1868.] 



Constitution of the Sun and Stars, 



21 



29. The first thing that strikes the eye in the part of the tahle appro- 

 priated to the iron lines is a continuous gradation of intensity from the 

 indigo to the red. The most refrangible iron lines mapped by KirchhofF 

 are those in the indigo, all of which he found of the deepest black, which 

 he represents by the number 6. Then follow the lines in the blue, in 

 which there appears to be a struggle between this intense blackness, and 

 the darkest shade short of blackness recorded by Kirchhoff, and to which 

 he assigns the number 5. In this part of the spectrum lines of the in- 

 tensity 6 are still predominant. In the next region, the bluish-green, 

 this struggle is continued, but now with a predominance of lines of the 

 intensity 5. About the middle of the green we for the first time meet 

 with an unexceptionable line of intensity 4, corresponding to the wave- 

 length 53*87 VHIth-metres. The last line of intensity 6 presents itself 

 at wave-length 55*77, after which, in the yellow, orange, and red, the in- 

 tensity of iron lines has for the most part sunk to 4 or 3. 



30. Now the iron lines seen in the solar spectrum originate in the upper 

 part of the iron atmosphere, each ray coming from a stratum of such a 

 thickness that it is opake for that particular ray. This thickness differs 

 from ray to ray, being greater for those rays which are caused by atomic 

 motions of feeble intensity. Such rays therefore will in part originate from 

 a greater depth in the solar atmosphere, and therefore from a region of 

 greater heat. They will therefore be brighter, or in other words less con- 



little informed on these subjects to speculate with any confidence on the cause, and 

 perhaps the following conjecture is the best that can yet be made. 



The effect may perhaps be due to the brief duration of the sparks. The enormous 

 temperature caused by each spark lasts for a very short time, and is not renewed until 

 after the lapse of an interval long in comparison. The electricity, when it passes, pro- 

 bably produces its direct effect in accelerating and controlling the directions of the 

 motions cf translation of the molecules of the gas ; and only indirectly, through the 

 resulting violence of the molecular collisions, excites those more subtle atomic motions 

 which give out the light. Those of the atomic motions therefore which are most in- 

 fluenced by each collision will be the first to reveal themselves, and the rest not until 

 after very many collisions shall have taken place, so that before they have had time to 

 culminate, the duration of the spark may be over : whereas when they have time fully 

 to unfold themselves, as they can in a continuous current, they may attain in some 

 cases a higher intensity, and consequently emit a greater brightness. 



In support of this explanation, we have the fact that the lines seen with Ruhmkorff's 

 coil have been observed to correspond to the most conspicuous lines in the solar 

 spectrum. Now those atomic motions which are most developed by a few collisions 

 will usually be those of which the periodic time is most subject to perturbation (see 

 Phil. Mag. 1868, vol. xxxvi. p. 132). They will therefore in such cases give rise to dilated 

 lines in the solar spectrum, and if the circumstances be such as to cause much of the 

 breadth of the line to appear quite black, as for example in many of the iron lines, it will 

 in consequence of its breadth appear much more intense. On the other hand it should 

 be remembered as against our conjecture, that if the Euhmkorff's sparks last as long as 

 the measures Wheatstone made of the duration of the spark of a Leyden Jar, viz. 

 four Yth- seconds, the number of collisions which take place during the continuance of a 

 spark must be so great as to take away much from the probability of the explanation. 



