178 On Stresses in Solid Bodies due to unequal Heating, fyc. 



An examination in the polariscope of the so-called toughened 

 glass, introduced a few years ago, is interesting. It was 

 understood to be prepared by a sudden cooling in oil while 

 still plastic with heat. When it is examined through the 

 thickness of the sheet, a great want of uniformity is mani- 

 fested. In spite of the shortness of the distance traversed, 

 there is in places considerable revival of light with inter- 

 mediate irregularly disposed dark bands. The course of 

 these bands is altered when by fracture any part is relieved 

 from the constraining influence of neighbouring parts. To 

 make an examination by light transmitted edgewise it was 

 necessary to immerse the glass in a liquid of nearly equal 

 refractivity (benzole with a little bisulphide of carbon) con- 

 tained in a small tank. The width, traversed by the light, 

 was about 1 cm. In this way, and with the aid of a mag- 

 nifier, the condition of the various layers could be well made 

 out. The dark bands of no double refraction seemed to be 

 nearer to the faces than according to the calculation made 

 above, but the whole thickness is so small that this observation 

 is scarcely to be relied upon. The interior was in a state of 

 tension, and the double refraction was nearly sufficient at the 

 middle to give the yellow or brown of the first order. By the 

 action of hydrofluoric acid on the lower end of one of 

 the strips the outermost layers were dissolved away. This 

 caused a drawing together of the dark bands towards the 

 middle, and though a good deal remained the light was much 

 reduced. 



The cause of the toughening has been sought in a special 

 crystalline condition due to the sudden cooling. There may 

 be something of this nature ; but it would seem that most of 

 the peculiarities manifested may be explained by reference to 

 the known condition of stress. The fracture of glass is 

 usually due to bending, and the failure occurs at the surface 

 which is under tension. If, initially, the superficial layers 

 are under strong compression, a degree of bending may be 

 harmless which otherwise would cause fatal results. It seems 

 possible also that the superficial compression may be the 

 explanation of the special hardness observed. 



A short length of glass rod in its natural imperfectly 

 annealed condition may be used to illustrate symmetrical 

 stress. The ends may be ground, and either polished or 

 provided with cover-glasses cemented with Canada balsam. 

 In the specimen examined by me the colours varied from the 

 black of the first order on the axis to the red of the second 

 order near the surface. The length of the cylinder was 1*6 cm, 

 and the diameter 1'8 cm. 



