898 PRINCIPLES OF CHEMISTRY 



coal into carbonic oxide 2,420 heat units are given out, and on burning to 

 carbonic anhydride 8,080 heat units. It is evident that on transforming 

 the charcoal first into carbonic oxide we obtain a gas which in burning 

 is capable of giving out 5,660 heat units for one part of charcoal. This 

 preparatory transformation of fuel into carbonic oxide, or producer 

 gas containing a mixture of carbonic oxide (about by volume) and 

 nitrogen ( volume), in many cases presents most important advantages, 

 as it is easy to completely burn gaseous fuel without an excess of air, 

 which would lower the temperature. 20 In stoves where solid fuel is 

 burnt it is impossible to effect the complete combustion of the various 

 kinds of fuel without admitting an excess of air. Gaseous fuel, such as 

 carbonic oxide, is easily completely mixed with air and burnt without 

 excess of it. If, in addition to this, the air and gas required for the 

 combustion be previously heated by means of the heat which would 

 otherwise be uselessly carried off in the products of combustion (smoke) 21 

 it is easy to reach a high temperature, so high (about 1,800) that 

 platinum may be melted. Such an arrangement is known as a regene- 

 rative furnace , 22 By means of this process not only may the high 

 temperatures indispensable in many industries be obtained (for instance, 



with a valve, into' the gas main U. The addition of fuel ought to proceed in such a way 

 as to prevent the generated gas escaping ; hence the space A is kept filled with the com- 

 bustible material and covered with a lid. 



80 An excess of air lowers the temperature of combustion, because it becomes heated 

 itself, as explained in Chapter III. In ordinary furnaces the excess of air is three or 

 four times greater than the quantity required for perfect combustion. In the best 

 furnaces (with fire-bars, regulated air supply, and corresponding chimney draught) it is 

 necessary to introduce twice AS much air as is necessary, otherwise the smoke contain* 

 much carbonic oxide. 



21 If in manufactories it is necessary, for instance, to maintain the temperature in a 

 furnace at 1,000, the flame passes out at this or a higher temperature, and therefore 

 much fuel is lost in the smoke. For the draught of the chimney a temperature of 100 

 to 150 is sufficient, and therefore the remaining heat ought to be utilised. For this 

 purpose the flues are carried under boilers or other heating apparatus. The preparatory 

 heating of the air is the best means of utilisation when a high temperature is desired (see 

 Note 22). 



28 Regenerative furnaces were introduced by the Brothers Siemens about the year 

 1860 in many industries, and mark a most important progress in the use of fuel, espe- 

 cially in obtaining high temperatures. The principle is as follows : The products of 

 combustion from the furnace are led into a chamber, I, and heat up the bricks in it, and 

 then pass into the outlet flue ; when the bricks are at a red heat the products of com- 

 bustion are passed (by altering the valves) into another adjoining chamber, II, and air 

 requisite for the combustion of the generator gases is passed through I. In passing round 

 about the incandescent bricks the air is heated, and the bricks are cooled that is, the 

 Jieat of the smoke is returned into the furnace. The air is then passed through II, and the 

 smoke through I. The regenerative burners for illuminating gas are founded on this 

 same principle, the products of combustion heat the incoming air and gas, the tempera* 

 ture is higher, the light brighter, and an economy of gas is effected. Absolute perfection 

 in these appliances has, of course, not yet been attained; further improvement is 

 etill possible, but dissociation imposes a limit because at a certain high temperature 



