﻿332 Prof. R. W. Wood on the Fluorescence 



existed in the state of a gas, giving the fine-line absorption 

 spectrum. With a given density of the vapour, a ceratin 

 amount of iodine could be vaporized without showing its 

 gas absorption spectrum : if a little more were added, 

 however, the fine lines at once appeared. This experiment 

 appears to me to be analogous to the one in which the 

 fluorescence of iodine in air at moderately low pressure can 

 be brought about by the vaporization of an excess of iodine. 

 In other words, we may say that at room temperature, air at 

 15 cms. pressure will dissolve all of the iodine which will 

 vaporize at this temperature, but if the temperature is raised 

 the excess of iodine remains undissolved, and it is this portion 

 only which fluoresces. Further evidence confirming this 

 theory has been -recently found by one of my students, 

 Mr. T. S. Elston, who has been working on the fluorescence 

 of anthracene. The vapour of this substance emits a brilliant 

 • blue-violet fluorescent light when it is formed in a vacuum, 

 or in hydrogen, nitrogen, or carbonic acid at atmospheric 

 pressure, but when formed in oxygen, sulpur dioxide, or 

 cyanogen at atmospheric pressure, there is absolutely no trace 

 of fluorescence. Heated to a higher temperature in anv one of 

 these gases, it reacts chemically w r ith them, and we may if we 

 choose use the term incipient chemical action instead of 

 solution. That the union is only of a temporary nature was 

 shown by vaporizing the anthracene in the oxygen, cooling 

 the bulb until the anthracene had crystallized on the walls, 

 and then opening the bulb. The oxygen was found to be at 

 atmospheric pressure, indicating that none of it had been 

 consumed, and on filling the bulb with hydrogen and 

 vaporizing the anthracene, the fluorescence at once appeared 

 again with undiminished intensity. Probably interesting 

 results will be obtained by measuring the vapour-pressure of 

 mixtures of anthracene and oxygen. If equal volumes at 

 the same pressure and temperature are mixed, it seems 

 probable that a diminution in the pressure will result. 



The fluorescence was bright enough to record its spectrum 

 on an isochromatic plate with an exposure of seven hours. 

 The spectrum is reproduced on PI. 1Y. (upper figure). The 

 dispersion of the three-prism spectrograph was insufficient to 

 show the structure of the bands, which extend from the 

 orange-red well down into the greenish-blue. I feel certain 

 that I shall be able to photograph the spectrum with the 

 12-foot concave grating, although it will require a very long 

 exposure. 



The magnetic-rotation spectrum and the temperature- 



