278 Messrs. R. G. Parker and A. J. Dalladay on a 



At temperatures very much below the melting-point the glass- 

 is still liquid enough to deform gradually under stress — hence 

 the slow permanent bending of a glass rod supported at each 

 end, in a horizontal position, and at ordinary temperatures. 

 Glass behaves, in fact, as a brittle elastic solid under sudden 

 strain, but as a viscous liquid to severe strain applied over 

 long periods when at ordinary or moderately elevated tem- 

 peratures ; with rise of temperature the elastic nature of the 

 glass becomes more and more masked by the decrease in 

 viscosity ; and at a temperature approximating to the melting- 

 point it becomes no longer observable. 



Annealing Temperature. — If a piece of glass having two 

 plane, parallel, and polished sides is placed in an electric 

 furnace so that it can be heated and examined by transmitted 

 polarized light at the same time, the strain caused by unequal 

 heating can be observed both as regards position and intensity 

 by its effect on the light. This strain, which would be very 

 evident at lower temperatures in a piece of glass in the form 

 of a J-inch cube, or larger, suddenly disappears over quite 

 a short range of temperature situated very much below the 

 melting-point. The glass has obviously readied a tempe- 

 rature at which it is sufficiently mobile to accommodate itself 

 rapidly to the stresses caused by unequal heating — it is 

 therefore deformed, and the stress disappears ; this is clearly 

 the annealing-point, in that a piece of badly annealed glass 

 would lose all internal stress at this temperature, and if 

 sufficiently slowly cooled would be perfectly annealed. 



Relation of the Temperature employed to the Annealing 

 Temperature. — The viscosity of a glass at its annealing tem- 

 perature is still very great, but it has softened sufficiently for 

 a pointed tool, pressed against it with considerable force, to 

 make its way slowly in. Therefore, an optically worked 

 surface of a piece of glass would not retain its form accu- 

 rately after heating to the annealing temperature, if some 

 clamping apparatus had touched the surface. 



The mobility of glass has been found to be an exponential 

 function of the temperature 



S T =S Ti A N , 



where St, S-Tj are the " mobilities " at the temperatures T, T ly 

 measured by the rate of deformation of glass under constant 

 stress, and A andN are constants for the glass. The quantity 

 N,is. comparatively small, so that for quite small values of 



T — Ti, the index — ^— - would become equal to 1, 2, 3 ... 



If, for example, we take N = 8, A = 2, and compare the- 



