140 LIQUID HYDROGEN. 



ished pressure. This thermometer, which gave the boiling point of 

 oxygen as 90.5° absolute, or —182.5°, gave for hydrogen 21° absolute, 

 or —252°. The three determinations that have been made are, then, as 

 follows: (1) Pure-platinum resistance thermometer, 35° absolute; (2) 

 rhodium-platinum resistance thermometer, 27° absolute; (3) hydrogen 

 thermometer, 21° absolute. From this it appears that the boiling 

 point of hydrogen is reallv lower than was anticipated, and must range 

 betAveen 20° and 22° absolute. Further experiments will be made 

 with thermometers filled with hydrogen prepared from different 

 sources. A hydrogen thermometer filled with the gas obtained from 

 the evaporation of the liquid hydrogen itself must be employed. 



The approximate density of liquid hydrogen at its boiling point was 

 found by measuring the vohime of the gas obtained by evaporating 10 

 cubic centimeters of the liquid, and is slightly less than 0.07, or about 

 one-sixth that of liquid marsh gas, which is the lightest liquid known. 

 It is remarkable that, with so low a density, liquid hydrogen is so 

 easily seen, has so well defined a meniscus, and can be so. readily col- 

 lected and manipulated in vacuum vessels. As hydrogen occluded in 

 palladium has a density of 0.62, it follows that it must be associated 

 with the metal in some other state than that of liquefaction. 



The atomic volume of liquid hydrogen at its boiling point is about 

 14.3, the atomic volumes of liquid oxygen and nitrogen being 13.7 

 and 16.6, respectiveh^, at their boiling points. The weight of a liter of 

 hydrogen gas at the boiling point of the liquid is about the same as 

 that of air at the ordinary temperature. The ratio of the density of 

 hydrogen gas at the boiling point to that of the liquid is approximately 

 1:60, as compared with a ratio of 1:255 in the case of oxygen under 

 similar conditions. 



The specific heat of hydrogen in the gaseous state and in hydrogen- 

 ized palladium is 3.4. I)ut may ver}- probably be 6.4 in the liquid sub- 

 stance. Such a li(iuid would )>e unique in its properties; but as the 

 volume of one gram of liquid hydrogen is about 14—15 cubic centimeters, 

 the specific heat per unit volume must be nearly 0.5, which is about 

 that of liquid air. It is highly probable, therefore, that the remark- 

 able properties of liquid hydrogen predicted by theory will prove to 

 be less astonishing when they are compared with those of liquid air, 

 volume for volume, at corresponding temperatures. 



With hj^drogen as a cooling agent we shall get to from 13° to 15° 

 of the zero of absolute temperature, and its use will open up an entirely 

 new field of scientific inquiry. Even so great a man as James^ Clerk- 

 Maxwell had doubts as to the possibilit}^ of ever liquefying hydrogen.* 

 He sa}^s : 



"Similar phenomena occur in all the liquefiable gases. In other 

 gases we are able to trace the existence of attractive force at ordinary 



^ See Scientific Papers, 2, 412. 



