19 



The liquid also is somewhat viscous and this prevents 

 the development of any considerable eddy currents, which, 

 in the stirring of ordinary liquids, have such an important 

 effect in producing homogeneity. The viscosity of the liquid 

 has a still more harmful influence in delaying the process of 

 diffusion, which eventually brings about homogeneity in all 

 liquids by the exchange of material between two adjacent 

 layers of slightly different composition. Above all, the very 

 high temperature at which the operation has to be carried 

 out severely restricts the mechanical devices which can be 

 adopted. 



In view of the above considerations, it is almost a matter 

 for wonder that any homogeneous glass is obtained from the 

 pot especially when one remembers the possibility of attack 

 on the pot continuing after the stirring is complete, and of 

 the products of attack being carried by convection currents 

 to other parts of the mass. 



When the stirring process is finished, the pot is removed 

 from the glass-making furnace, and is then placed in another 

 furnace, in which it is allowed to cool. 



When cold, the crucible and the glass are usually found 

 to be considerably fractured. Occasionally almost the 

 entire mass of glass will be in one single lump, and in such 

 a case is under a condition of great stress, and when broken 

 with a hammer flies into inconveniently small pieces. 



The blocks of glass are broken up by workpeople into 

 lumps of the particular size desired, and are roughly sorted 

 for obvious defects. The pieces are moulded into the 

 required shape by being slowly heated to the softening point 

 and then either pressed into shape or allowed to run by 

 gravity into a suitable clay mould. 



The blocks so obtained must then be cooled very slowly, 

 in order to avoid a condition of serious internal stress being 

 set up in the glass. When such stress is present in a glass 

 block, it causes the glass to develop optical properties similar 

 to those of many crystalline substances. Light, on passing 

 through the glass, travels with slightly different velocity 

 according to its plane of vibration or, in other words, the 

 glass may be said to differ very slightly in refractive index 

 towards light- waves having different planes of vibration. 

 This results in the familiar phenomenon of double refraction 

 when the strained glass is examined in polarised light, and, 

 when present in unduly large amount, renders the material 

 unfit for optical instruments. 



The development of this stress is due to the contraction 



