IRON 957 



rature must evidently be impaired by the coldness of the air injected upon the fuel. 

 The heat developed in combustion is distributed into three portions : one is communi- 

 cated to the remaining fuel ; another is communicated to the nitrogen of the atmosphere 

 and to the volatile products of combustion ; and a third to the iron and fluxes, or other 

 surrounding matter, to be afterwards dissipated by wider diffusion. This inevitable 

 distribution takes place in such a way, that there is a nearly equal temperature 

 over the whole extent of a fire-place, in which an equal degree of combustion exists. 



We thus perceive that if the air and the coal be very cold, the portions of heat ab- 

 sorbed by them might be very considerable, and sufficient to prevent the resulting tem- 

 perature from arising to a proper pitch ; but if they were very hot they would absorb 

 less caloric, and would leave more to elevate the common temperature. Let us sup- 

 pose two furnaces charged with burning fuel, into one of which cold air is blown, and 

 into the other hot air, in the same quantity. In the same time, nearly equal quantities 

 of fuel will be consumed with a nearly equal production of heat ; but notwithstanding 

 this, there will not be the same degree of heat in the two furnaces, for the one 

 which receives the hot air will be hotter by all the excess of heat in its air above 

 that of the other, since the former air adds to the heat while the latter abstracts from 

 it. Nor are we to imagine that by injecting a little more cold air into the one fur- 

 nace, we can raise its temperature to that of the other. "With more air indeed we 

 should burn more coals in the same time, and we should produce a greater quantity 

 of heat, but this heat being diffused proportionally among more considerable masses 

 of matter, would not produce a greater temperature ; we should' have a larger space 

 heated, but not a greater intensity of heat in the same space. 



Thus, according to the physical principles of the production and distribution of 

 heat, fires fed with hot air should, with the same fuel, rise to a higher pitch of tem- 

 perature than fires fed with common cold air. This consequence is independent of 

 the masses, being as true for a small stove which burns only an ounce of charcoal 

 in a minute, as for a furnace which burns a hundred-weight; but the excess of 

 temperature produced by hot air cannot be the same in small fires as in great, be- 

 cause the waste of heat is usually less the more fuel is burned. 



This principle may be rendered still more evident by a numerical illustration. Let 

 us take, for example, a blast-furnace, into which 600 cubic feet of air are blown 

 per minute ; suppose it to contain no ore but merely coal or coke, and that it has 

 been burning long enough to have arrived at the equilibrium of temperature, and 

 let us see what excess of temperature it would have if blown with air of 300 C. 

 (572 R), instead of being blown with air at C. 



600 cubic feet of air, under the mean temperature and pressure, weigh a little more 

 than 45 Ibs. avoirdupois; they contain 10*4 Ibs. of oxygen, which would burn very 

 nearly 4 Ibs. of carbon, and disengage 16,000 times as much heat as would raise by 

 one degree per cent, the temperature of 2 Ibs. of water. These 16,000 portions of 

 heat, produced every minute, will replace 16,000 other portions of heat, dissipated by 

 the sides of the furnace, and employed in heating the gases which escape from its 

 mouth. This must take place in order to establish the assumed equilibrium of caloric. 



If the 45 Ibs. of air be heated beforehand up to 300 C., they will contain about |th 

 part of the heat of the 16,000 disengaged by the combustion, and there will be there- 

 fore in the same space th of heat more, which will be ready to operate upon any 

 bodies within its range, and to heat them |th more. Thus the blast of 300 C. gives 

 a temperature which is |ths of the blast at zero C., or at even the ordinary atmospheric 

 temperature; and as we may reckon at from 2,200 to 2,700 F. (from 1.200 to 

 1,500 C.), the temperature of blast-furnaces worked in the common way, we perceive 

 that the hot-air blast produces an increase of temperature equal to from 270 to 360 F. 



Now, in order to appreciate the immense effects which this excess of tempera- 

 ture may produce in metallurgic operations, we must consider that often only a 

 few degrees more temperature are required to modify the state of a fusible body, 

 or to determine the play of affinities dormant at lower degrees of heat. Water is 

 solid at 1 tinder 32 F. ; it is liquid at 1 above. Every fusible body has a deter- 

 minate melting point, a very few degrees above which it is quite fluid, though it may 

 be partly below it. The same observation applies to ordinary chemical affinities. 

 Charcoal, for example, which reduces the greater part of metallic oxides, begins to 

 do so only at a determinate pitch of temperature, under which it is inoperative, but a 

 few degrees above it is in general lively and complete. It is unnecessary in this article 

 to enter into any more details to show the influence of a few degrees of heat more or 

 less in a furnace upon chemical operations, or merely upon physical changes of state. 



Figs. 1230, 1231, exhibit the apparatus of a hot blast as mounted at the Codner Park 

 works, belonging to William Jessop, Esq., in every requisite detail. The drawings 

 from which the wood-cuts are faithfully copied were kindly furnished for this work 

 by Mr. Joseph Glyn, F.R.S., the distinguished engineer of the Butterly Iron Works. 



