122 On the Manufactui'e of Glass. 



cutting room, where it gradually cools to the temperature of 

 the atmosphere. 



Perhaps there 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 which 

 the diamond operates, in dividing plates of glass. When a 

 diamond is drawn across a sheet of glass, so as to produce a 

 good cut, the line which it makes is scarcely perceptible, yet 

 the fracture extends through the plate. The cutter judges 

 of the perfection of the cut, rather 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 the surface, separating mi- 

 nute fragments of the glass which are conchoidal. In selecting 

 diamonds,! prefer those that are perfect, 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, 

 and this difierence is so slight, that to the eye it appears 

 perfect. 



Cylinder 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 Ipss, upon all specimens of 

 this kind of glass, belong to the class of inevitable evils ; 

 most of these however, can be greatly mitigated, by peculiar 

 management and care. 



The inferior lustre, is occasioned, by the necessity of heat- 

 ing the glass again, in the process of flattening. 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- 



