The Nadir of Temperature, and Allied Problems. 361 



point of hydrogen, gives as the melting point 16° absolute. This 

 value does not differ greatly from the value previously deduced from 

 the use of hydrogen gas thermometers, viz., 16° - 7. The lowest tem- 

 perature recorded by gas thermometry is 14°*5, but with more com- 

 plete isolation and a lower pressure of exhaustion, it will be possible 

 to reach about 13° absolute, which is the lowest practicable tempera- 

 ture that can be commanded by the use of solid hydrogen. Until 

 the experiments are repeated with a helium thermometer filled with 

 helium, previously purified by cooling to the lowest temperature that 

 can be reached by the use of solid hydrogen, the gas being under 

 compression, no more accurate values can be deduced. 



The latent heat of liquid hydrogen about the boiling point as 

 deduced from the vapour pressures and helium-thermometer tempera- 

 tures, is about 200 units, and the latent heat of solid hydrogen cannot 

 exceed 16 units, but may be less. 



The order of the specific heat of liquid hydrogen has been deter- 

 mined by observing the percentage of liquid that has to be quickly 

 evaporated under exhaustion in order to reduce the temperature to 

 the melting point of hydrogen, the vacuum vessel in which the experi- 

 ment is made being immersed in liquid air. It was found that in the 

 case of hydrogen the amount that had to be evaporated was 15 per 

 cent. This value, along with the latent heat of evaporation, gives an 

 average specific heat of the liquid between freezing and boiling point 

 of about 6. When liquid nitrogen was similarly treated for comparison, 

 the resulting specific heat of the liquid came out 0*43 or about 6 

 per atom. Hydrogen therefore appears to follows the law of Dulong 

 and Petit, and has the greatest specific heat of any known substance, 

 near its melting point. 



The same fine tube used in water, liquid air, and liquid hydrogen 

 gave respectively the capillary ascents of 15'5, 2 and 5*5 divisions. 

 The relative surface tension of water, liquid air, and liquid hydrogen 

 are therefore in the proportion of 15*5, 2, 0*4. In other words, the 

 surface tension of hydrogen at its boiling point is about one-fifth that 

 of liquid air under similar conditions. It does not exceed one thirty- 

 fifth part the surface tension of water at the ordinary temperature. 



The refractive index of liquid hydrogen determined by measuring 

 the relative difference of focus for a parallel beam of light sent through 

 a spherical vacuum vessel filled in succession with water, liquid oxygen, 

 and liquid hydrogen, gave the value 1*12. The theoretical value of the 

 liquid refractive index is I'll at the boiling point of the liquid. This 

 result is sufficient to show that hydrogen, like oxygen and nitrogen in 

 the liquid condition, has a refractivity in accordance with theory. 



Free hydrogen, helium, and neon have been separated from air by 

 two methods. The one depends on the use of liquid hydrogen to boil 

 the dissolved gases out of air kept at a temperature near the melting 



2 c 2 



