426 
MR. vr. CROOKES OK THE VISCOSITY 
ratio of the viscosity of hydrogen to that of air. In 1876 I found the ratio to be 
0‘508. In 1877 I reduced this ratio to 0'462. Last year, with improved apparatus, 
I obtained the ratio of (P458, and I have now got it as low as 0'4439. This is much 
lower than Graham’s number 0 - 4855 deduced from transpiration. Graham calls this 
number theoretical, but in discussing the subject it appears that he was cpiite prepared 
to find hydrogen depart from the theoretical law which seemed to govern some other 
of the gases. Thus at page 17 9 of the collected edition of his works, already quoted, 
Graham writes : “ The times of oxygen, nitrogen, carbonic oxide, and air are directly 
as their densities, or equal weights of these gases pass in equal times. Hydrogen 
passes in half the time of nitrogen, or twice as rapidly for equal volumes.The 
circumstance that the transpiration time of hydrogen is one-half of that of nitrogen 
indicates that the relations of transpirability are even more simple in their expression 
than the relations of density among gases.” 
OBSERVATIONS ON THE SPECTRUM OF HYDROGEN. 
698. In the description of the viscosity apparatus (641) I mentioned that a 
spectrum tube with capillary bore was attached to it at k. With this apparatus 
I have taken observations on the spectrum of hydrogen during the exhaustion, using 
a coil giving a 6-inch spark, and examining the spectrum with a dispersion sufficient 
to well separate the sodium lines. 
No points of special interest were noticed. The red line (A=656 , 2), the green line 
(\=486T), and the blue line (\=434) were seen at their brightest at a pressure of 
about 3 millims., and after that exhaustion they begin to diminish in intensity. As 
exhaustion proceeds a variation in visibility of the three lines is observed. Thus 
at 36 millims. the red line is seen brightly, the green faintly, wdiilst the blue fine 
cannot be detected. At 15 millims. the blue fine is seen and the three keep visible 
till an exhaustion of 418 M is reached, when the blue fine becomes difficult to see. 
At 38 M only the red and green fines are visible, the red being very faint. It is seen 
with increasing difficulty up to an exhaustion of 2 M, when it can be seen no longer. 
The green fine now remains visible up to an exhaustion of 0 - 37 M, beyond which it 
has not been seen. 
It is worthy of remark that, although when working with pure hydrogen the green 
fine is always the last to go, it is not the first to appear when hydrogen is present as 
an impurity in other gases. Thus when working with carbonic anhydride insufficiently 
purified, the red hydrogen fine is often seen, but I have never seen either the green or 
the blue fine. 
INFLUENCE OF AQUEOUS VAPOUR ON THE VISCOSITY OF AIR. 
699. In the foregoing experiments many discrepancies were traced to the presence 
of moisture in the gas (688). The influence of aqueous vapour does not appear to be 
great when present in moderate amount in gas of normal density, but at high 
