211 



3. Dcrifri/ifafioji: When one raises the temperature of 

 a vitreous substance, it crystallizes. In the ^lass industry, 

 this process is called devitrification or recrystallization. 



Among the various methods of ascertaining the passage 

 from the vitreous to the crystalline state, the simplest is 

 that of observing the decrease in transparency. When a 

 large number of small crystals are formed in a body in the 

 vitreous state, light is scattered in all directions, the trans- 

 parency is attenuated and, in some cases, the body becomes 

 completely opaque. 



This method, however, is not always infallible. In de- 

 vitrifying concentrated solutions of magnesium chloride, 

 we observed that the transparency remained altogether 

 unaltered, although the material reestablished light be- 

 tween crossed nicols. The method of analysis by polarized 

 light is, therefore, necessary in certain cases. 



The best method of diagnosing the commencement of 

 devitrification would be the analysis by X-rays. This 

 might allow one to perceive crystallites of very small 

 dimensions in most of the substances which we consider 

 glasses. 



The possible existence, in a body in the vitreous state, 

 of these crystallites or of what we previously called nuclei 

 of crystallization is suggested by the fact that, during de- 

 vitrification, a glass passes from the condition of trans- 

 parency to that of opacity by a continuous and uniform 

 darkening. One cannot distinguish separate centers of 

 crystallization in this case as one can when one induces 

 crystallization from the liquid state. If the opacity of a 

 devitrified body is due to the presence of crystals, the 

 gradual darkening ought to be explained by an increase in 

 the dimensions of the crystals already formed or by the 

 formation of new crystals. So, when a preparation begins 

 to lose its transparency, it does so either because nuclei of 

 crystallization are forming, or because the already pre- 

 formed nuclei, at first too small to cause the obscuration of 

 the vitreous substance when it is observed with ordinary 

 light, are growing in size. 



