ox THE MEASUEEMEXT OF TEMPERATUlfE. 
171 
'svliile the stop-cock was open; on passing the current through the solenoid the piece 
of iron d did not move, indicating that its lower end was embedded in solid hydrogen. 
The stop-cock leading to the pump was then closed, and wlien the hydrogen in the 
tube a had melted it was again 023ened and the melting pressure was again 
determined. This operation was rej^eated several times; the melting pressure was 
invariably found to be 49 or 50 millims. This pressure corresponds to a temperature 
of 14°T on the helium scale, which is the melting-point of hydrogen. 
Appendix II.— Xote on ihe Preporation of Pure Hydrogen. 
It is obvious that the simplest method for the preparation of pure dry hydrogen 
would be to liquefy some of the gas in a bulb immersed in liquid hydrogen and allow 
the gas to evajiorate directly into the apparatus which it is intended to fill. At the 
tenqierature of liquid hydrogen all possible impurities would be practically non¬ 
volatile, and the gas would not require further purification. 
The hydrogen employed in these researches was obtained by the action of dilute 
suljihuric acid on pure platinised zinc. The gas was passed through a solution of 
potassium permanganate, and through a tube containing pentoxide of phosjihorus 
into a tube fitted with a stop-cock coutaining pieces of palladium sponge. When the 
palladium was saturated, the stop-cock was closed and the tube connected with it 
was sealed to the appaintus for filling the thermometer. The gas from the palladium 
tube passed through a tube, about 40 centims. long, containing pentoxide of 
phosphorus, through a large bulb filled with glass beads and immersed in liquid air, 
to the inlet tube of the thermometer. The apparatus also communicated througli a 
stop-cock with the mercury punqi and with a mercury manometer. Before filling the 
thermometer the vliole apparatus was exhausted and a considerable quantity of 
hydrogen alhjwed to escape from the palladiimi. The actual operations involved in 
filling the thermometer have already been described (p. 117); it is only necessary to 
state here that every precaution was taken to avoid contamination of the gas by 
moisture or by other impurities. 
Appendix III. — Note on the Preparation of Pure Helium. 
As has been jioiided out by one of us (Travers, ‘ The Experimental Study of 
Gases’), the constant-^■olume helium thermometer furnishes the only reliable means 
of measuring low tenqieratures. For the purpose of measuring the temperature of 
liquid hydrogen Ijoiling under reduced j)ressure, it is of particular importance that 
tlie helium should be free not only from argon, but even from neon; for as that 
element has a vapour pressure of 12‘8 juillims. of mercury at the boiling-point of 
liquid hydj'ogen, and 2'4 millims. at 15°'65 abs., its vapour pressure at 14° abs. would 
z 2 
