264 Dr. Beddoes's Observations on the Process for 
be considered as air bubbles caught in a solid film of lead. 
Perhaps white cast iron would not yield air so readily ; pos- 
sibly iron holds its charcoal with more force as it contains less. 
I intend to make some comparative experiments upon the va- 
rieties of cast iron, but so curious an appearance as these 
blisters, will always be rather the bounty of accident, than the 
effect of skill or labour. The obvious method to produce 
them, would be to cover the iron with lead. All the unmelted 
lumps of iron were porous, and open in their texture. 
2. I am at some loss how to explain the occasional discharge 
and cessation of air, in one experiment in which a crown glass 
retort was used, and in another with an unglazed earthen tube. 
There was no flaw in the lute, nor in the vessels, for it was 
discharged for the space of several hours under a small pres- 
sure. Either, then, it was forced through the softened glass in 
the first, and the dilated pores of the tube in the second case ; 
or it was absorbed by the substance of the vessels ; or it was 
not extricated from the iron. Of these suppositions the third 
seems to me the most probable. It is not likely that an hole 
should be made through the melted glass, under the pressure 
of the half, and closed under that of perhaps the eighth of an 
inch ; or that pores in the tube should open and shut in con- 
formity to such a variation of circumstances : and, with re- 
gard to the tube, there can be no question as to absorption. 
One principal difficulty, as it appears to me, in the manufac- 
ture of iron, is to get rid of the charcoal. The oxygene 
readily enough unites with a small portion ; but the attraction 
of the iron on the one hand, and on the other, the little dis- 
position of the charcoal to put on the elastic form, in compa- 
rison with many other less fixed substances, together form a 
