164 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



but the experimental difficulties were so great that very little was 

 accomplished. A small box was made of brass and square plates of 

 quartz. The inside was varnished and blackened with soot, a drop 

 of mercury introduced, and the box exhausted. The camera with its 

 quartz objective was now trained on the box, and a beam of light 

 from a mercury lamp (quartz) focused at the center of the box. 

 Though the eye could see no trace of the cone of rays, the photograph 

 brought it out as distinctly as if the box was full of smoke. An ex- 

 posure of only 1 second was necessary, and with a 10-second expo- 

 sure the spectrum of the light scattered by the vapor was secured. 

 It was found to consist of a single line only (the 2536 line), though the 

 light entering the box was the total radiation of the mercury arc, the 

 spectrum of which contained hundreds of lines. The pressure of the 

 mercury vapor was about 0.001 millimeter, in other words, 7-5-^77 

 of the pressure of the air in the room. It seems most extraordinary 

 that a vapor at such a very low pressure and at the temperature of 

 the room should glow so brilliantly with invisible light. A little fur- 

 ther experimenting resulted in a further discovery. It was found 

 that if the box was filled with air at atmospheric pressure, the cone 

 of rays glowed feebly in the mercury vapor with which tlie air was 

 saturated. As the pressure was reduced the glow increased in brilliancy, 

 reaching its maximum at a pressure of about 5 millimeters. As the 

 exhaustion was pushed further the mercury vapor outside of the 

 cone became luminous, and at the highest vacuum attainable the glow 

 filled the entire box. This is secondary resonance radiation excited 

 by the primary radiation of the mercury vapor, which is excited by 

 the cone of focused rays. The brilliancy of the cone remained about 

 the same, so that we can not attribute the bursting out of this sec- 

 ondary fluorescence to a mere increase in the brilliancy of the directly 

 excited vapor. 



Experiments are now in progress to determine why the presence of 

 a few millimeters of air destroys all trace of the secondary radiation. 

 Photographs of the glowing vapor in air at pressures of 5 millimeters, 

 1 millimeter, and are reproduced in plate 6, d, e,f. 



If we put the drop of mercury in a small flask with very thick 

 walls, exhaust the air, and seal the neck of the flask with the oxy- 

 hydrogen flame, we are in a position to study this interesting type 

 of radiation in mercury vapor at high pressures. I found that as 

 the temperature of the flask was raised the radiation came from a 

 region nearer and nearer the front surface, which was illuminated by 

 tht rays from the lamp, and that when the pressure was about 10 

 atmospheres the ray from the lamp, which had a wave length of 

 2536, was selectively reflected from the surface of the vapor, pre- 

 cisely as if the inner surface of the bulb were plated with silver. The 

 other rays passed through the bulb with then usual facility. I am 



