86 A SYSTEM OF PHYSICAL CHEMISTRY then proceeds to consider the spectrum between the limits io/a and 20/t, ascribing all the band heads therein to rotations, n being given consecutive values from lo to i6. The corresponding wave-length series (in ^t) is given by the expression— ^U A = 3 X ID- 3 X ID 14 n X 1 73 X lo12- The following table indicates the degree of concordance obtained by this means ;— n.
ABSORPTION SPECTRUM OF WATER VAPOUR 87 region. The validity of the expression can of course be tested by comparison with actual measurements in the far infra-red where the bands are due to rotation only. From what has been said above, it follows that the product ^1 • ^2 . ^T should be a constant, provided the theoretical considerations are sound. Making use of some data obtained by Paschen in the region 5 to in dft. the case of water vapour, E. von Bahr has found that the two heads of the double band (due to molecular rotations compounded with atomic vibration) alter their relative position in accordance with the above expression. /c.