x.] WIDENING BY QUANTITY. 129 



until, in spite of considerable dispersion, the two lines blended 

 into one. The source of heat being now removed, the same 

 changes occurred in inverse order; the broad band split into 

 two lines, gradually the black thread alone was left, and finally 

 that vanished, and the two bright lines were restored. 



(2.) This experiment was then varied in the following way. 

 Some pieces of metallic sodium were introduced into a test- 

 tube, and a long glass tube conveying coal-gas passed to the 

 bottom, an exit for the gas being also provided at the top. The 

 sodium was now heated and the flow of coal-gas stopped. In a 

 short time the reversal of the D lines was complete. The gas 

 was now admitted, and a small quantity only had passed when 

 the black lines were reduced to threads. 



In this manner we were able to artificially reproduce the 

 thickness observed in the D lines when they are thinnest in a 

 high prominence and thickest in a deep spot, and this in a way 

 which eliminated the effect of temperature. 



When we consider the widening of the lines seen in spots we 

 are in presence of another cause than pressure to which that 

 widening may be ascribed. There are many reasons for believ- 

 ing that a spot is the seat of a downrush of comparatively cool 

 vapours, so that in it we have an accumulation of those vapours, 

 and the increased quantity of any particular substance leads to a 

 widening of its spectral lines independently of the change in 

 the same direction that would be produced by the pressure 

 which also is doubtless increased. 



Experimentally we can vary the thickness of the D line of 

 sodium so as to exhibit the effects to a large audience, and if 

 we then examine the conditions under which this is effected, we 

 get an additional strengthening of the above explanation of the 

 thickening of the lines. 1 



1 It is the more necessary to insist upon this, as quite recently the thickening 

 has been attributed by M. Fievez to temperature. This would make the tempera- 

 ture of the sun-spots lower than that of the sodium vapour in the experiment 

 referred to above ! 



K 



