COMPOUNDS OF CARBON WITH OXYtiKN AND NITROGEN 387 



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 1800) that platinum may 

 be melted. Such an arrangement is known as a regenerative furnace?* 

 By means of this process not only may there be obtained the high 

 temperatures indispensable in many industries (for instance, glass- 

 working, steel-melting, &c.), but also great advantage 23 as regards 

 the quantity of fuel, because the transmission of heat to the object to 

 be heated, conditions being equal, is determined by the difference of 

 temperatures. 



The transformation of carbonic anhydride, by means of charcoal, 



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

 A furnace at 1000, 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, as by this means the highest 

 temperature is attained and the quickest heating and greatest economy are effected. 



22 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 com- 

 bustion 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 combustion 

 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 cool that is, the heat 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 temperature is 

 higher, the light brighter, and an economy of gas is effected in lighting. Absolute per- 

 fection in these appliances has of course not yet been attained; amelioration is still 

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

 combinations do not ensue, possible temperatures being limited by reverse reactions. 

 Here, as in a number of other cases, the ultimate conception of the subject ought to be 

 limited to the sphere of general external and visible advantages. 



25 At first sight it appears absurd, useless, and paradoxical to lose nearly one-third of 

 the heat which fuel develops by turning it into gas. Actually the advantage is enormous, 

 especially for producing high temperatures, as is already seen from the fact that fuel rich 

 in oxygen (for instance, wood) when damp is unable, with any kind of hearth whatever, to 

 give the temperature required for glass-melting or steel-casting, whilst in the gas-producer 

 they furnish exactly the same gas as the driest and most carbonaceous fuel. In order to 

 understand the principle which is here involved, it is sufficient to turn one's attention to 

 the fact that the only heat employed with advantage is that which is concentrated in the 

 shape of products of combustion having a high temperature. A large amount of heat, 

 but having a low temperature, in many cases is of no use whatever. We are unable here 

 to enter into all the details of the complicated matter of the application of fuel, and 

 further particulars must be sought for in special technical treatises. The following 

 footnotes, however, contain certain fundamental figures for calculations concerning com- 

 bustion. 



cc2 



