BRIDGMAN. — WATER UNDER PRESSURE. 481 



rium was approached from only one side. By proceeding in this way 

 from high to low temperatures, it was possible to prolong the curve 

 considerably into the region of stability of 11. A point was thus found 

 at — 62°.0, showing a subcooling of 28°. If the pressure had been in- 

 creased on ice I at *his temperature, ice II and not ice III would have 

 separated out. 



The actual data are given in Table X. Those of February 16 were 

 obtained with the same apparatus as that used for the mercury ; all the 

 others with the smaller apparatus for lower temperatures. The plus 

 or minus signs after the pressures show whether the value was ap- 

 proached from above or below. Three different manganin coils were used 

 in making the measurements. The data of April 6 were obtained with 

 a new coil which had not yet been sufficiently seasoned. For this reason 

 the data of April 6 are slightly in error as to the absolute value of the 

 pressure, but are competent to show the shape of the equilibrium 

 curve. The data of April 20, taken with especial care to show the 

 actual shape of the equilibrium curve, give more significant figures 

 than one is entitled to for the absolute pressure, but the variation of 

 pressure is accurately indicated by the figures given. 



The equilibrium points are shown in Figure 9, in which are shown 

 also the other equilibrium curves in the vicinity. The shape of the 

 curve is given by the points of April 6 and 20, which agree, but the 

 best value for the absolute pressure is to be given by the mean of 

 the points at the upper end of the curve. On the same pressure and 

 temperature scale as the complete diagram for ice and water this 

 equilibrium curve appears nearly vertical, but on the scale shown here 

 it is seen to have unmistakable curvature, being concave toward the 

 temperature axis, and at one part parallel to that axis. At this point 

 of parallelism, the latent heat of transformation is zero ; above it is 

 positive, below negative. This point is approximately at —40° and 

 2150 kgm. 



The equilibrium curve I-III as found by Tammann shows the same 

 general shape, concavity toward the temperature axis. But Tammann's 

 vertical tangent is between —40° and —50°. The total change of 

 pressure from the triple point at —22° to the maximum is practically 

 the same, 50 kgm., but Tammann does not find so pronounced a cur- 

 vature below the maximum as is found here. On the whole, however, 

 the agreement in the shape of the curve as found in these two indepen- 

 dent determinations is as good as could be expected, particularly when 

 it is remembered that the discrepancies are at the low temperatures 

 where the reaction runs very slowly. But the actual value of the 

 pressure as found by Tammann at —22° is higher than that found 



VOL. XLVII. — 31 



