June 4, 1920] 



SCIENCE 



571 



have equal interest for all of its readers. It 

 is a matter beyond the control of the manage- 

 ment but one of which it is fully mindful and 

 the editor very properly points out that if the 

 magazine is to be a well-balanced one those 

 members who are particularly interested in 

 certain special phases of mammalian life must 

 be largely responsible for furnishing the mate- 

 rials relating to their respective fields. In the 

 opinion of the reviewer the management is 

 to be congratulated upon the manner in which 

 the journal has been launched. That the 

 magazine will be indispensable to the active 

 worker in the domain of mammalogy is a 

 matter of course, but it seems also eminently 

 worthy of a place in the libraries of all our 

 schools and colleges where biological subjects 

 are taught, for a sufficient number of articles 

 of non-technical nature are assured to furnish 

 highly profitable reading of a kind that can 

 not help but be an incentive to a wider and 

 more intelligent interest in mammalian life. 



Charles E. JonNSON 

 Department of Zoology, 

 XJniveesity of Kansas 



SPECIAL ARTICLES 



FLUORESCENCE, DISSOCIATION AND IONIZA- 

 TION IN IODINE VAPOR 



I. FLUORESCENCE AND IONIZATION" 



Early attempts to account for fluorescence 

 as due to radiation produced by the return to 

 the parent molecules of electrons which were 

 photoelectrically emitted by the exciting light 

 have been unsuccessful, since the fluorescence 

 of gases and vapors is not generally accom- 

 panied by ionization. Consequently, the recent 

 viewpoint is that the primary effect of the ex- 

 citing light is to cause one or more electrons 

 of a molecule to take positions or conditions of 

 abnormally large potential energy, without be- 

 ing necessarily removed from the parent mole- 

 cule. This additional energy is absorbed from 

 the exciting light, and is reemitted as radia- 

 tion when the electrons return to their initial 

 stable configurations. This fluorescent radia- 

 tion may be of the same, of longer, or of shorter 

 wave-length than the exciting light according 

 as the return is accomplished in a single step. 



in several steps, or in a single step following 

 the absorption of additional radiant energy. 



We have obtained experimental evidence of 

 the correctness of this viewpoint from measure- 

 ments of the minimum energy required to 

 ionize an iodine molecule in the normal state 

 as compared with that required to ionize a 

 fluorescing molecule. This energy is expressed, 

 as usual, in terms of the -minimum ionizing po- 

 tential, whicjh is found to be close to 10 volts 

 for the normal molecule and 7.5 volts for the 

 fluorescing molecule, excited by the green mer- 

 cury linfc (whose wave-length is the same as 

 that of the green absorption band of iodine, 

 and which excites strong fluorescence). The 

 difference, 2.5 volits, corresponds to the quan- 

 tum of energy of the frequency of the exciting 

 light by the quantum relation eV^h". This 

 offers direct evidence, therefore, of the exist- 

 ence of molecules whose electrons possess ab- 

 normal potential energy as a result of the ex- 

 citing light The existence of such unstable, 

 and therefore active, molecules has particular 

 bearing on the explanation of photochemical 

 reactions, and suggests the process of chemical 

 action recently proposed by Perrin. 



n. DISSOCIATION AND IONIZATION 



Two types of ionization were discovered in 

 iodine vapour, a very weak ionization at 8.5 

 volts, attributed to the ionization of atoms 

 present because of the hot filament which 

 served as the source of the bombarding elec- 

 trons, and a very intense ionization at 10 volts, 

 attributed to the ionization of the molecules. 

 This was tested by carrying out ionization ex- 

 periments in a pyrex glass tube which could be 

 highly heart;ed in an electric furnace so that 

 various degrees of dissociation of the iodine 

 vapor could 'be obtained. The results thus ob- 

 tained were consistent with the above assump- 

 tions that the ionizing potential of the iodine 

 atom is 8.5 volts and that of the iodine mole- 

 cule is 10 volts. 



The interesting feature of this result is that 

 the difference, 1.5 volts, corresponds exactly to 

 V in the relation eV=W, where W is the 

 heat of dissociation of iodine reckoned for a 

 single molecule. In other words, the ioniza- 



