272 PROGRESS IN MICROSCOPY 



rise is slightly decreased. Shortly before melting the crystals lose their 

 shape, their outlines rounding off. The temperature at which the last 

 crystal melts is considered as the melting point. 



When dealing with an unknown substance, the melting point is 

 determined approximately in a preliminary test, followed by the fore- 

 going procedure. In most organic crystals, transition from the solid 



Fig. 11.5. Melting of azobenzene crystals. 



\4 



^ 





i 





Fig. 11.6. Melting of azobenzene crystals. 



to the liquid state is well defined and leaves no room for uncertainty. 

 Melting is to occur at a definite point. If melting extends over several 

 degrees, this means that the substance is not pure. The photographs 

 11.5, 11.6 and 11.7 show the melting of azobenzene crystals. 



