1914] on The Coming of Age of the Vacuum Flask 249 



Thus the hydrogen volumes measured are reduced to V at N.T.P. 

 and simply multiplied by the constant factor ()-0uol792, thereby 

 giving the specific heat. 



In an earlier Paper* the value of the latent heat of hydrogen 

 was taken as 122 "92. Five observations were given varying ±5 

 about this mean value. This was determined on the basis of (r0291 

 being accepted as the mean specific heat of lead from 15° C. to 

 20° Abs. Several AYillard Gibbs vapour pressure formula, however, 

 calculated from the vapour tensions of hquid hydrogen, give a mean 

 value of 115 for the latent heat of hydrogen, so that the specific heat 

 of lead over this range would seem to be 0'0272. 



In connection with the computation of the latent heat of lique- 

 faction of hydrogen it is essential to know the value of the specific 

 volumes of saturated hydrogen between 15° and 20° Abs. This was 

 determined in the following simple manner : a carefully calibrated 

 bulb A was charged with hydrogen to saturation at liquid hydrogen 

 temperatures, and the volume of the hydrogen so required determined 

 by pumping out the bulb. The figure (Fig. 5) shows the bulb A 

 immersed in liquid hydrogen in vacuum vessel B, which is further 

 isolated in the usual way in an external vacuum vessel C of exhausted 

 liquid air. The hydrogen evaporating from the liquid was used to 

 saturate the space to the same pressure as existed in B, the tube 

 dipping into the bubbling bottle of hydrogen D was lengthened to 

 more than barometer height when the liquid hydrogen pressure was 

 reduced below atmospheric to obtain temperatures below the boiling 

 point ; for this purpose the exhaust was connected by a stopcock on E. 

 The various connecting stopcocks and collecting and measuring 

 apparatus shown indicate the necessary manipulations. 



The values obtained at the boiling point and near the melting 

 point respectively are shown in the following Table : — 



Specific Volume of Saturated Hydeogen. 



- ^ 



T^«,T.^^„+„^^ V,. .=„ « ' "^'ol. of Unit W^eight Ratio to Density of H. at the 



Temperature Pressure (Specific Volunil) same Pressure and 0° C. 



20-4^ Abs. 760 mm. 

 15-3- „ 115 „ 



747-4 14-94 



3914 IS -84 



However, the relative weights of liquid and saturated vapour thus 

 determined, only involve a correction of about 1% in the calculated 

 latent heat, which is within the experimental variations. 



Taking the value of 115 for the latent heat of hydrogen, the 

 esulting value of the specific heat of lead from 80' Abs. to 20° Abs. 

 s now 0*02399. This is the rnean value of a series of nearly 



Roy. Soc. Proc, A., Ixxvi,, p. 325. 



