516 PROCEEDINGS OF THE AMERICAN ACADEMY. 



is sufficiently sharp to give trustworthy measurements of the change of 

 volume. The presence of impurity in the water will be shown by a 

 rounding off of the corners of the melting curve and by a change in the 

 value of the equilibrium pressure with the relative proportions of liquid 

 and solid present. Figure 31, chosen at random from the earlier meas- 

 urements, shows the sharpness of freezing and the consequent freedom 

 from impurity. In the majority of subsequent measurements, the ques- 

 tion of absence of impurity having been settled, it was customary to 

 find only one value for the equilibrium pressure, corresponding to a 

 mixture varying from one third to two thirds liquid. 



The equiHbrium points obtained by the change of volume method 

 were found to be consistently lower than the supposed transition points 

 found by the method of change of resistance. There can be no ques- 

 tion whatever but that the change of volume points are the correct ones. 

 The cause of the discrepancy probably is the formation of the eutectic 

 mixture of ice and salt. 



Measurements made with the electrolytic conductivity method after 

 the completion of the rest of the experiments, with the new apparatus 

 giving also the piston displacements, showed that the flat minimum in 

 the conductivity curve previously referred to occurs at nearly the pres- 

 sure of the beginning of freezing. Evidently at first pure ice separates 

 out, leaving the remaining liquid richer in whatever salt produces the 

 conductivity. The total quantity of this salt remains nearly constant, 

 but its concentration and so its association are increasing, so that on 

 the whole we have a decrease in the conducting power. When the 

 pressure has been carried so far that the concentration reaches the 

 eutectic value, the solid salt separates out with the pure ice and we 

 have the sudden jump in the resistance. That this was the case was 

 also suggested by a fact observed once or twice when using water in 

 glass with a mercury seal. This was that the sudden discontinuity 

 might be either a short circuit or an open circuit. Evidently the short 

 circuit is due to the penetration of the mercury through the mass of 

 ice along the core left by the freezing of the pure ice to the walls of the 

 tube. 



The points on the VI-L curve were obtained with two different pieces 

 of apparatus. The points between —-20° and 20° were found with the 

 apparatus already described as used for the middle temperature range. 

 For the higher pressures, up to 20,500 kgm., another form was used. 

 The particular point of weakness in the middle temperature apparatus 

 is the connecting tube. The new form for the highest pressures was 

 made in one piece, therefore. This has been already described as the 

 third piece of apparatus. It is to be noticed that with this there is no 



