1085 
a sufficiently high degree of purity, was boiling ') under a pressure 
which was kept constant to a few mms of mercury by the labora- 
ratory pumps, and was only a little higher than the atmospheric 
pressure *)); the emptying of the apparatus could be very quickly 
accomplished by the evaporation of the liquid, after removing the 
external vacuum glass, which, as in the experiments with liquid air, 
contained liquid air. For the internal vacuum-vessel, which contained 
the liquid hydrogen, a completely silvered glass was taken, in which 
only two opposite windows were left open, so as to enable us to 
observe the height of the liquid when filling the glass; as the 
external vessel was silvered with only a transparent strip left open, 
it was only necessary to turn this outside glass a little way in order 
to protect the liquid hydrogen practically completely from external 
radiation. 
2. Technical difficulties in connection with the use of liquid 
hydrogen and the much lower temperature (+ 20° K.) did not arise ; 
the only thing was, that, as was to be expected, the damping of 
the oscillations was very small (about 6 times less than in liquid 
air) in consequence of which the internal friction of the suspension 
wire acquired a very high and unpleasant degree of importance. 
This friction, in fact, proved to be not only comparatively large, 
but to depend to a high degree upon accidental circumstances, 
difticult to estimate and control; consequently, although for each 
experiment separately an accurate logarithmic decrement could be 
deduced, only a very moderate agreement could be found between 
the various experiments®). In the first place it was found that 
unstretching and re-stretching of the wire (by exchanging the cylinders 
C’) altered the viscosity of the wire very greatly (usually increasing 
it); in the second place the viscosity was a function of the time, 
which only decreased slowly, in an approximately exponential manner, 
and required some days to become constant; in the third place the 
gas in which the wire was placed proved to have a great influence 
upon its viscosity; pumping a vacuum, filling with air, replacing 
1) The boiling. which was entirely superficial, without the formation of bubbles, 
took place very slowly, thanks to the screens. 
2) The small difference of pressure between the vapour pressure in the apparatus 
and the air pressure outside, was read on the small open manometer Ma (see 
figure in Comm. N°. 1495). 
3) This is clearly also the cause of the differences which were observed in the 
experiments with liquid air (see Comm. N°. 149) V). This instability in the internal 
friction in the wire has given similar difficulties to previous investigators: see for 
instance ZEMPLÉN, Ann. d. Phys. 19, 802, 1966. 
69 
Proceedings Royal Acad. Amsterdam. Vol. XIX. 
