CHAP, x.] WIDENING BY PRESSURE. 127 



it was possible experimentally to vary the thickness of lines in 

 the way we find them thickened in the sun. The means by 

 which this was effected was an increased quantity or pressure 

 of the vapour producing them. We found, for instance, that 

 by increasing the pressure, say of hydrogen, and rendering 

 the gas incandescent by means of the passage of an electric 

 current, we thickened the lines, and especially the F line, 

 exactly as it is thickened in the lower region of the chromo- 

 sphere, and as some lines are thickened in spots. We carried 

 on the experiments to a pressure of twenty atmospheres, and the 

 spectrum was continuous with maxima indicating the places of 

 the lines. 



Since by varying the pressure of hydrogen we can thus vary 

 the thickness of the lines, it is possible to observe the spectrum 

 of hydrogen in a tube, and to place the spectrum we get from 

 the hydrogen in the sun, side by side with the spectrum we 

 obtain from the hydrogen in the tube. We can vary the pres- 

 sure of the hydrogen in the tube so that its spectrum exactly 

 fits, so to speak, the spectrum of the hydrogen in the sun ; and 

 hence we are enabled to determine the pressure of the atmosphere 

 at the sun. And this, no doubt, will some day be done, but the 

 thing is not quite so easy as this. There are more chemical 

 substances than hydrogen in that part of the sun which so 

 conveniently gives us these bright lines ; and we have in all 

 questions of pressure not only to take into account the actual 

 pressure of the hydrogen, but the combined pressure, so to 

 speak, of all the vapours which exist in that stratum ; and so 

 we have a very great inquiry before us, before a final estimate 

 of the pressure can be given. 



very visibly augmented, whilst at ten atmospheres' pressure the light emitted by 

 a jet about one inch long is amply sufficient to enable the. observer to read a 

 newspaper at a distance of two feet from the flame, and this without any reflect- 

 ing surface behind the flame. Examined by the spectroscope, the spectrum of this 

 flame is bright and perfectly continuous from red to violet." Proc. Roy. Hoc. 

 vol. xvi. p. 419. 



