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The earliest observations on the infra-red absorption spectrum of nitrogen peroxide were made by Warburg and Leithauser, who found that at ordinary temperatures the mixture of the tetroxide and dioxide had strong absorption maxima at 3•4 μ, 5•7μ, and 6•1μ. By varying the temperature, and so the degree of dissociation of tetroxide molecules into dioxide molecules, they were able to attribute the absorption band at 5•7μ, to the nitrogen tetroxide molecule, and show that the other two bands were due to the molecule of nitrogen dioxide. A later investigation by Eva von Bahr confirmed these observations and disclosed another band due to the dioxide at 7•3μ. None of these investigators examined the region beyond 8 μ, and all the work was done using low dispersion. Recently Strong and Woo have examined the spectrum between 23 μ, and 150 μ, and find two main regions of absorption, one near 26 μ and 150 μ and the other around 80 μ.Experimental. The spectrometer used in the first survey of the spectrum was a prism instru­ment having a 60° rock salt prism in the Wadsworth mounting. Subsequently several of the bands were examined on grating instruments of high resolving power. The absorption cell used in the preliminary work was of brass, 20 cm. in length, and with rock salt windows. This cell was lagged with asbestos and could be heated electrically until the temperature of the gas which it contained was as high as 160° C. The absorption cell used in the examination of the individual bands was of glass, 10 cm. long, and with mica windows. For work in the region beyond 7 μ. the mica windows were replaced by ones of rock salt. The nitrogen peroxide was taken from a very pure sample specially prepared for research work. It was kept in a sealed pyrex tube as a liquid (under its own pressure) and the gas was transferred to the cells as required, drying tubes containing phosphorus pentoxide being used to avoid con­tamination from water vapour in the atmosphere.

The infra-red absorption spectrum of nitrogen tetroxide and the structure of the molecule