158 
DK. M W. TRAVERS, MR. (A SENTER, AND DR. A. TAQUEROT) 
8. The AIaiuind((tioa of Liquid Hyd/rof/en. 
In each experiment we employed about 400 centim,s. of liquid liydrogen, which, 
wlien sufficient liquid air (about 8 litres) liad been accumulated to cool the apparatus, 
could be obtained in less than 1 liour from the time of commencing the operations. 
Though perhaps it is little to be ex])ected, it is a fact that it is much easier to 
make measurements of the boiling-point and vapour pressures of liquid hydi'ogen 
than of liquid oxygen. As we have already stated (]i. 140), liquid air tends to 
become supei'heated, and only with difficulty can it be made to boil steadily. Liquid 
hydrogen, on the other hand, can be made to boil quite steadily at any temperature 
between its boiling-point and melting-point. Possibly this may be due to the 
presence of finely divided particles of solid air suspended in it, for it does not appear 
that tlie ])henojnenon of superheating is dependent npon any intrinsic property of a 
liquid. 
'fliat the value of tlie latent heat of vaporisation is very high (Df.wae, ‘ Hoy. 
Soc. Proc.,’ June 13th, L901, vol. (18, 3GI) is also in fiivour of the experimenter; for 
as the boiling-point of hydrogen is more than sixtv degrees lower than the tempera¬ 
ture of licpiid air, the operation of cooling a thermometer ladb, previously cooled in 
liquid ai]-, to the boiling-point of licpiid hydrogen, necessitates the absorpti(m of a 
considerable quantity of heat. 
When the vacuum vessel which received the li([uid hydrogen from the liijuetier was 
tilled with tlie liquid it was at once removed from the a])paratu.s, ])lugged with 
animal wool, and enclosed in an outer vessel containing liquid air. The mouth of a 
vessel containing liquid hydrogen cannot be left open, as in that case air enters the 
vessel, solidities, and rapidly evaporates the liijuid. A plug of natural wool is preferable 
to a cork or rubbei' stop])er, for the interstices of avooI become filled with cold vapour, 
and the plug acts as an excellent insulator, whereas the heat radiated from the 
solid stopper helps to evaqioiate the liipiid. 
4. Method of Detenniinii<j the Wipour Pressures of Liquid Ilydivf/eu. 
The thermometers emj)loyed in these researches were identical Avith those used in 
determining the vajiour jiressures of liijuid oxygen, and in nearly eveiy case in which 
:i measurement was made with liquid hydrogen, a point on the vapour-pressure curve 
of oxygen was also determined as a check njion the residts. The apparatus as 
arranged for these experiments is shown in section in tig. 1. The thei'inoiueter 
lias already been fully described in Part II. of this investigation, so that to .simplify 
the description only the bulb o of the main thermometer, and the bulb rd of the 
auxiliary thermometer, by means of which the temperature ot the .stem of the main 
thermometer was determined, are shown in the present diagram. 
