an the different kinds of Coal 325 



burning air, which operates upon the surface of the combustible, 

 as takes place under the muffle. The intumesced cokes main- 

 tain the mass in such a state of motion, on account of the aug- 

 mentation of their proper volume, that the passage of the de- 

 composed air is never for a moment disturbed or interrupted. 



The conglutinated cokes already form a more compact and 

 more firm mass. With regard to the pulverulent cokes, whether 

 froin the commencement of the operation they were already re- 

 duced into small pieces^ or whether in the combustion, which is 

 gradually operated, they diminish in size, all the interstices are 

 so obstructed that the decomposed air finds no issue, and then 

 the combustion is arrested not from want of access of air, but 

 from cessation of the current of air. The powder of wood- 

 charcoal when heaped up so as to form a large mass, owing to 

 its closeness burns with difficulty, notwithstanding the briskest 

 affluence of air. From this manner of burning it might almost 

 pass for a glance-coal. 



Whatever may be the reasons of preference which we have 

 stated in favour of coals with intumesced coke, such coals can- 

 not be employed in certain cases and for certain objects. Cokes 

 that are too much intumesced, if they are heaped together in 

 large pieces^ fall into cinders or fragments, and this arises partly 

 from their weight. This reduction to small fragments still in- 

 creases, if such cokes are to be burnt in fourneaiix a cuve, or 

 if they are stratified with the substances which it is intended to 

 melt or reduce. Thus, coals with highly intumesced coke do 

 not furnish a suitable combustible for the treatment of iron-ore 

 in the high furnace Qe haut Jburneau) ; but in the cases in 

 which the pressure is less considerable, where, consequently, 

 there is no reason to dread the reduction of the coke into small 

 fragments, they may be employed, even for the use oifourneaux 

 a cuve, such as Wilkinson's {urnaces., Jburneaux a manclte, and 

 ^emi-hauts-fourneaux ; then such cokes always answer best. In 

 general, it is the state of loose aggregation, or the lightness of 

 cokes, which entirely decides as to their employment in the 

 Jburneaux a cuve. 



A coal with intumesced coke, when it passes into coal with 

 conglutinated coke, furnishes an excellent combustible for the 

 use of high furnaces for melting iron-ore ; but coals with intu- 



