THE NATURE OF FLUORESCENCE. 471 



save by a process of evolution upward ? How could their numerous 

 relations with each other, and their regular serial arrangements into 

 groups, be better explained ? In this, as in other problems, the hy- 

 potliesis of evolution is the simplest, most natural, and best in accord- 

 ance with facts. Toward it all the lines of argument presented in 

 this article converge. Atomic weights, specific volumes, and spectra, 

 all unite in telling the same story, that our many elements have been 

 derived from simpler stock. 



I know that all this is only speculation, but surely it is not base- 

 less. Science is constantly reaching forward from the known to the 

 unknown, partly by careful experiment, and partly by the prophetic 

 vision of thought. It first discovers facts, and then seeks to interpret 

 them, although oftentimes the interpretation is not capable of abso- 

 lute proof. So with the material of this article. We have seen that 

 many relations connect in some mysterious way those bodies which 

 we commonly regard as simple, and we have sought to determine 

 their meaning. What can they mean, save that the elements are not 

 elementary ? How could the elements have originated but by a pro- 

 <5ess of evolution ? 



THE NATUKE OF FLUORESCENCE/ 



By Dr. EUGENE LOMMEL, 



PROFESSOK OF PHYSICS IN THE TJNIVEBSITY OF ERLANGEN. 



THE question now arises. What becomes of the rays that have un- 

 dergone absorption ? Are they in lact, as they appear to be, 

 annihilated ? A series of phenomena now to be considered will give 

 ns an answer to these questions. 



If water containing a little esculine, a substance contained in the 

 bark of the horse-chestnut in solution, be placed in a flask, and the 

 rays of the sun or of the electric lamp, con- 

 centrated by a lens situated at about its focal 

 distance from the vesel (Fig. 1), be directed 

 upon it, the cone of light thrown by the lens 

 into the interior of the fluid will be seen to 

 shine with a lovely sky-blue tint. The parti- 

 cles of the solution of esculine in the path of 

 the beam become spontaneously luaunous,^'**-^--^"^^^"'"'^'-^"'"^^" 

 and emit a soft blue light in all directions. 



The cone of light appears brightest at the point Avhcre it enters into 

 the fluid through the glass, and quickly diminishes in brilliancy as it 

 penetrates more deeply. 



There are great numbers of fluid and solid bodies which become 



From "The Nature of Light," No. XIX. of the " International Scientific Series." 



