December 29, 1916] 



SCIENCE 



907 



and must be determined by the electrical 

 state of the radiator. A molecule may be 

 neutral or for a moment charged by the 

 loss or gain of an electron. This type of 

 ionization must actually occur, as indi- 

 cated by the conduction of electricity 

 through the vapor of a compound which 

 shows no evidence of chemical dissociation. 

 What causes the light emission? It may 

 accompany the loss or gain of an electron 

 by a neutral molecule, in which case the 

 emission center would be charged. It may 

 be due to the shock of elastic collision with 

 an electron or ion, or to the reunion of an 

 electron with a positively charged molecule, 

 in which cases the emission center would be 

 neutral. Luminous vapors emitting band 

 spectra usually appear to be neutral at the 

 instant of emission, so that it seems prob- 

 able that band emission is due either to 

 elastic shock or to the recovery of a lost 

 electron. It is to be remarked that as a 

 rule band spectra are not subject to the 

 Zeeman, Stark or Humphreys-Mohler ef- 

 fect ; in the exceptional cases it is probable 

 that those subject to one of these effects 

 are subject to all. It would be of interest 

 to examine these cases with reference to 

 the nature of the molecular charge. 



Luminous vapors emitting line spectra 

 appear, in many cases at least, to be posi- 

 tively charged. A sodium flame is at- 

 tracted to the negative plate of a con- 

 denser. A metallic salt introduced near 

 the cathode of a spark discharge colors the 

 spark only in that neighborhood; if intro- 

 duced near the anode, the color flashes en- 

 tirely across the spark. The most promis- 

 ing method of verifying such conclusions 

 appears to be by the study of canal or posi- 

 tive rays. Sir Joseph Thomson, from a 

 study of the deflections produced by mag- 

 netic and electric fields, found that, with 

 very few exceptions, no molecules of either 

 elements or compounds carry a negative 

 charge, while those with positive charges 



are common. No molecule acquires more 

 than one positive charge. The atoms of 

 but few elements are found with a negative 

 charge, but all may acquire positive 

 charges and many may be multiply 

 charged. For example, krypton may have 

 as many as five and mercury eight posi- 

 tive charges. Hydrogen never has more 

 than one charge, which accords with Bohr's 

 view that it has but one detachable elec- 

 tron. 



Stark has reached similar conclusions 

 from a study of the spectra of canal rays. 

 In many cases the motion in the line of 

 sight gives a Doppler effect. There is an 

 undisplaced line due to the stationary gas 

 and a displaced line due to the canal rays. 

 A distinct separation between the dis- 

 placed and stationary lines shows that the 

 canal rays can not radiate until their 

 kinetic energy reaches a threshold value, 

 which Stark first interpreted in favor of 

 the quantum theory, but which he now be- 

 lieves to represent the energy necessary 

 for ionization. There may be two or even 

 three displaced lines, with separations con- 

 sistent with the view that the luminous 

 centers are doubly or triply charged. The 

 radiation is evidently due to collisions, 

 for a reduction of pressure in the canal 

 ray chamber causes a reduction of lumi- 

 nosity. In general, all series lines are sub- 

 ject to the Doppler effect. Pulcher has 

 shown that nitrogen canal rays give the 

 negative pole band spectrum, with dis- 

 placements, but no other bands have been 

 found to give this effect. The series lines 

 of hydrogen show displacements, but they 

 are not observed in the many-line spec- 

 trum except to a slight extent in a few 

 cases. Stark concludes that the series lines 

 are emitted by positive atom ions, and the 

 lines of the secondary spectrum by neutral 

 atoms. He thus associates the compound 

 spectrum with band spectra, which he sup- 

 poses to be due to neutral systems. It may 



