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PHYSICS: BRINSMADE AND KEMBLE 
THE OCCURRENCE OF HARMONICS IN THE INFRA-RED 
ABSORPTION SPECTRA OF DIATOMIC GASES 
By James B. Brinsmade and Edwin C. Kemble 
JEFFERSON PHYSICAL LABORATORY. HARVARD UNIVERSITY 
Communicated by E. H. Hall. May 1.1917 
The study of the infra-red absorption bands of gases has within the 
last few years received a great impetus through Bjerrum's^ explanation 
of their structure as the result of the superposition of the molecular ro- 
tations on the vibrational motion of the charged atoms, which is the pri- 
mary cause of the absorption. Bjerrum's suggestion was quickly fol- 
lowed by the discovery of v. Bahr^ that these bands are, in reality, clusters 
of comparatively narrow lines, corresponding to the discrete angular 
velocities predicted for the molecular rotations by the older form of the 
quantum theory. In a recent number of the Physical Review^ Kemble 
has called attention to the fact that if the energy of the vibrational mo- 
tion also takes on only the quantum values the corresponding infra-red 
absorption bands in the spectra of diatomic gases ought to be accom- 
panied by faint harmonics. An examination of the literature showed 
only one case, viz., carbon monoxide, in which more than one band was 
known to exist and this was a faint one at very nearly half the wave 
length of the fundamental. The present article is a preliminary report 
on an experimental search for similar harmonics in the infra-red absorp- 
tion spectra of other diatomic gases and an examination of their struc- 
ture. First harmonics have been found in the spectra of hydrochloric 
acid and hydrobromic acid and their structure as well as that of the faint 
carbon monoxide band has been shown to be in general agreement with 
the theory. Search was also made for possible second and third har- 
monics in the spectra of carbon monoxide and hydrochloric acid, but 
without result. 
Apparatus and Method of Procedure. — The work was done on a prism 
spectrometer with a combination Wadsworth-Littrow mounting similar 
to that described by Gorton.^ This arrangement gives an unusually 
high resolving power by a double passage of the energy through the 
prism. The energy was measured with a thermopile. The prism was 
of quartz, the refractive indices given by Carvallo and Rubens^ being 
used in computing the wavelength scale. The absorption tube was 78 
cm. long and had a minimum inside diameter of 3.5 cm. The ends were 
closed with windows of fused quartz. 
Each galvanometer deflection with the absorption tube in place was 
'sandwiched' between two deflections with the tube drawn to one side. 
