282 On the Effect of Pressure on the Spectra of Gases. [May 1, 



About half a litre of the mixed gases was now passed through the 

 apparatus, submitting them to the action of the electricity. The con- 

 tents of the two bulbs were next transferred to two test-tubes ; and after 

 adding excess of potash to each, Nessler's test was applied. The first 

 solution gave a faint yellow coloration, the second a rather thick reddish- 

 brown precipitate. 



No attempt was made to estimate the quantity of ammonia formed, as 

 it would vary with many of the conditions of the experiment. 



Since writing the account of the above experiment, which was made 

 in Dr. Odling's laboratory at Oxford on March 24, I have seen in the 

 6 Comptes Eendus ' for April 22, 1873, a note of an experiment by Messrs. 

 Thenard of Paris, in which they observe the formation of traces of 

 ammonia by the action of electricity on a mixture of hydrogen and 

 nitrogen ; but no details of the mode of operating are given. 



III. " On the Effect of Pressure on the Character of the Spectra of 

 Gases." By C. H. Stearn and Gr. H. Lee. Communicated 

 by W. Huggins, F.R.S. Received March 19, 1873. 



The variations in the spectra of gases which accompany changes of 

 density have been studied by Pliicker and Hittorf, Prankland and 

 Lockyer, Wiillner and others. 



It appears to us that one cause to which these changes may be due 

 has been overlooked, and that many of the observed variations are entirely 

 independent of the density of the gas. If a Leyden jar be placed in the 

 circuit, and the current from an induction-coil be passed through a 

 Plucker's tube containing nitrogen with the traces of hydrogen generally 

 present, the following well-known phenomena are observed. 



When the gas is near atmospheric pressure, the line-spectrum of 

 nitrogen is brilliant, and the IT line of hydrogen is broad and nebulous. 

 As the pressure is reduced, the lines of nitrogen gradually fade out, and 

 the band-spectrum appears, while at the same time the P line of hydrogen 

 becomes narrow and well defined. If fresh gas be admitted, the line- 

 spectrum reappears, accompanied by a widening of the P line. 



That these changes are not dependent on the density of the gas, 

 appears from the following experiment : — 



A sealed tube containing nitrogen, with traces of hydrogen at a 

 pressure of about 2 millims., was placed before the spectroscope. A 

 second tube was connected with the air-pump, and the current passed 

 through both tubes, a Leyden jar being placed in the circuit. 



When the pressure in the second tube was high, the line-spectrum of 

 nitrogen appeared brilliantly in the sealed tube, and the P line was broad 

 and nebulous ; as the exhaustion proceeded these lines faded out, and the 

 P line became narrow, in precisely the same manner as if the sealed tube 

 had been in process of exhaustion. The exp]anation appears to be that 



