LIGHT AND HEAT 247 



salt and acid. Evidences of series of " aggregates " were obtained by 

 spectropliotographs of chemical reactions, spectrograms of the absorp- 

 tion spectra of a solution of a given salt being taken as increasing 

 amounts of some other kind of acid was added to the solution. 



Cexters of Phosphorescent Spectra 



Lenard, Klatt, Urbain and others have studied the phosphorescence 

 of various calcium phosphates of bismuth, manganese, nickel, etc. Le- 

 nard and Xlatt have proposed the view that these light centers or 

 " dynamids " store electrons, the state of motion of the electrons depend- 

 ing upon the temperature. At high temperatures the electrons possess a 

 much greater freedom of motion than at low temperatures. They visual- 

 ize the states of motion as being " gaseous," " liquid " and " solid." In 

 the " gaseous " state the electrons can occasion the conduction of electri- 

 city between the atoms if the latter exist in the same way as they do in 

 metals. In the " liquid " state the electrons are in a state of motion sen- 

 sitive to light vibrations and therefore they take part in light absorption. 

 In the " solid " state the electrons take part neither in conduction nor in 

 absorption. At low temperatures the spheres of action of the "djma- 

 mids " are considered to extend to greater distances than at high tem- 

 peratures and the free paths of the electrons are therefore greatly 

 reduced. 



To each phosphorescent band Lenard and Klatt assign three phases : 

 An upper momentary or heat phase; a permanennt phase possessing 

 quite definite temperature limits ; and a lower momentary or cold phase. 

 These phases succeed each other as the temperature falls. The upper 

 momentary phase results when the d}Tiamids do not store electrons. 

 Whenever electrons are stored these return afterwards to the atom from 

 which they were expelled by the light-wave, thus producing the perma- 

 nent phase of the phosphorescent band. At low temperatures a few 

 electrons return to the atoms from which they were expelled and these 

 cause the lower momentary phase. 



The phenomena of phosphorescence are generally conceded to be due 

 to some kind of electrolytic dissociation or ionization of the dissolved 

 substance in the medium about it. Among the first to hold this view 

 were Wiedeman and Schmidt. The theory explains the law of Stokes 

 and many of the other phenomena of phosphorescence. 



The Light Cextees of Oegaxic Compounds 



During recent years a very large number of investigations have been 

 carried out concerning the nature of the absorption light centers of 

 organic compounds, both pure and in a state of solution. These centers 

 have been roughly defined as chromophores, the chromophores consistino- 

 of radicles of the given compounds that are found necessary and suffi- 

 cient to produce the given absorption. Among the chromophores that 



