Fluorescence 399 



burning in nitrous oxide not only to affect a photographic plate 

 but also to excite fluorescence. Like the electric spark, the bisul- 

 phide flame is rich in ultraviolet light. T, R. Robinson (1858) 

 thought it of suflicient interest to record that a particularly intense 

 display of the aurora borealis on March 14, 1858, would make a 

 drop of quinine sulphate solution as well as potassium platino- 

 cyanide, luminesce. G. Seelhorst (1869) studied fluorescing liquids 

 in Geissler tubes. 



W^ien it was realized around I860 that something from the 

 cathode of an electric discharge tube was causing a green lumines- 

 cence of the glass of the tube, the word " fluorescence " was soon 

 applied to the light, which lasted only while the cathode was active, 

 i. e., while the electrode emitted what came to be known as cathode 

 rays. Similarly, the excitation of various substances by anode rays, 

 X-rays, and radium rays was also designated fluorescence at the time 

 these new radiations were discovered (see Chapter XII) . 



The term " fluorescence " was almost too widely used. It was 

 applied to the luminescence of solids, liquids,^^ and gases, even to 

 the light from residual gas in a vacuum tube, by which the path of 

 cathode rays can be observed. In the enthusiasm of a new discovery 

 the question arose whether all substances might not be fluorescent, 

 whether when illuminated by visible light they might not emit in 

 the infrared. F. Salm-Horstmar (1861) and Dammer (1862) thought 

 they had observed such effects but the phenomenon turned out to 

 be no more than the warming of the irradiated material by absorbed 

 sunlight. 



In addition to positive fluorescence, which was now recognized 

 as of common occurrence, a more remarkable claim was made for 

 the existence of negative fluorescence, described by H. Emsmann 

 (1861), by C. K. Akin ^* (1865) and by John Tyndall (1865). The 

 designation itself is not a good term and the above writers studied 

 different phenomena having nothing to do with fluorescence. Since 

 Stokes had observed ultraviolet converted to visible radiation and 

 called this change a fluorescence, the converse change of infrared to 

 visible light might be called negative fluorescence, i. e. negative 

 fluorescence would be excited by " heat rays " as positive fluores- 

 cence is the result of " chemical rays." 



Emsmann gave as examples of negative fluorescence the change 



^^ Substances fluorescent as solids may not be fluorescent in solution. This point led 

 to a discussion between Stokes (1855) and R. Bottger (1855, 1856) . 



"■* Rep. Brit. Assoc, 33-105. 1863, and Phil. Mag. (4) 29:28-43, 1865. Akin spoke of 

 infrared as Herschel's rays, the visible a.s Newton's rays and the ultraviolet as Ritter's 

 rays. 



