PRESSURE IN FORMATION OF ROCKS AND MINERALS 739 



will yield substances hitherto unobtained or even unknown, some 

 of which might prove to be of technical importance. An illustra- 

 tion of this is the new form of phosphorus recently discovered by 

 Bridgman;^ this form is grayish black like graphite, has a high 

 density (2.69, as against 2.34 for the red variety and 1.9 for the 

 yellow), is ignited with difficulty by a match, and cannot be 

 exploded by a blow from a hammer, as red phosphorus can. Never- 



EDOr 



KG. SQCM. 



Fig. I. — Equilibrium diagram for water — ^liquid and five solid forms 



theless it is probable that uniform pressure has been essential to 

 the formation of but few ordinary minerals, except in so far as it 

 was effective in securing an adequate concentration of some volatile 

 component (or components) in the magma from which the minerals 

 separated. 



So far we have taken up the effects of pressure upon a single 

 pure substance; when we deal with systems of more than one 



^P. W. Bridgman, Physic. Rev. Ill (1914), iS?- 



