LIQUEFACTION OF HYDROGEN AND HELIUM. 265 



deserve my especial gratitude for their handsome donations to the 

 conduct of this research. Unfortunately, its prosecution will demand 

 a further large expenditure. It is my duty also to acknowledge that 

 at an early stage of the inquiry the honorable company of Goldsmiths 

 helped low temperatures investigation by a generous donation to the 

 research tund. 



During the whole course of the low temperature work, carried out at 

 the Royal Institution, the invaluable aid of Mr. Robert Lennox has 

 been at my disposal, and it is not too much to say that but for his 

 engineering skill, manipulative ability, and loyal perseverance the 

 present successful issue might have been indefinitely delayed. My 

 thanks are also due to Mr. J. W. Heath for valuable assistance in the 

 conduct of these experiments. 



ADDENDUM. 



Since the above paper was written, both the boiling point and specific 

 gravity of hydrogen have been determined. The boiling point in the 

 meantime given by the use of a platinum resistance thermometer 

 involves, however, extrapolation of the curve correlating temperature 

 and resistance. The result is that the boiling point of hydrogen is minus 

 228° 0. or 35° absolute. At this temperature, the tension of liquid air 

 (which, of course, becomes solid) is less than 0.002 millimeter. The 

 resistance of the thermometer used was 5.338 ohm at the melting point 

 of ice, and this was reduced to 0.129 ohms when placed in boiling 

 hydrogen. The absolute zero in platinum degrees of this thermometer 

 was minus 263.27°, and the temperature measured on this scale is minus 

 256.29° or 6.38° from the point where the conductivity of the platinum 

 would become infinite. The resistance of the platinum in the liquid 

 hydrogen is reduced to nearly one-eleventh of what it is in liquid 

 oxygen. It will be necessary to find out the electric conductivity of the 

 fluid itself, and to repeat the observations with other thermometers 

 before we can arrive at more definite conclusions. The vapor of hydro- 

 gen at its boiling point is about eight times denser than the gas at 

 ordinary temperatures, or it has about half the density of air, while 

 the vapor coming off from liquid air at its boiling point is somewhat 

 less than four times the density of air at the ordinary temperature. By 

 evaporation in a vacuum, the temperature of liquid hydrogen will be 

 lowered from 10° to 15°, but it will be practically impossible (so far as 

 we can anticipate the results of experiment) to reach a lower tempera- 

 ture than minus 250° C. or 20° absolute by this means. At present we 

 can see no way of bridging over the last 20° or 25°, and therefore the 

 approach to the zero of absolute temperature and the study of matter 

 and energy under such conditions must be confined to temperatures 

 above 25° absolute. 



The density of liquid hydrogen has been approximately determined 

 by evaporating some 10 cubic centimeters of the liquid, and collecting 



