Fluorescence produced by Ultra- Schumann Rays. 453 



to a difference in the constitution of the gas just leaving 

 the tube T, and that immediately around the stream, even 

 though the jet-chamber was entirely closed, and had been 

 washed out with a stream of nitrogen delivering a litre and 

 a half per minute for four minutes before beginning the 

 exposure. Figs. 1, 2, and 3 (PI. YI.) show the type of 

 spectrum obtained with the tube close up to the slit and the 

 jet-chamber washed out in advance. 



To increase the difference in constitution between the gas 

 in the stream and that surrounding it, an opening was made 

 in the jet-chamber to admit air. Purified nitrogen was 

 blown across the slit and an exposure made. The result is 

 shown in fig. 4. It is seen that the water band is again 

 interrupted where the nitrogen crosses it, and that the 

 nitrogen bands show only in the stream. 



An attempt was also made to explain the interruption of 

 the water band found when a stream of air was blown over 

 the slit S, and the jet-chamber was filled with air (/. c. p. 905, 

 fig. 13), as being due to a residual difference between the air 

 in the stream and that in the jet-chamber. When air from 

 the room, which of course also filled our apparatus, was blown 

 by means of a bellows through the tube T over the slit, no 

 interruption of the water band was shown. When the air was 

 moistened by passing through wet cotton still no interruption 

 was shown ; but when the air from the bellows was dried and 

 blown over the slit, and a source of moisture was provided in 

 the jet-chamber, so that the stagnant air around the air 

 current might take up moisture, the photograph of fig. 5 was 

 obtained. Moreover, when a strong current of dried air 

 (about 2*5 litres per minute) was blown into the apparatus 

 for five minutes before exposure was begun, and no source of 

 moisture was provided in the jet-chamber, the fluorescence 

 was so faint that it could not be photographed in fifteen 

 minutes, which is about the time of exposure of the other 

 photographs. It thus appears that water vapour is necessary 

 to obtain the fluorescence of the water band, while the presence 

 of oxygen mixed with nitrogen, either in large or small 

 quantities, will not give it. 



An exposure was also made in which nitrogen purified and 

 -subsequently moistened was blown across the slit S. This 

 plate shows the water band uninterrupted where the current 

 of nitrogen crosses the fluorescent jet; shows faintly two of the 

 three nitrogen bands 33G9 and 355G on the long wave-length 

 side of the water band, and a fourth band or line on the 

 •short wave-length side, probably the fainter water band 2811. 

 The fact that the water band is interrupted when the stream 



