306 Obfervations on the Flints of Chalk-beds, 
perforated proceffes, or mamille, and the holes 
throagh them, muft have been really produced by 
the extrication of fome elaftic fluid. The few 
imperforated hollow nodules I have feen, are much 
more nearly globular than the others. In thefe,. 
what is now the compact femi-tranfparent coat, muft 
have yielded fo much during the effervefcence, as 
‘to afford {pace enough for the whole of the extricated 
elaftic fluid. When the effervefcence was rapid, or 
when the air was produced in large quantity, it 
burft its way out, producing an elongated mamillary 
procefs; and carrying along with it the effervefcing 
fubftance within, as far as the orifice or beyond it. 
In the fpecimens containing powder, the effervefcing 
matter muft have become concrete, while its parts 
were difunited by the iffuing air. Something of the 
fame kind frequently happens to bars of caft iron, 
ufed as a grate for reverberatory furnaces. I have 
feveral times feen fuch bars, after having lain for 
weeks or months in the furnace, converted fuperfici- 
ally into malleable iron, and within containing a grey 
powder. In two papers, printed in the Philofophical | 
Tranfactions, I have fhewn, that air is extricated 
during the converfion of caft into malleable iron, 
Now, in the bars which are found to contain pow- 
der, the application of heat occafions throughout 
the whole fubftance of the bar, an effort towards the 
extrication of air. But from fome curious circum- 
fiances, defcribed at length in the latter of the two 
papers above-mentioned, it appears, that the air 
iffues 
