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ON THE GASES EVOLVED FROM IRON FURNACES. 179 
The small pressure of the blast also effects a slow combustion, so that the 
fuel frequently takes twice or three times the period to pass through the 
same region of the furnace, 
On the application of Furnace-Gases to practical purposes. 
In this division of the subject we have to consider the useful purposes to 
which these gases may be applied when employed as fuel. Their practical 
value does not so much depend on the amount of heat capable of being ge- 
nerated by their combustion, as upon their maximum temperature ; and both 
these conditions may be exhibited by examining the composition of the vari- 
ous mixtures of gases obtained from different depths in the furnace. But it 
would be erroneous to suppose that the values thus obtained by calculation 
expressed in all cases the average practical effects capable of being derived 
by their application on the large seale, for such a conclusion would only be 
justifiable when the numbers obtained by analyses expressed the average 
value. That this is not the case has been shown in the above considerations 
as to the proportion between the nitrogen and oxygen, for we observed at the 
depth of fourteen feet, when the gases were richest in combustible materials, 
this relation was the least observed. Even in the highest layer of the 
gaseous mixture, the constituents of which may be viewed as most intimately 
mixed, we obtain a combustible value more than one-third greater than the 
above, when we estimate it according to the composition of the materials 
thrown into the furnace, and the products of distillation of the coal. In 
order, therefore, to found our calculations on a firm basis, we will estimate 
the practical value of the gases according to the results obtained in our 
former calculations, as to the limits of variation in the value of the gases, 
when deduced from the composition of the materials with which the furnace 
is supplied. This mode of proceeding will safely lead us to numbers ex- 
pressing the average value, and will enable us to repose upon them with 
confidence, from the assurance that the results capable of being obtained on 
the large scale, must be much greater than those expressed by calculation. 
Our experiments have proved the combustibility of the entire column of gas, 
even when cold, from a depth of twenty-four feet to the mouth ofthe furnace. 
Hence it follows that the gas collected from any point to this depth is capable 
of being applied asfuel. It would be objectionable, however, to conduct the 
gas from a deep region of the furnace, because we should thus draw off the 
heat necessary to support the process of coking the coal in its upper part. 
The gas might be collected from the upper part of the furnace without any de- 
triment to the process, and with additional advantage as fuel, because, while it 
contains all the combustible products of distillation, it is not deteriorated by 
having taken up any incombustible ingredients. This cireumstance greatly 
facilitates the application of the combustible gases, for it removes the obstacle 
to their use in furnaces fed with charcoal. In the latter, the zones of distilla- 
tion and reduction lie more closely together, and the proportion of ore and 
limestone to the coal is so much greater than in this country, that the amount 
_ of carbonic acid evolved obliges the withdrawal of the combustible gases 
from a low region of the furnace, where the reduction of the ore and evolu- 
_ tion of carbonic acid have been completed. But the withdrawal of the gaseous 
fuel from a region below that of the reduction necessarily produces such 
disturbance in the operations of the furnace, that only a small part of these 
gases dare be removed, while the largest portion must still be allowed to re- 
main and administer to its necessities. The advantage to be derived from 
the use of the gaseous fuel in this country will therefore be the more obvious, 
when we consider that its apvlication cannot derange any of the processes 
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