('oniparing these with (30), (31), we see that the only dilference is that /. and 7 of those equations are uow replaced l>y
lleuce, instead of (37), we should have
and the saine general conclusions follow.
In the preceding ealculations we have supposed that the sulid is tree froni stress at a uniform Standard teinperature wlien i', w vanisli. In the case of unannealed glass, it would require a variable teinperature lo relieve the material froni stress. To meet t li is, 4 in the above equations would have to be reekoned froni the variable teinperature corresponding to tlie state of case, rather than froni a uniform standard teinperature.
Soine of the questions above considered are easily illustratcd experiinentallv. .V slab of glass about s cm. square and 1 cin. thick, polished upon opposite edges, wlien placed in the polariseope shows hut little revival of liglit so long as the teinperature is uniform. The contact of the hands with the two faces suftices to cause an alinost instantaneous illuniination, rising to a maxiinuni at the middle of tlie tliickness after a few seconds. Dark bands situated about halfwav between the middle and the faces are a eonspicuous feature. After about 30 or 10 seconds the light lades greatly, a result more rapidly attained if the hands be renioved after 10 or 2lt seconds' contact. In the earlier stages of the heating the outside layers are the warnier, and heilig prevented froni expanding f uil v are in a condition ot compresniun. I he inner Invers at the saine time are in tension, a conclusion that may be veritied by interposition of anothcr piece of glass, of whicli the niechanical condition is kuown, and of whicli the effect may be eitlicr an augmeutation or a diminution of the liglit.
An examination in the polariseope of the so-called tunghened glass, introduced a few vears ago, is interesting. lt nas understood to be prepared by a sudden cooling in oil wliile still plastic with heat. ^ hen it