908 Fluorescence of Gases excited by Ultra-Schumann Waves. 



through fluorite lenses and prisms. The radiations which 

 excite the nitrogen bands appear to be reduced to about 20 

 per cent, of their original intensity after passing through a 

 quartz plate 1 mm. in thickness. Very few experiments 

 have been made with the plates, and the apparatus was not 

 very well adapted to the work, as it is necessary to keep the 

 plates in motion or clean them every ten or fifteen seconds. 

 With a properly designed apparatus it is probable that much 

 more reliable data can be secured. It will also be possible 

 to work with gases at a low pressure, in which case the 

 fluorescence is likely to be very much brighter, judging 

 from the behaviour of iodine, which ceases to fluoresce under 

 the stimulation of visible light when at a pressure of a few 

 millimetres only, its greatest luminosity appearing when it 

 is at a pressure of about 0'2 mm. 



A number of photographs of the jet were made with a 

 quartz spectograph of medium size, an exposure of several 

 hours being necessary. Fig. 15, PI. XY. shows the spectrum 

 of the jet in nitrogen with a current of nitrogen delivered 

 across the slit. The reduction of the intensity of the water- 

 band, where it is crossed by the nitrogen stream, is very 

 marked. The line to the left of the water-band is the head 



of the nitrogen band at wave-length 3159. It falls within 



. . . 



the less refrangible and fainter portion of the water-band ; 



(see fig. 17 for complete water-band) then come the two 



strong copper lines 3292 and 3217 (in the continuous 



spectrum of the light reflected from the slit jaws), then the 



nitrogen band 3369 (A), and the bands 3527-3576 (B) — 



3755-3802 (0)— 3914 (D). 



Fig. 17 was made with a hydrogen current crossing the 

 jet. A faint continuous band appears above the strong 

 copper lines (which show also probably as the result of the 

 formation of a slight trace of fog or cloud, which sometimes 

 happens if a very slow gas-current is u?ed). The water-band 

 is very well shown in this photograph, and can be detected 

 above the region where it is interrupted by the hydrogen 

 stream. In making this experiment the side of the box was 

 opened and the hydrogen allowed to escape without filling 

 the jet- chamber. 



Fig. 19 shows the spectrum obtained with a current of 

 C0 2 blowing across the jet. It appears to be identical with 

 the band found with hydrogen, so far as its position is con- 

 cerned, but on plates made three years ago the distribution 

 of intensity in the two bands was quite different. 



Laboratory of Physical Research, 

 Sorborme, 



Paris. 



