184 
value of k is important because of its entrance into several of the funda- 
mental equations of therniodynamics, and also because it furnishes an ex: 
cellent criterion for the correctness of the assumptions made in the kinetic 
gas theory, concerning the distribution of the energy within the molecule. 
In view of these intimate correlations of the value of k with other funda- 
mental factors it is important to study its variation under different con- 
ditions of pressure and temperature for the same gas. 
For constant pressure Wiillner* found practically a constant value of 
k between O° and 100° C., for air. Witkowski* has found evidence of a 
variation in k with both temperature and pressure, from theoretical con- 
siderations. Luduc* shows that k should decrease with rising temperature 
and with falling pressure. Stevens’ finds a value of 1.34 for k at 1000°, and 
Kalihne® shows that & decreases with rising temperature, reaching a value 
1.89 for 900°. S. R. Cook,’, working with liquid air temperatures, finds 
the value of & for air to be 1.35 (nearly), and Valentiner® in an exhaustive 
study of the dependence of /: in nitrogen upon pressure, at liquid air tem- 
peratures, finds the value of k to increase, approximately in proportion to 
the ratio of the pressure to the saturation pressure for the temperature 
used. 
In this connection it was suggested to the writer by Professor Réntgen, 
that a study of the value of k should be undertaken for constant pressure 
and liquid air temperature, and under his direction the present work was 
carried out during the winter and spring semesters of 1901, in the Physi- 
cal Institute at Munich. Two series of observations were carried through: 
I. For constant pressure, the ratio of values of k for the tem- 
peratures of melting ice and boiling water was determined, the gas 
used being air, free from moisture and CO,. Values under these con- 
ditions had been determined by Wiillner (loe. cit.), and were here 
repeated as a means of checking the method. 
II. For constant pressure, the same ratio was determined over 
a range of temperature from that of the room, about 20° C., to that 
7 Ann. der Physik 4, 1878. 
3Sci. Abs. 3, 1900, p. 387. 
4Sci. Abs. 3, 1900, p. 29. 
5Scl. Abs. 4, 1901, p. 847. 
® Ann. der Physik 11, 1903, p. 225. 
™Phys. Rey. 23, 1906, p. 232. 
8 Ann. der Physik 15, 1904. 
