548 



Mr. Richard Threlfall 



[March 19, 



parently cau exist together indefinitely at ordinary temperatures and 

 pressures are an illustration of the same property. 



As a final illustration we may note the results for water down to 

 - 80° C, from which it appears that it possesses three allotropic 

 crystalline forms with at least litwo melting-points. 



Rhombic 



Liquid 



Vapour 



Temperature 



Fig. 3 



The melting-curves of from thirty to forty substances have been 

 investigated, mainly by Tammann, up to about 8000 kg/sq.cm. 

 = 19*05 tons/sq. inch, and the general result has been to show that 

 there is a tendency for the rate of change of melting temperature with 

 pressure to fall off as the temperature rises, and also that many sub- 

 stances, which at ordinary pressures crystallise in one form only, can 

 be caused to assume allotropic modifications under high pressure. 

 This tendency to form allotropic modifications appears to be associated 

 with the extent to which a substance can be under-cooled without 

 crystallising. 



