LIGHT AND ITS ARTIFICIAL PRODUCTION. 279 



So-called ^^coldjlames.^^ — To these belong all those free-burning flames, 

 such as that of carbon bisuliDhicle, which emits a bluish, weakly light 

 even at the relatively Ioav temperature of 150° C. The classical repre- 

 sentatives of this class are, however, the lightning bug and the will-o'- 

 the-wisp. The phosphoresence at sea is also included in this class. 

 Those who have not yet seen this natural phenomenon can form no idea 

 of its magnificence, especially in southern seas. As soon as night falls 

 and the stars appear in the heavens bright spots begin to glisten in the 

 water, first only here and there at the bow of the ship; these increase 

 in numbers; they come and go; they enlarge almost without limit, 

 until finally spiral strands of gold seem to rise from the depths, often 

 condensing to large nuggets. 



This phosphorescence is also produced by living organisms, for bil- 

 lions of infusoria unite their weak light to produce the magnificent 

 effect. Its origin is as unknown to us as that of the faint light emitted 

 by wood in the process of rotting and other phenomena which have 

 become generally known since the discovery of the X-rays. 1 refer to 

 the luminosity of gases in Geissler tubes. 



You are all familiar with the action of the Euhmkorff coil used so 

 much in medicine for the treatment of partial muscular paralysis, etc. 

 If an electric current from such a coil — that is, one of high tension and 

 high frequency — is sent through a highly exhausted Gleissler tube, the 

 residual gas shines with a magic blue-violet light. [Experiment.] By 

 touching the tube we can assure ourselves that its temperature is not 

 high. The peculiar striation of the light in the neighborhood of the 

 electrode is an indication of the presence of a rarefied gas. In this 

 second Geissler tube we have, in addition to the light emitited by the 

 gas, the colored light of the glass, and especially at those parts of the 

 tube made of uranium glass. We say that the glass fluoresces, with- 

 out attempting to give an explanation of the curious phenomenon. 

 Now, if a Geissler tube is still further exhausted the stratification of 

 the glass becomes more indistinct and the luminosity of the gas finally 

 ceases entirely.^ Curious rays (cathode rays) radiate from the cathode — 

 that is, from the negative terminal of the circuit — into the almost 

 completely exhausted interior of the tube. 



We can not see these rays, but they can be made visible by placing 

 in their path fluorescent materials. In this tube asbestus is caused to 

 fluoresce. [Experiment.] In this second tube the glass fluoresces at 

 those parts on which the cathode rays impinge. You can plainly see 

 the green surface of the glass wall which is turned toward you, and 

 within this surface is a dark cross. That is the shadow produced by 

 the screening of these mysterious cathode rays by a metal cross in the 

 interior of the tube. If tiiis latter is overturned the shadow disapj^ears 

 and the whole surface shines brilliantly. But the phenomenon appears 



' This tube is called by the lecturer a Hittorf tube, but is similar to that commonly 

 known in America as a Crooke's tube. (Translator.) 



