[ 5° 2 ] 
ekftricitas omnibus cylindris tributa in maceriam, 
quae eandem hand confervat, quantum opus eft, propa* 
gctiir 
Hxc quanquam nonnifi ad dele&andosoculosfpeftare 
videan'tur, attamcn vobis, Iiluftriffime Pr&fes, Viri II- 
lufiriftimi, qui in ejufmodi deledamentis ele&ricis 
vim naturae mirabilem latere per fplcitis, haud ingrata 
tore confido. Qua fpe fretus, veftro omnium favori, 
quem impenfe veneror, me fubmiffe commendo. 
^Scribebam Lipfit , 
die 5 I MartiL 
1 747 * 
XVII. Some Obfervations upon Gems or Pre- 
cious Stones 3 more particularly fuch as the 
Ancients ufed to engrave upon, by Robert 
Dingley, EJq- y 
Read May EMS or precious Stones, of all Species, 
I747 ‘ are f ome times’found of regular Shapes, 
and with a natural Polifh 5 and fometimes of irregu- 
lar Shapes, and with a rough Coat. The firft Sort 
may be confider'd as of the Pebble-Kind j and they 
are faid to be found near the Beds of Rivers, after 
great Rains: The others are found in Mines, and in 
the Clefts of Rocks. 
The Gems of the firft Sort were what the Ancients 
moft ufually engraved upon: Thefe are commonly 
called Intaglios ; and they are moftly of a long oval 
Figure, inclining to a Point at each End, convex as 
well on the engraved Face, as on the others, with a 
Ridge 
