388 



PROCEEDINGS OF THE AMEHICAN ACADEMY. 



greater vapor density the green radiation from a mercury tube was used, 

 with which fringe shifts as high as one hundred fringe-widths were ob- 

 served, corresponding to a shift of twenty-five hundred helium fringes. 

 In this way it has been possible to obtain measures of the optical density 

 of the vapor under the specified conditions from a temperature of 280°, 

 at which point the fringes commenced to move, to above 650°, at which 

 temperature the dense vapor was luminous, the entire section of the tube 

 appearing filled with a dull red glow. Plotting the fringe shifts as 

 abscissae, and the temperatures as ordinates, we obtain curves of form 

 similar to the density curve obtained by Jewett, though by no means 

 coincident with it, for reasons which I have already given. 



The observations were made both when heating and cooling. At first 

 it was found that the shift obtained on cooling was greater than the 

 one observed during the heating ; in other words, the fringe system did 

 not return to the starting point. This was found to be due to hydrogen 

 gas liberated from the sodium, which from the circumstance that it shifts 

 the fringes in the opposite direction from the shift produced by the 

 sodium vapor, retards the motion of the system during its liberation, and 

 by remaining unabsorbed in the tube prevents the fringes from returning 

 to the starting point. This trouble was eliminated by heating the tube 

 to a high temperature, and setting the mercurial pump in operation. 



