414 
MR, W. CROOKES OK THE VISCOSITY 
a result closely coinciding with O'1124, the experimental result for ah Up to an 
exhaustion of about 30 M the same proportion between the viscosities of air, oxygen, 
and nitrogen is preserved with but little variation. From that point great divergence 
occurs between the individual curves of the three gases. The difficulties of obtaining 
concordant observations at these high vacua are very great, and the unavoidable 
errors of experiment are, I think, sufficient to account for any divergence between 
theory and observation. 
OBSERVATIONS OK THE SPECTRUM OF NITROGEN. 
675. Spectrum observations during exhaustion give the following results :— 
At 55 millims. pressure the band spectrum of nitrogen commences to be visible. 
The red and yellow bands are easily seen, and the green and blue are exceedingly 
faint. As the pressure grows less the bands become more distinct, until at 1T4 millim. 
the band spectrum is at its brightest. 
At a little higher exhaustion a change comes over the spectrum, and traces of the 
line spectrum are observed. 
At 812 M both the band and the line spectrum can be seen very brilliantly. 
At 450 M the line spectrum is seen in great parity. As the exhaustion becomes 
higher the lines commence to disappear at the two ends of the spectrum. 
At 188 M the lines below X 610 at the red end, and above X 400, cease to be 
visible. 
At 94 M a bright greenish-yellow line is visible at about X 567. 
At 55 M this greenish-yellow line is still very prominent. The red lines have 
disappeared altogether, and the highest blue line visible is one at X 419. The line 
X 567 varies much in visibility; sometimes it cannot be seen, whilst at others it is very 
visible. Thus— 
At 40 M the line X 567 has quite disappeared. 
At 17 M line X 567 is visible again, being the most prominent line left. 
At 12 M line X 567 is not seen, although several other green and blue lines are left. 
At 3 M only three lines are visible in the green, and these are very faint. 
At 2’8 M line X 567 is detected again. 
At 2 M only traces of one or two lines can be seen, the faint light of the lines being 
overpowered by the green phosphorescence of the glass. 
Line X 567 has been seen on several occasions at high exhaustions when the gas 
under examination has been mixed with a little air. It is probably a nitrogen line, 
for one of the most brilliant nitrogen lines has a wave length of X 567*8 (Thalen), 
X 568*0 (Huggins), or X 568*1 (Plucker), and my interpolation curve is not sufficiently 
accurate to enable me to say that the line I have entered in my notes as being at X 567 
