130 ILLINOIS STATE ACADEMY OP SCIENCE 



radiometer as the receiving instrument. The grating 

 was one of 2500 lines per inch and had a ruled surface 

 2x2 inches. A cell 10 cm. long with mica windows was 

 so arranged before the slit of the spectrometer that it 

 could be moved in or out of the beam of light brought to 

 focus on the slit. Such an arrangement made it possible 

 to determine the per cent absorption. The bands as ob- 

 served for ammonia are not equally spaced, the wave- 

 length interval between them increasing from about 140 

 A. U. on the short wave side of the band to 200 A. U. on 

 the long wave side. In addition to the system of narrow 

 bands a deep band was found at 299,000 A. U. which cor- 

 responds to the transition of n-= m = Oton = m = l as 

 indicated above. This appears to be the first substance 

 found to show the zero branch of the double absorption 

 band. Eight of the narrow bands were found on each 

 side the zero branch. Inasmuch as the grating used in 

 the investigation was a 2500 line per inch grating it did 

 not have sufficient resolving power to make accurate 

 quantitative determinations possible. It is planned to 

 examine these bands under larger resolving power. 



The frequency difference between the narrow bands is 

 given from the expression of Lenz as 



h 



df = 



4ttI 

 From this expression it is possible to get an estimate of 

 the moment of inertia of the ammonia molecule. Substi- 

 tuting wavelengths in the expression and solving for I we 

 have 



1U 2 



1 = 



4ttccU 

 where c is the velocity of light, X the wavelength of a 

 band and dX the difference of wavelengths between two 

 bands. Making the substitutions, we find 2.8 x 10" 40 

 gm. cm 2 as the moment of inertia of the ammonia mole- 

 cule. 



Northwestern University 

 May 4, 1924. 



