BRIDGMAN, — "WATER UNDER PRESSURE. 447 



the specific heat, which involves a second derivative of the p-v-t rela- 

 tion, can evidently be found much less accurately than the compressi- 

 bility, for example. Therefore, while some hold is obtained by the 

 data of this paper on all the thermodynamic quantities, some of these 

 will be found more accurate than others. 



Two methods were used in determining the compressibility. The 

 first is the same as that used in determining the compressibility of 

 mercury, and has already been fully described and critically discussed 

 in the previous paper. Briefly, it consists in enclosing the water to be 

 measured in a steel bottle, communicating at the upper end through a 

 very fine channel with a mercury reservoir. The bottle and the reser- 

 voir are completely immersed in a fluid to which pressure is applied. 

 The water contracts under pressure, mercury from the reservoir runs 

 in to take its place, falls to the bottom of the bottle, and is weighed 

 after pressure is removed. Compressibility was measured by this 

 method at two temperatures, 0° and 22°, and the same method evi- 

 dently might be used at any temperature above zero. Under pressure, 

 however, water may exist as the liquid at temperatures considerably 

 below zero, so that for a complete knowledge of the p-v-t surface of 

 water, the volume must be measured throughout this extended region. 

 The method just described is evidently not applicable here without a 

 troublesome variation of both temperature and pressure together dur- 

 ing a single measurement, and a second method was adopted. 



The second method, which was used to obtain the changes of volume 

 below zero, assumes as correct the compressibility at zero as found by 

 the first method. The data given by this second method are the 

 thermal dilatation from zero at constant pressure for a number of dif- 

 ferent pressures. Combined with the known volume at zero, this is 

 evidently sufficient to give the volume at any temperature and pressure. 



The apparatus used was the same as that already described in deter- 

 mining the data on the freezing curve of mercury, and consists of a 

 lower cylinder placed in a thermostat, and an upper cylinder in which 

 the pressure is produced by a moving piston. The water under ex- 

 periment is placed in a cylindrical steel shell in the lower cylinder. 

 The lower cylinder also contains the manganin resistance coil with 

 which pressure is measured. The measurements of pressure by this 

 method have been described in a previous paper. The remainder of 

 the lower cylinder and the entire upper cylinder is filled with gasolene, 

 by which pressure is transmitted to the water. 



The experimental procedure was as follows. The temperature of 

 the lower cylinder was maintained at some value below zero by the 

 thermostat, and the pressure varied over the region of existence of 



