SPECTRA OF IGNITED GASES AND VAPOURS. 
29 
The spectrum of chlorine is taken under similar conditions with the spectrum of 
bromine. The spectral tube most carefully exhausted was several times filled with 
chlorine and exhausted again. The final tension of the remaining gas was about 6 centi- 
metres, as it was in the former case. 
P exhibits the spectrum of phosphorus (64). 
We conclude with a general remark regarding more or less all the spectra of the 
second order represented in Plates I. & IT. The intensity attributed to the different 
bright lines constituting these spectra corresponds to the condition in which they are 
best developed.. There seems to be a general rule that all luminous lines become 
brighter and are finally expanded, when the heating-power of the discharge continually 
increases. But for different lines the intensity does not rise in the same ratio : thus lines 
less brilliant at first than others may afterwards surpass them in brilliancy. The inten- 
sity attained by the different luminous lines before they are expanded greatly differs ; 
lines may disappear by expansion, while others of the same spectrum do not yet appear. 
The least-refracted lines generally resist expansion the most. 
MDCCCLXV. 
F 
