An investigation of the physical properties of active nitrogen in low-density flow indicates that active nitrogen may be used for flow visualization without introducing excessive disturbances in the stream. Quantitative measurements of density in the flow may be obtained from certain restricted regions of the spectrum of the afterglow. The results are shown to disagree with Mitra's theory of the afterglow. The results appear to require that active nitrogen contain metastable molecules. Methods are described for the use of active nitrogen in shock tubes.