ON THE STRESSES IN SOLID BODIES DUE TO UNEQUAL HEATING^ &C, 41 



C'oinpaviiig thèse witli (;3(l)j (;31), we see tliat tlie onlj différence is that 

 7. and y of those équations are uow replaced by 



Heuce^ instead of (-"i?), \ve should hâve 



P-(^=.^^{^-|/>v/r}, 





and the sanie gênerai conclusions follow. 



In the preceding calculations \ve hâve supposed that the solid is free 

 IVom stress at a uniforni standard teraj)erature when a, r, w vanish. In 

 the case of nnannealed glass, it Avould reqnire a variable température 

 to relieve the inaterial froni stress. To nieet this^ Ù in the above équa- 

 tions wonld bave to be reckoned from the variable température corre- 

 sponding to the state of ease, rather than from a uniform standard 

 température. 



Some of the questions above considered are easily illustrated experi- 

 mentally. A slab of glass about 8 cm. square and 1 cm. thick^ polished 

 upon opposite edges, when jjlaced in the polariscope shows but little 

 revival of light so long as the' température is uniform. The contact of 

 the hands with the two faces sufîices to cause au almost instautaneous 

 illumination^ rising to a maximum at the middle of the thickuess after 

 a few seconds. Dark bands situated about halfway betweeu the middle 

 and the faces are a conspicuous feature. After about oO or -10 seconds 

 the light fades greatly^ a resuit more rapidly attained if the hands be 

 removed after 10 or 20 seconds' contact. In the earlier stages of the 

 heating the outside layers are the warmer^ and being prevented from 

 expandiug fully are in a condition of compression. The imier layers at 

 the same time are in tension, a conclusion that may be veritied by inter- 

 position of anotlier pièce of glass_, of which the mechanical condition is 

 known, aiul of which the etfect may be either an augmentation or a 

 diminution of the light. 



An examination in the polariscope of the so-callcd ionglieaed glass, 

 introduced a few years ago, is interesting. It was understood to be pre- 

 pared by a sudden cooling in oil while still plastic with beat. When it 



