of Metals for reflecting flelef copes. zgg 
venience immediately took place. This therefore is the 
maximum in point of hardnefs. 
Thus much of the two firft confiderations, the hard- 
nefs and whitenefs of the metal; the next, and indeed 
the raoft effential, property is its compadlnefs, or its 
being without pores. 
This compofition (though complete in the former 
refpedls) was, as well as Dr. smith’s, fubjedt every now 
and then to be porous ; fometimes, indeed, I fticceeded in 
calling a lingle metal, or perhaps two or three, without 
this imperfedlion ; at other times, and moft frequently 
indeed, they were attended with this defedl, without my 
being at all able to form a probable conjedlure at the 
caufe of my fuccefs or difappointment. The pores were fo 
very fmall that they were not difcoverable when the metal 
had received a good face and figure upon the hones, nor 
till the laft and higheft polifh had been given ; and then 
it frequently appeared as if dulled over with millions of 
microfcopic pores, which were exceedingly prejudicial 
in two refpedls ; for firfc, they became in time a lodg- 
ment for a moillure which tarnilhed the furface; and 
fecondly, on polifhing the fpeculum, the putty necefia- 
rily rounded off the edges of the pores, fo as to fpoil a 
great part of the metal, by the lofs of as much light and 
Q q 2 fharpnefs 
