205 



not ciitirely eomparable to devitrificatioii. Melting and 

 boiling- are reversible while devitrification is not. A gas 

 becomes a liqnid on cooling and a liqnid becomes a gas on 

 warming; a liquid crystallizes when one lowers its tem- 

 perature and a crystal melts when its temperature is 

 raised ; while a glass crystallizes on warming but a crystal 

 does not vitrify on cooling. 



Furthermore, melting and boiling take place at tempera- 

 tures which are practically points, while devitrification 

 occurs over a larger range of temperatures. To illustrate 

 this diiference, we indicated in the diagram (Fig. 29) the 

 boiling and melting points by the lines of separation B and 

 M, and the range of devitrification temperatures by the 

 zone D. 



According to these principles, the behavior of silicates is 

 not exceptional. Their vitreous zone (V in Fig. 29) ex- 

 tends from the absolute zero to about 1000° C. Since the 

 atmospheric temperature, which prevails about us, is 

 within this zone, we are familiar with the vitreous state of 

 silicates. A silicate glass heated to the devitrification 

 temperatures (below the melting point) becomes opaque 

 by crystallization. At a higher temperature, the crystals 

 melt. 



From observations made on some silicates which devit- 

 rify very slowly, becoming opaque in the course of several 

 years, it has been concluded that the vitreous state is un- 

 stable and that if enough time, i.e., centuries or thousands 

 of years, were allowed, devitrification would always take 

 place. The fact that natural flint has an opalescent ap- 

 pearance, due probably to the presence of minute crystals, 

 is sometimes considered an indication of the instability of 

 the vitreous state, it being assumed that crystallization is 

 still being completed. But there is no evidence that the 

 opacit}^ of flint has developed during the millions of years 

 which have followed its vitrification and that the crystals 

 noM^ observed were not formed at the same time as flint 

 itself. 



It has been mentioned above that glasses are regarded 



