The colour of the stone was now changed to a blacky 
vitrescent, slightly porous mass, hard and refractory., I 
have not given the specific gravities of the two last natures 
of stones : iron-stones containing equal portions of iron, 
in similar states of preparation, vary little in their specific 
gravities. 
It will no doubt be observed, tliat the increase of 
weight in these statements tally not with the sums former- 
ly given : the amounts there adduced are results from 
iron-stones which had been previously deprived of most 
of their oxygen; but in these, the extra quantity of oxy- 
gen taken up by the stone is only given, forming an ag- 
gregate, with the original existing quantity, as shall here- 
after be shown, nearly corresponding to the sums first 
giveii. From these experiments, singled out to convey 
a just idea of the changes to which iron stone may be sub- 
jected, it becomes obvious, that all the varieties of iron- 
stone are capable of decomposing atmospheric air at a cer- 
tain temperature, and of fixing a portion of its oxygen* 
whereby weight is gained, by each, nearly equal to one- 
eighth of its original quantity. 
It must also from this appear obvious, that the burning 
of iron-stone is an operation— though hitherto conducted 
by chance, exposed to all weathers— of the greatest nice- 
ty , and consequence, to the certain and economical manu- 
facture of cast-iron; wherein a small addition of fuel, by 
exciting a high temperature, exposes the iron to the com- 
bination of a hurtful principle, in quantity (as will hereaf- 
ter be shown) almost equal to what the metal was original- 
ly precipitated in. The extra proportion of fuel, therefore, 
requisite under circumstances where a severe mode of tor- 
refaction is either universally adopted, or where it is fre- 
quently the result of inattention and want of skill, though, 
as yet unascertained upon a large scale, must be very con- 
siderable . 
