HIGH-PEESSURE RESEAECH BRIDGMAN 165 



of the phenomena of polj^morphism under high pressures will be of 

 assistance in understanding this fundamental matter, if only once 

 somebody can find the loose end of the skein, and I even hope that 

 it may be helpful in finding the loose end. 



One particular transition produced by pressure is so interesting 

 that I shall mention it in detail. Under proper conditions of pressure 

 and temperature, ordinary yellow phosphorus is transformed into a 

 black variety much like graphite in its properties, and of a density 

 nearly 50 per cent greater than that of the parent yellow phosphorus. 

 The change is irreversible and permanent, and is the only example 

 of such a permanent change produced by high pressure that I have 

 found. The mechanism of the change is not at all understood, 

 although a number of attempts have been made at explanation. 

 The great diiference of density points is something unusual. I wish 

 to direct special attention to the conditions initiating the change 

 from yellow to black. For a time varying from 10 to 80 minutes 

 before the change occurs, some preliminary change takes place 

 throughout the entire interior of the mass of phosphorus, which is 

 accompanied by a slight loss of volume, and which proceeds at an 

 accelerated rate until the entire structure becomes unstable and col- 

 lapses into the new modification. This preliminary change can not 

 be produced in the usual ways by external agents. For instance, 

 wo can not inoculate a mass of yellow phosphorus with black and 

 thereby either hasten or delay the transformation. There is nothing 

 else that I know of like this behavior. Certainly the ordinary 

 formation of a new phase out of one that has become unstable by 

 the formation of nuclei and the growth of these nuclei at a definite 

 rate has nothing in common with this. 



The electrical resistance of metallic elements in general decreases 

 under pressure, the rate of decrease itself decreasing by a sort of 

 law of diminishing returns as the pressure rises. The magnitude 

 of the decrease under 12,000 kg. varies from 1 per cent or less for 

 metals like cobalt and tungsten to more than TO per cent for potas- 

 sium and rubidium. There is no simple connection with the change 

 of volume, as the change of resistance is of the order of tenfold 

 greater. The resistance of a fcAV metals increases under pressure, 

 and it is true of all of these that the rate of increase itself increases 

 with the pressure. I have now, in addition, two unique metals: 

 Cfesium, whos-e resistance at first decreases but later passes through 

 a minimum and then increases; and antimony, whose resistance, at 

 right angles to the trigonal axis, at first increases, but later passes 

 through a maximum and then decreases. The effects of pressure are 

 thus much more complicated than those of temperature (which are 

 nearly the same for all metals), but they may, I believe, be even 

 7G041— 2G 12 



