SECT. iv. EFFECTS OF PRESSURE. 145 



coincident with dark solar lines, and nitrogen, which 

 exhibits a complicated series of bands. 



The experiments of the Rev. Dr. Robinson on a 

 variety of gases and vapours, inclosed in glass tubes, 

 show that a greater change is produced by pressure 

 than by heat. At the ordinary atmospheric pressure, 

 the spectra show a number of bright lines on a coloured 

 ground, the light of which is, in general, stronger 

 towards the red than the violet end, and strongest in 

 the green. In some the ground is so bright as to efface 

 all but the most luminous lines. This is especially the 

 case with hydrogen. On gradually exhausting the 

 tube in which the vapour is contained, the spectra 

 rather suddenly fade away, leaving only a suspicion of 

 one or two lines, but upon exhausting the tube still 

 more, these transition spectra become bright again, 

 fresh lines appear, and they are changed into new 

 spectra which are never so bright as those at ordinary 

 pressure. Fewer lines are visible in the rarefied spec- 

 tra, and of these four-tenths are not found in the spectra 

 of atmospheric pressure. The difference between the 

 common pressure spectra, the transition, and the rarefied 

 spectra shows, that the character and even the existence 

 of certain lines depend upon the mere density of the 

 media, the chemical circumstances remaining unchanged. 

 Dr. Robinson also observed that spectra are not super- 

 posed without a change ; the spectrum of atmospheric 

 air does not always exhibit all the lines of oxygen and 

 nitrogen, and occasionally there are some lines not 

 visible in either of them. It appears also that for 

 certain lines the actions of bodies may be antagonistic. 



Metals do not always give the same spectrum, what- 

 ever may be the combinations in which they are found. 

 Among various instances M. Mitscherlich mentions that 

 the spectra of copper and the chloride and iodide of 

 copper present essential differences, and Mr. Roscoe 



VOL. I. L 



