122 On the Manufacture of Glass. 
cutting room, where it gradually cools to the temperature of 
the atmosphere 
Perhaps sherds is nothing connected with this manufacture, ° 
that causes so much pleasure and surprise to the spectator, 
as the facility with which an experienced glass cutter per- 
forms his work; but in reality, no mere manual art requires 
more time and patience to acquire the requisite skill than this. 
There have been several opinions as to the manner in whi 
the diamond operates, in dividing plates of glass. When » 
diamond is drawn across a sheet of glass, so as to produce 
good cut, the line which it makes is scarcely perceptible, oo 
the fracture extends through the plate. The cutter judges . 
of the perfection of the cut, yather by the ear than the eye. 
A peculiar creaking sound is produced when it is perfectly 
done. If a rough white line is made, accompanied with a 
tearing sound, you may be sure that the glass is not cut. In 
this case it would seem, that the fracture, instead of descend- 
ing vertically from the point of the diamond, extends later- 
ally from it and returns again to to the surface, y repre. mi- 
ng 
dia de prefer those that are fame with triangular rhom- 
boidal faces, the edges not strait but slightly convex, either 
octahedrons or dodecahedrons, The peculiar delicacy re- 
quired in the cutting edge of the diamond, is such, that by 
constant use, (although so very hard,) it is soon destroyed, 
ane tee difference is so slight, that to the eye it appears 
es oe glass as usually met with, is far inferior to crown 
glass. Some of its imperfections are necessarily connected 
with the manner in which it is made, and cannot be entire- 
ly obviated. Others there are remediable, with due care 
and skill. 
The inferior lustre or polish, the irregular reflection of light 
from its surface, and the slight abrasions and scratches, 
which are perceptible, more or less, upon all specimens of 
this kind of glass, belong to the class of inevitable evils; 
most of these SeReteey can be greatly mitigated, by pecsliar 
management and c 
The inferior i is occasioned, by the necessity of heat- 
ing the glass again, in the process at f Sierine Should the 
temperature be raised no higher than is absolutely necessary 
to render the glass flexible, the diminution of the lustre 
would be so slight, as to be scarcely perceptible : but in con- 
