EFFECT OF PRESSURE UPON ARC SPECTRA. 225 



10. SUMMARY OF RESULTS. 



The spectrum of the copper arc in air has been examined in the region X = 4000 to 

 X = 4600 A.U. at the following pressures 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 100, 

 125*, 150*, 175*, 203* atmospheres (excess above one atmosphere). 



/. Broadening: 



Within the region X 4000 to X 4600 : 



1. All lines are broader under high pressures than under atmospheric pressure. 



2. The broadening increases with the pressure ; it has not been determined whether 



the increase is continuous and linear with the pressure. 



3. The broadening of all lines is uusymmetrical, being greater on the red side. 



4. The amount of broadening is different for different lines. 



5. Two types of broadening have been observed : some lines at first become faint 



and hazy, almost resembling bands which are completely dissipated under high 

 pressures (series lines) ; others, though much broadened, remain well-defined 

 lines (non-series lines). 



6. No simple relation has been found between the width of a line under pressure 



and its original intensity. 



7. The intensity curves of the sharp lines under pressure are steeper towards the 



violet than are those of the nebulous lines. The sharp and nebulous lines 

 retain their characteristic hard and soft appearances at all pressures. 



8. The nebulous and sharp non-series lines broaden to about the same extent for 



the well-defined lines, the width may be as great as 12 A.U. at 203 

 atmospheres.* 



9. The broadening at first appears to increase more rapidly than the displacement 



at first, making measurements at low pressures less accurate than those at 

 high pressures. 



//. Displacement : 



Within the region X 4000 to X 4600 : 



1. Under pressure the most intense portion of every line is displaced from the 



position it occupies at a pressure of 1 atmosphere. 



2. The displacement is in the direction of greater wave-length. 



3. The displacement is real and not due to unsymmetrical broadening, i.e., the line 



is broadened about a displaced position. 



4. The displacement of each line is, within the limits of accuracy of the experiments^ 



continuous and linear with the pressure. 



5. The rates of increase of the displacement with the pressure are different for 



different lines. 



* Added October 19, 1908. G. D. 

 VOL. CCIX. A. 2 G 



