Figure 3- 10 shows the variation with temperature of the molal specific heat capacities cp and c. for copper, at a constant pressure of I atm. At low temperatures the two arc nearly equal, and near absolute zero both drop rapidly to zero. (Compare with the graph of expansivity in Fig. 2- 16.) This behavior is characteristic of most solids, although the temperature at which the sharp drop occurs varies widely from one substance to another. At high temperatures, cp continues to increase while c. becomes nearly constant and equal to about 25 x 10• J kilomole-• K- 1• It is found that this same value of c. is approached by many solids at high temperatures and it is called the Dulong* and Petitt value, after the men who first discovered this fact. Although there seems to be little connection between the heat capacity of solids and the properties of gases at low pressure, it will be recalled that the gas constant R equals 8.31 x 10' J kilomoJe-• K- 1, and 25 x 10' J kilomolc-1 K-1 is almost exactly three times this; that is, the specific heat capacity at constant volume is nearly equal to 3R at high temperatures. We shall show in Section 9-8 that on theoretical grounds a value of 3R is to be expected for c. for solids at high temperatures.