BRIDGMAN. — THERMODYNAMIC PROPERTIES OF LIQUIDS. 93 



Heat of Compression. — The heat of compression, that is the 

 quantity of heat in kgm. in. which flows out of a substance as it is 

 compressed isothermally, is shown in Folder 4; for the twelve liquids 

 separately in F'igures 60 to 71, and the average for the twelve liquids 

 collected into a single diagram in Figure 72. The differences of the 

 curves for different temperatures are sufficiently great so that the total 

 heat of compression for each temperature could be plotted and the 

 difference found with sufficient accuracy directly from the curves, 

 without the necessity of drawing difference curves as was the case 

 for the work of compression. The zero of each curve has been dis- 

 placed upwards one square for the successive temperatures. In the 

 case of those liquids which boil at low temperatures, the zero of the 

 curves for all temperatures has been taken at the same pressure, 500 

 or 1000 kgm., although it would have been possible to extend the 

 curves to atmospheric pressure for the lower temperatures. In the 

 case of acetone and ethyl chloride the curves for the lower tempera- 

 tures have been extended backwards from the origin (1000 kgm.) to 

 atmospheric pressure. 



The heat of compression is positive, that is, as a substance is com- 

 pressed isothermally, heat flows out to the surroundings. Examina- 

 tion of the curves in detail shows also that the total heat always 

 increases with increasing pressure. This is a direct consequence of 

 the fact that the thermal expansion is always positive. The curves 

 therefore, show less pronounced irregularities than some, such as 

 those for compressibility for example. 



In general, the concavity is toward the pressure axis, that is, the 

 increase of the heat of compression becomes less rapid at the higher 

 pressures. The curves for the different liquids at different tempera- 

 tures show that the heat is not universally greater at the higher 

 temperatures, although such is generally the case. This is shown 

 by the curves drawing together at high pressures in some cases to 

 within less than the one square which separated them at the origin. 

 An example of this is afforded by amyl alcohol between 20° and 40°, 

 and by ethyl chloride and ethyl iodide between 60° and 80°. In 

 general, a drawing together of the curves with increasing pressure 

 means that the expansion is less at the higher temperature. There 

 are many instances of this, although it is not usual that the drawing 

 together is great enough to bring the curves to within less than the 

 original arbitrary distance of separation. 



Figure 72, in which are collected the average results for all twelve 

 liquids, shows again that the twelve liquids are alike in character. 



