450 History of Luminescence 



attention during the nineteenth century (see Chapter VII) , it was 

 the studies of E. P. Lewis (1899, 1900) on nitrogen and the new 

 views on emission of light from " excited " atoms and molecules 

 that again led to the twentieth-century discussion and controversy 

 over the nature of " active nitrogen," so called because of chemical 

 activity. The principle study of the conditions favoring formation 

 of active nitrogen was carried out by R. J. Strutt, later Lord Ray- 

 leigh, beginning in 1910. 



Another gas, hydrogen, exhibits an after-glow of much shorter 

 duration (0.2 second) under conditions similar to those of nitrogen, 

 as shown by R. W. Wood in 1920. Both active hydrogen and active 

 nitrogen are of particular interest, not only because of their self- 

 luminosity but because they excite luminescence of material they 

 come in contact with— gases or metals like sodium or mercury, as 

 well as certain phosphors and fluorescent substances. All these bodies 

 then glow with their characteristic spectra. It has been established 

 that this effect is not due to any ultraviolet light emitted from the 

 active gas, but by energy transfer to the luminescent body. 



Another important gaseous luminescence is that connected with 

 ozone. ^* Although a peculiar smell had been noted in the vicinity 

 of electrical discharges in the eighteenth century, it was C. F. Schon- 

 bein in (1840) who found that " I'odeur electrique " also accom- 

 panies the electrolysis of water and appeared to be a distinct gas, 

 which he called ozone, from the Greek ozo, to smell. Later, Schon- 

 bein (1845) noted that slow oxidations like that of phosphorus also 

 gave rise to the smell and he rather specifically connected it with 

 the luminescence, as indicated previously. 



As a strong oxidizing agent, ozone not only causes chemilumi- 

 nescence of organic compounds, but there is also evidence that 

 gaseous ozone itself may emit light. As early as 1881, it was demon- 

 strated by A. Schuller that ozone heated to 200-500° C, would 

 decompose with luminescence, and a number of later workers have 

 confirmed the observation.^^ Again the light is less marked with 

 pure ozone, and is much influenced by various gases, some increas- 

 ing and some decreasing the light. 



In 1888 J. Dewar allowed dried air to pass through an electric 

 discharge and then enter an evacuated space by a capillary orifice, 



^* See the excellent summary of ozone properties by F. Fonrobert, Das Ozone, Stutt- 

 gart, 1916). 



^^Trautz and Seidel {Ann. der Physik 67:527-572, 1922) find no decomposition of 

 ozone at high temperature if it is pure and free of organic vapors. That gases do not 

 emit light merely as a result of heating was demonstrated by Thomas Wedgwood 

 {Phil. Trans. 82:272, 1792) who observed no light when air was passed through a 

 red hot tube, although a strip of gold in the tube became red hot from the heated air. 



