HEAT OF COMPRESSION' 4 1 



the actual coefficients corresponding to the mercury and glass together 

 must be used in the calculations. 



The heat of compression of a cubic centimeter of mercury over the 

 pressure range of 500 atmospheres is thus about 13.5 x 1.2 X 0.033 

 X 4.2 = 2.3 joules. This value is probably too low rather than too 

 high. 



In the case of water the pressure-difference between the adiabatic 

 and isothermal readings was much smaller, being only 9.5 units of 

 pressure for a pressure range of 416 units. Since in the experiment 

 with this same jacket containing water, 97.5 units of pressure corre- 

 sponded with 1.034 grams of added mercury, the difference of 9.5 

 units must have signified an addition of 0.10 gram or 0.0075 milli- 

 liter of mercury. The volume of water was 18.75 milliliters, hence 

 the percentage expansion was 0.040. But a rise of temperature of i 

 would cause a percentage expansion of about 0.02, hence the rise of 

 temperature must have been about 2 . This signifies the evolution of 

 nearly 9 joules of heat under compression to 416 units of pressure, or 

 about 11 joules under compression to 500 units of pressure. The 

 amount is large, being nearly five times as great as the corresponding- 

 heat of compression of mercury. In this case, as in the preceding one, 

 the cooling effect probably introduces a large error ; the results are 

 given as preliminary examples of an application of the apparatus, 

 rather than as a precise evaluation of the effect. With greater precau- 

 tions a more exact result might be obtained ; and we hope to test the 

 method further. 



A High Pressure Manometer and the Unit ok Pressure. 



The properties of a few pure substances serve as the most convenient 

 and generally useful means of defining by comparison the properties 

 of all substances and the various dimensions of energy. Thus specific 

 gravities and specific heats usually serve as the means of determining 

 densities and heat capacities ; the temperature scale is defined by the 

 triple or quadruple or other fixed points of a few elements or simple 

 compounds and subdivided by the tension-increase of hydrogen in con- 

 stant volume ; electromotive force is found by comparison with a 

 Clark or Weston cell ; electrical quantity is determined by the weight 

 of a pure metal which it can deionize, and so forth. It seems to us 

 desirable to define the measurement of high pressures also in an 

 equally convenient way by reference to the compressibility of one or 

 more easily obtained pure liquid substances. The problem is a diffi- 

 cult one, because the apparatus used for containing the material may 



