Iron, 
71 
stone, becomes iixed, of a brown, dark brown, or deep 
claret colour. These indicate the almost entire expulsion 
of the water, acid, &c. In this state, all iron-stones are 
possessed of magnetic attraction, and exhibit the various 
phenomena already described, as being peculiar to their 
respective natures,. 
The methods commonly in use for torrefying iron-stone, 
in the large way, are of two different kinds : that of burn- 
ing in kilns or conical furnaces ; and that of exposing 
iron-stone in open air, stratified with coals, to combus- 
tion. The former is used in some places in Wales ; 
the latter is almost universally adopted in England, and 
totally so in Scotland. 
In the operation with furnaces, they become filled en- 
tirely with iron-stone, except a stratum of coals in the bot- 
tom, which is afterwards inflamed. The combustion is 
then carried on by means of a current of air passing 
through the furnace, raid forcing the heat along with it. 
When the iron-stone is deemed sufficiently burnt, the re- 
gister is shut up ; and the combustion, no longer main- 
tained by means of external air, soon dies away, and leaves 
the furnace to cook 
The most common method, however, of burning iron- 
stones consists in levelling a piece of ground, and covering 8 
it with a layer of small pit-coals. This is of various thick - 
nesses, 4, 6, or 8 inches, according to the height the pile 
is to be built, and the nature of the iron-stone. Upon 
this stratum of coals the pieces of iron-stone are imbed- 
ded, as near to the same size as possible, in order that all 
may be equally acted upon. These are reared to various 
heights, 18, 20 to 24 inches, the determination of its 
height depending upon circumstances. The surface is a 
second time levelled, by introducing small pieces of iron- 
stone betwixt the interestices occasioned by the angles of 
the larger. This again receives a covering of small coals. 
