LIQUID hydrogp:n. 263 



gave the boilino- pojnt of 21 absolute or —252= C, and a similar 

 helium thermometer g-ave the same result. The critical temperature 

 is about 32^ absolute or —24:1 C, and the critical pressure about 15 

 atmospheres. If a closed vessel is full of hydrogen gas at atmospheric 

 pressure, then, unlike the air vessels, it shows no condensation Vvhen a 

 part of it is cooled in liquid hydrogen. To produce liquefaction we 

 must increase the pressure of the gas or reduce the boiling point of the 

 licjuid hydrogen by exhaustion. Pure hydrogen liquefied in a closed 

 vessel is perfectly clear, showing no trace of color or any appearance 

 of absorption bands in the position of the spectrum lines. Electric 

 sparks passing in the liquid when examined with the spectroscope 

 show the ordinary line spectrum without any reversals. The vapor 

 of boiling hydrogen is alujut lifteen times denser than that of the 

 ordinary gas, thus bringing it up to the density of air. The liquid 

 hydrogen, at its boiling point, is about sixty times densei- than the 

 vapor coming otf. In the case of oxygen the density of the liquid is 

 255 times that of the vapor at its ])oiling point. 



If a piece of cotton wool in the form of a little ball is attached to a 

 thread, placed in 'liquid hydrogen, and then l)rought into the magnetic 

 held, it is found to be strongly magnetic. This is simply due to the 

 condensation of solid and liquid air in the pores of the wool. _ This 

 substance we know is magnetic on account of the oxygen it contains. 

 Pure licjuid hydrogen is not magnetic, but when the solid air snow is 

 in suspension in the lluid, then the magnetic character of the latter 

 becomes apparent when the vessel is placed in the magnetic held. 



All the phosphorescent etl'ects produced at low temperatures formerly 

 described are intensified at the much lower temperature of boiling 

 hydrogen. To stimulate phosphorescence at the temperature of liquid 

 air, ultraviolet light had to be employed, and then the solid bod3% 

 organic or inorganic, allowed to rise in temperature. It was during 

 the rise of temperature that the marked luminous emission took place. 

 Amongst inorganic bodies the platino-cyanide of ammonia is very 

 remarkable in this respect, and generally the group in organic chem- 

 istry known as the ketonic bodies. In the case of bodies cooled in 

 liquid hydrogen, it appears that some show phosphorescence by simple 

 stimulation with the light coming from an ordinary carbon filament 

 electric lamp. The light in this case coming through glass contains 

 only, we may say. the visible spectra, so that the ultraviolet rays are 

 not now essential. It is strange to find photographic action still rela- 

 tively considerable. At the boiling point of liquid air the photo- 

 graphic intensity is reduced by 80 per cent of the value at the ordinaiy 

 temperature. The photographic effect on a sensitive film immersed in 

 liquid hydrogen as compared with the same placed in liquid air is as 

 1 to 2, so that 10 per cent of the action at ordinary temperatures still 

 remains. As every kind of chemical action so far examined is non- 

 existent at this extreme temperature, these experiments suggest that 

 S3I 1900 19 



