204 



crystalliiu' down lo llic absolute zero. \\\\\ if, 1)\- alti'ii])! 

 cooliiiu', one can brinj;- a iKiiiid 1 lii'on,i;ii llic /one (' before 

 it lias the time to crystallize, it takes the vitreous state and 

 stays vitreous at h)\ver temperatures. If a l)ody iu tlic 

 viti-eous state at a low temperature is warmed up slowly, 

 it devitrifies, that is, it becomes crystalliue when it reaches 

 the devitrification temperatures 1), it becomes liquid at the 

 melting ])oiut M and it is transformed into a gas at the 

 boiling point H. 



V C L G 



A.Z. DM B 



Fig. 29. Diiifji.-nii rc'incsi'iithig, on a teiiiperaturo scale of which the ori- 

 gin is the absolute zero, A. Z., tlie four states of matter: gas G, liquid L, 

 ervstalliue C and vitreous V, and the three changes of state: boiling B, melt- 

 ing M and devitrification D. The upper arrow indicates tlie change of tem- 

 perature for i)assing from the liquid to the vitreous state, and the lower arrow 

 the change of temi)eiature involved in subcooling. 



A glass is amorphous, that is, its molecules are dis- 

 tributed at random, while, in a crystal, there is an ordered 

 arrangement of the atoms. A glass is isotropic, its physi- 

 cal properties being the same in all directions, w^hile most 

 of the crystals are anisotropic, their physical properties 

 having higher coefficients in some directions than in others. 

 In polarized light, between crossed nicols, a glass is opaqne, 

 while most crystals are brightly illuminated. A glass dif- 

 fers from a li(iuid in that it is hard and breakable, while 

 a liquid is fluid and deformable. 



In the passage from one state to another, an intermedi- 

 ate state can, as we said above, be skipped over. A crystal 

 can become a gas w^ithout taking the intermediate liquid 

 state, a process wiiich is known as sublimation. Similarly, 

 when the crystalline state is skipped ovei- by rapid cooling, 

 the body passes from the liquid to the vitreous condition, 

 it is said to vitrify. If the crystalline state is avoided by 

 rapid warming, the vitreous body becomes liquid, a pi'ocess 

 which we call vitromelting or vitrofusion. 



The two transition processes, melting and boiling, are 



