12 Professor Dewar [Jan. 20, 



platinum, containing 10 per cent, of the former. Alloys had been 

 shown by Professor Fleming and the author to differ from pure 

 metals in showing no sign of becoming perfect conductors at the 

 absolute zero of temperature, and a study of the rhodium-platinum 

 alloy had shown that the change in conductivity produced by cooling 

 from 0° to the boiling point of liquid air is regular and may be 

 represented by a straight line. As determined by the rhodium- 

 platinum thermometer, the boiling point of hydrogen was found to 

 be — 246° or some 8° lower than the platinum thermometer gave. 

 Two ways of explaining the discrepancy between the two values 

 suggested themselves. Pure platinum, although its resistance may 

 be represented by a straight line almost clown to the solidifying 

 point of air, shows signs of a departure from regularity at about; 

 this point, and the curve may become asymptotic at lower tempera- 

 tures. On the other hand, the resistance of the rhodium-platinum 

 alloy diminishes less rapidly at these lower temperatures and is much 

 higher than that of pure platinum under similar conditions. It 

 follows that its resistance curve, in all probability, deviates less from 

 a straight line than is the case with platinum. Either cause would 

 explain the differences observed, but the lower boiling point ( — 246° 

 or 27° absolute) seemed to be the more probable as it agreed very 

 fairly with the value for the boiling point calculated by the author 

 from Wroblewski's results. As the use of other pure metals or 

 alloys was not likely to lead to more satisfactory results, the problem 

 had to be attacked in a different way, namely, by means of an " air " 

 thermometer containing hydrogen under diminished pressure. 



A first attempt has been made at determining the boiling-point by 

 a constant-volume hydrogen thermometer working under diminished 

 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 tho 

 boiling point of hydrogen is really lower than was anticipated, and 

 must range between 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 ob- 

 tained 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 volume of the gas obtained by evapo- 

 rating 10 cc. 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 denned a meniscus, and can be so 

 readily collected and manipulated in vacuum vessels. As hydrogen 

 occluded in palladium has a density of 0*G2, it follows that it must 



