Precise frequency values have been obtained for the NH, stretching vibrations of a number of primary aliphatic and aromatic amines in solution and in the vapour state.The greater intensity of the band due to the symmetric NH, vibration in aliphatic amines is explained in terms of hybridization changes a t the nitrogen atom during vibration.ExperiunentaZ.-Amines (from British Drug Houses Ltd.) and " AnalaR " carbon tetra-The measurements were made with a Grubb-Parsons G.S. 2 double-beam grating spectro-chloride used were dried and redistilled several times. meter, the band centres being measured within &2 cm.-l.