DEGLI Sl'ETTROSCOPISTI ITALIANI 135 



is iiecessarily altered by increasing the thickness of the stratum , the assnmption 

 seems also devoid of true theoretical foandation. 



One thing is clear , that if the assumed continnons spectrura is ever areched 

 by increased thickness , as by incroased density , it must be reached throngh the 

 « short-line » stage. 



To test this point I bave made the following experiraents: — 



1. An irou tube aboiit 5 feet long was filled with dry hydrogen; pieces of sodinni 

 were carefully placed at intervals along the whole length of the tube, except dose 

 to the euds. The ends were closed with glass plates. The tube was placed in two 

 gas-furuaces in line and heated . An elctric lanip was placed at one end of the 

 tabe and a spectroscope at the other. 



Wheu the tube was red-hot and filled with sodium-vapour tgroughout, as nearly 

 as possible, its whole length, a stream of hydrogen slowly passing through the tube, 

 the liue D was seen to be absorbed; it was no thicker than when seeu under si- 

 inilar conditions in a test-tube, and far thinner thau the line absorbed by sodium- 

 vapour in a test-tube, if the density be only slightly increased, 



Only the longest » fnndamental » line was absorbed. 



The line was thicker than the D line in the solar spectrum, in ivhich spectrum 

 ali the short lines are reversed, 



2. As it was difficult largely to increase either the temparature or the density 

 of the sodium-vapour, I bave made another series of expesiments with iodine-vapour. 



I bave already poited out the differences indicatcd by the spectroscope between 

 the quality of the vibrations of the « atora » of a metal and of the « subatom » 

 of a metalloid (by which terni I defìne that mass of matter which gives us a spe- 

 ctrum of channelled spaces , and builds up the continuous spectrum in its owq 

 way). Thus, in iodiue, the short lines, bronght about by increase of density in aa 

 atomic spectrum, are represented by the addition of a system of well-defined « beats » 

 and broad bands of continuous absorption to the simplest spectrum, which is one 

 exquisitely rhythmical, the intervals increasing from the blue to the red , and in 

 which the beats are scarcely noticeable. 



On increasing the density of a very small thickness by a gentle heating , the 

 beats and bands are introduced, and, as the density is stili further increased, the 

 absorption becomes continuous thronghout the whole of the visibile spectrum. 



The absorption of a thickness of 5 feet 6 inches of iodine-vapour at a tempe- 

 rature of 59» F. has given me no indication of bands , whiie the beats were so 

 faint that they were scarcely visible. 



