226 CARNEGIE INSTITUTION OF WASHINGTON. 



center of the tube-arc was found to give a maximum intensity for 

 the hnes most characteristic of the spark discharge, such as those of 

 hydrogen and carbon. The effect points to a distinct physical dif- 

 ference in the conditions for the production of enhanced and non- 

 enhanced Hnes. Each of the elements showed a definite behavior 

 in the degree to which its spectrum lines were favored by the radia- 

 tion of this central region, and it was possible to classify the elements 

 studied, dividing them into groups on this basis. 



A general but somewhat variable tendency toward dissymmetry 

 was observed in the structure of tube-arc lines. This is not noticeable 

 for the enhanced lines, but the majority of the wider arc hnes show a 

 greater strength on the red side, giving strong dissymmetry in the 

 case of many reversed lines. This effect increases in the part of the 

 tube-arc where the enhanced lines are strongest and seems to be similar 

 in nature to the dissymmetry often given by the spark discharge. 



A comparison of these results for the tube-arc with the known 

 phenomena of arc, spark, and furnace spectra shows that the effects 

 may be unified by a consideration of the electro-luminescence which 

 should be present as a result of the ejection of corpuscles from the 

 highly heated carbon, the resultant effect of which may be expected 

 to reach a maximum in the portion of the tube-arc which emits 

 enhanced lines most strongly. 



Effect of Pressure on Ionization. 



A series of experiments was made by Mr. King on ionization effects 

 at various pressures, which bears on solar phenomena as well as on 

 those of the furnace and tube-arc radiation. When two insulated 

 carbon electrodes were supported within the heated furnace tube, 

 according to the method of Harker, the negative electricity passing 

 from the hotter to the cooler electrode gave currents of about 1.5 

 amperes for a low pressure. As the pressure increased, the ioniza- 

 tion current fell rapidly until a pressure of 4 atmospheres was reached, 

 after which the change was slower, but a measurable current per- 

 sisted for as high as 20 atmospheres. These experiments showed the 

 magnitude at higher pressures of the effects which were known to 

 exist in vacuum and at atmospheric pressure. 



Analytical Study of Arc Spectra. 



Mr. St. John and Mr. Babcock have begun an investigation of the 

 spectrum of different parts of the iron arc, with special reference to 

 the availability of lines as standards. Two series of photographs are 

 now awaiting reduction, namely, one in which no sUt was used, a 

 minified image of the arc being focused in the plane usually occupied 

 by the slit, and a second with the sHt, in which light from a point very 

 close to the negative pole was compared with light from the center of 

 the arc. Important results have been obtained from the few plates 



