44 A/r. king’s Account of a 
curious fails, which he permits me now to mention in 
this paper. 
On paffing through the ftreets of London in his 
walks, before the fign-irons were taken down, he per- 
ceived, that on the broad Hone pavements, whenever he 
came juft under any fign-irons, his cane gave a different 
found, and occafioned a different kind of refiftance to 
the hand, from what it did elfewhere; and attending 
more particularly to this circumftance, he found, that 
every where, under the drip of thofe irons, the ftones 
had acquired a greater degree of folidity, and a wonder- 
ful hardnefs, fo as to refill; any ordinary tool, and gave, 
when ftruck upon, a metallic found : and this faff, by 
repeated obfervations, he was at length molt thoroughly 
convinced of. 
Taking the hint, therefore, from hence, he thought 
fit to make feveral experiments ; and, amongft the reft, 
placed two pieces of Portland ftone in the fame afpedfc 
and fituation in every refpeil, but walhed the one fre- 
quently with water impregnated with rufty iron, and left 
the other untouched: and in a very few years he found 
the former had acquired a very fenfible degree of that 
hardnefs before defcribed, and on being ftruck gave the 
metallic found ; whilft the other remained in its original 
ftate, and fubjeit to the decays occafioned by the changes 
of 
