OF GASES AT HIGH EXHAUSTIONS. 
421 
At 211 M these fine lines are distinctly visible. The brightness of this spectrum is 
now near its maximum. 
At 100 M the general spectrum is growing faint, but a sharp green line at X 534 
makes its appearance by fits and starts. This is coincident with Plucker’s bright 
oxygen line X 534. 
After this degree of exhaustion the spectrum rapidly gets fainter. The line X 534 
soon disappears, and the carbon lines also go one after the other, until at an exhaustion 
of 4 M only two lines are visible, X 560 and X 519. 
VISCOSITY OF HYDROGEN. 
687. Hydrogen gas is prepared in the apparatus shown in fig. 12 (p. 408) by the 
electrolytic decomposition of well-boiled water acidulated with pure sulphuric acid, 
a plate of amalgamated zinc being used for the positive pole, and platinum for the 
negative pole. The gas is collected in a bell receiver and passed into the apparatus 
when required, exactly as described under Oxygen (666). Hydrogen is also introduced 
into the apparatus in another way. In the tube, I (Plate 55, fig. 1), are pieces of 
palladium-foil electroh tically saturated with hydrogen ; the foil retains the hydrogen 
even in a vacuum, and when desired gives it out on being heated with a lamp. 
Before commencing experiments with hydrogen the apparatus must be exhausted 
up to the highest point, and then very slowly filled with the gas. Exhaustion is 
again carried out to a high point, and the slow filling with hydrogen repeated. It is 
then advisable to allow all to remain undisturbed for at least twenty-four hours, to 
permit the hydrogen to soak into the apparatus and displace any other gas which may 
be condensed on the surface of the glass. The whole is again exhausted, and then a 
third time filled with hydrogen. 
Preliminary observations are now taken for viscosity; and the logarithmic 
decrement at standard pressure and temperature, noted. The pumping out and 
refilling with hydrogen should now be repeated, observing the logarithmic decrement 
each time, until it is found that it no longer decreases in value ; when this is the 
case accurate observations are commenced. 
688. It has been found that hydrogen has much less viscosity than any other gas ; 
the fact of the logarithmic decrement not decreasing by additional attempts at 
purification is the test of its being free from admixture. This method of ascertaining 
the purity of the gas, by the uniformity of its viscosity coefficient at 760 millims., is 
more accurate than collecting samples and analysing them eudiometrically (706). 
Several series of observations in hydrogen have been taken. For a long time it 
was considered that hydrogen, like other gases, showed the same slight departure from 
Maxwell’s law of viscosity being independent of density ; for the logarithmic decre¬ 
ment persistently diminished as the exhaustion increased, even at such moderate 
pressures as could be measured by the barometer gauge. Had it not been that the rate 
of decrease was not uniform in the different series of observations, I should have con- 
3 i 2 
