on mixing with a further supply of 

 air, introduced above the fire. 



Examples of the simplest form of the class of furnace in which solid 

 fuel is mixed directly with the matters to be heated are the heaps in 

 which brick clay is buined to make ballast, and in which iron and 

 other ores are often calcined. In these, the ore or dried clay, in pieces 

 of convenient size, is thrown into a heap, together with a little coal ; 

 and the mass, being lighted at one end, burns through to the other. 

 In calcining in such heaps, there is a considerable waste of heat, as 

 a great proportion of the burned gases from the fire pass off at a high 

 temperature ; and when the calcination is completed, all the heat that 

 the red-hot mass contains is lost. 



In an ordinary limekiln, and in kilns for calcining iron ore, such as 

 are used in Wales and Cleveland, a much larger proportion of the 

 heat produced by the combustion of the fuel is utilized, and the amount 

 of this required is proportionately reduced. The process in such a kiln 

 is carried on continuously ; the coal and raw stone being filled in, 

 together, at the top, and the calcined material drawn at intervals 

 from the bottom. By this mode of working, the air that maintains 

 the combustion of the fuel is drawn up through the material already 

 burned; cooling this down from a red heat to a temperature not 

 much above that of the atmosphere, and being itself considerably 

 heated : while the burned gases and excess of air, passing 

 away, rise through the mass of cold freshly charged material, and 

 leave in it the greater part of their available heat, escaping nearly 

 cool. 



In such a kiln, according to Gjers, calcining 800 tons of Cleveland 

 iron ore per week, the consumption of small coal is about one ton to 

 twenty-four or twenty-five tons of ore.* In kilns of somewhat smaller 

 size, both in Wales and in Cleveland, in which the regenerative action 

 is less complete, and in which the loss of heat also, by radiation and 

 conduction, is greater, the consumption is stated by Phillips to be one 



* Journal of the Iron and Steel Institute, i8ji, p. 213. 



