478 PROCEEDINGS OF THE AMERICAN ACADEMY. 



within which the viscosity is sufficient to keep the reaction from run- 

 ning, even if the equilibrium conditions are not satisfied. This has been 

 stated to be the fact by Tammann. The equilibrium points at the low 

 temperatures were found by approaching as close as practicable to 

 equilibrium from each side, and taking the mean. The tables of the 

 actual data show how broad the band of indifference is and how rapidly 

 it increases in breadth at the low temperatures. 



For these determinations with this new piece of apparatus, ice III 

 was usually obtained from ice I. The procedure was to freeze water to 

 ice I at a low pressure, and then increase the pressure on ice I at a 

 temperature in the neighborhood of —30°. It was always necessary to 

 pass considerably beyond the transition curve before the reaction 

 started. On several occasions at — 28° the pressure was carried so far 

 over the I-III curve as to arrive at the prolongation of the I-L curve, 

 at which melting took place immediately, it never being possible to 

 carry a solid into the domain of the liquid. I melts to the liquid even 

 if the liquid is relatively unstable with respect to another solid phase. 

 At the temperature —28° it was never possible to keep the unstable 

 liquid long enough to make an accurate determination of the equilib- 

 rium pressure. The rough values fell within the limits of uncertainty 

 on the smooth prolongation of the I-L curve. When the phase III 

 did appear at temperatures above —30°, the reaction from I to III ran 

 with explosive rapidity. Sometimes it was possible to hear a sharp 

 click in the apparatus announcing the sudden change of volume. 



Aside from the fact that it is possible to pass over the boundary line 

 between two solid phases without the reaction running, the actual 

 amount of possible trespassing has little significance. It is changed by 

 alterations in the size and shape of the containing vessels, is affected 

 by slight impurities, and most of all depends on the element of time. 

 It may be mentioned, however, that under pressure the viscosity of the 

 substances seems to be so great that mechanical shock has little if any 

 part in starting the reaction, as it does under ordinary circumstances. 

 It was never found possible to start a reaction from an unstable phase 

 by smart blows on the outside of the apparatus with a hammer. This 

 was tried both when the unstable phase was a solid and a liquid. 



After the phase III had once been obtained, the other points on the 

 equilibrium curve were obtained with decreasing temperature. At 

 every new temperature, the piston was pushed in a little, the pressure 

 then falling back to the equilibrium value, and then the piston was 

 withdrawn slightly, pressure rising to the equilibrium value. The 

 mean of these was taken as the equilibrium pressure. Sometimes at 

 the higher temperatures, where the reaction velocity is high, equilib- 



