EFFECT OF PRESSURE UPON ARC SPECTRA. 223 



the pressure cylinder. The insulated stuffing-hoxes, which are the special feature of 

 Dr. PETA.VEL/S cylinder, worked so excellently that the pressure only dropped 

 15 atmospheres in 24 hours. 



Using a wide slit, three photographs of the copper spectrum, under 200 atmospheres, 

 were taken with exposures of about 1 J hours each. 



When the pressure was reduced the window-tube was removed and examined. 

 The quartz was at first as clear as when placed in position, but gradually small 

 splinters extended across and through it, and faint metallic "pings" could be heard 

 as the strain was released. The window thus became gradually less and less 

 transparent until, after some hours, it scarcely allowed light to pass through, and 

 when finally removed from the window-txibe it was in several pieces. This rapid loss 

 of the windows has made the high-pressure work tedious and expensive. In 

 subsequent experiments glass windows have proved more satisfactory, though thev, 

 too, chip, however carefully the pressure may be released frequently a matter of 

 two or three hours. 



The Photographs. 



In addition to the photographs enumerated upon p. 207, single photographs have 

 been obtained in region X 4000 to X 4600, at pressures of 125, 150, 175, and 203 

 atmospheres. The exposure varied from 40 minutes at 125 to 90 at 203 atmospheres. 

 Photographs were also taken at 200 atmospheres with a small 1 -metre grating 

 spectrograph. These afford valuable confirmation of the results obtained with the 

 21^-ft. Rowland grating. 



General Features of the Results, 100 to 200 Atmospheres. 



No discontinuity in the nature of the spectrum was observed ; it remains a line 

 spectrum up to 203 atmospheres, though there is some continuous spectrum from the 

 poles of the arc. 



Broadening, The lines are broader, but the broadening does not seem to increase 

 as fast as the displacement this is especially difficult to estimate at the highest 

 pressures, because there is some general fogging due to scattered light in the room 

 consequent upon a long exposure, and also because there is some continuous spectrum 

 from the hot poles of the arc. 



Displacements. In the accompanying Table VIII. are given the measurements 

 made at the pressures named at the top of each column ; and Diagram 4 shows the 

 relation between the pressure and displacement throughout the whole range. Though 

 the readings between 100 and 200 atmospheres are rather lower than were expected 

 from the previous measurements, the difficulty in making the former is sufficient to 

 account for the slight apparent departure from a linear relationship between the 

 pressure and the displacement. 



