222 DE. W. GEOFFREY DUFFIELD ON THE 



" median" lines, and this classification is of some use in co-ordinating KING'S results 

 with those obtained in this research. In Table VII., PA = polar lines in the arc 

 spectrum, PS = polar lines in the spark spectrum, and it will be seen that the PA 

 lines are weakened under pressure and the PS lines strengthened. With the possible 

 exception of the lines of the 1st sub-series of KAYSER and RUNGE these correspond to 

 the sharp and nebulous lines respectively. The table contains a resume of the work 

 that has been done upon this part of the spectrum for the purpose of classifying the 

 lines. 



It is interesting to note that in the copper arc spectrum no marked differences 

 have been observed in the behaviour of individual members of a pair under pressure 

 as have been found to exist in a strong magnetic field, and, though there is some 

 apparent structure on the wings of line d at 20 atmospheres, it is not certain that it 

 is not due to irregularities in the ruling of the grating. 



KING'S conclusion that the weakening of the series lines in an atmosphere of 

 oxygen* is due to diminished vapour pressure is not confirmed by their disappearance 

 at high pressure. 



9. 100 TO 203 ATMOSPHERES. 

 [Added October 23, 1908.]. 



The importance of extending the range of pressures was urged upon me by the 

 unexpected behaviour of the silver arc under pressure, already mentioned paren- 

 thetically in the present paper (p. 209), and it seemed of great interest to examine if 

 an increase of pressure would similarly cause the copper line spectrum to vanish and 

 give place to a banded spectrum. This possibility was held to justify the extension 

 to higher pressures. 



The cylinder had originally been tested up to 400 atmospheres (liquid pressure) 

 with a metal screw in place of the glass window. A second test, with the window in 

 position, was made up to 350 atmospheres (liquid pressure). The glass held 

 satisfactorily, but was too much strained when the pressure was reduced for objects to 

 be clearly seen through it. In the hope that fused quartz would be less affected, a 

 window of that material was inserted in the window-tube and air admitted to the 

 cylinder from two gas-holders, the first being pumped up to 120 atmospheres and the 

 second to 200 atmospheres. 



A fair photograph was in this way obtained at 185 atmospheres, but the quartz 

 window chipped to such an extent under the combined action of the pressure and the 

 heat from the arc that it was useless for subsequent experiments. A third window 

 (also of fused quartz) was next employed, and with the aid of a third gas-holder 

 (kindly lent by Mr. Chas. W. Cook, of the Manchester University Engineering 

 Works), containing air at 210 atmospheres, 200 atmospheres were obtained within 



* A. S. KING, ' Astrophyeical Journal,' XVIII., p. 129 (1903). 



