94 MOLECULAR MOTION AND ITS ENERGY 40 



to distinguish in a gas, viz. the coefficient of volume- 

 increase and the coefficient of pressure-increment ; the 

 former determines the increase of volume that occurs with 

 rise of temperature when the pressure remains constant, and 

 the latter measures the increase of pressure that is produced 

 by heating without change of volume. By his experiments 

 Regnault proved the difference of these coefficients ; he 

 found, indeed, that with most gases the volume-coefficient 

 is somewhat larger than the pressure-coefficient, hydrogen 

 alone showing the reverse property. Regnault 1 further 

 observed that the two coefficients are not entirely indepen- 

 dent of the pressure and temperature of the gas, but that 

 they increase with the pressure and diminish when the 

 temperature rises. 



This behaviour of gases may be also indirectly recognised 

 from the observations that have been made on the specific 

 gravities of gases. These are usually referred to the density 

 of atmospheric air, at the same pressure and temperature, as 

 unity. It is therefore sufficient to determine the specific 

 gravity of a gas at different temperatures in order to learn 

 whether this gas has the same mean coefficient of expansion 

 as air, or a different one that varies with the temperature or 

 the pressure. 



An instructive example is given by the experiments made 

 by E. Ludwig 2 on the density of chlorine. For the densi- 

 ties of the gas, compared with air, he found the following 

 values : 



2-481 at 20 C. 



2-478 50 



2-468 100 



2-461 150 



2-450 200 



The falling-off in these numbers shows that chlorine expands 

 more than air. At 200 chlorine attains the same density 

 as it should have according to Gay-Lussac's law in 30 ; 

 presumably from this temperature upwards its specific 



1 M6m. de I'Acad. de Paris, xxi. p. 96 ; xxvi. p. 565. 



2 Ber. d. deutsch. chem. Qes. i. 1868, p. 232. 



