216 Centenary Commemoration, 1799-1899. [June 7, 



liquefaction we must increase the pressure of the gas, or reduce the 

 boiling point of the liquid hydrogen by exhaustion. Pure hydrogen 

 liquefied in a closed vessel is perfectly clear, showing no trace of 

 colour 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 vapour of boiling hydrogen is about fifteen 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 denser than the vapour coming off. In the case of oxygen 

 the density of the liquid is 255 times that of the vapour at its 

 boiling 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 brought into the 

 magnetic field, 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 liquid hydrogen is not magnetic, but when the 

 solid air snow is in suspension in the fluid, then the magnetic 

 character of the latter becomes apparent when the vessel is placed 

 in the magnetic field. 



All the phosphorescent effects 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, ultra-violet light had to be employed, and then the solid 

 body, 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 

 chemistry 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 ultra-violet 

 rays are not now essential. It is strange to find photographic action 

 still relatively considerable. At the boiling point of liquid air the 

 photographic intensity is reduced by 80 per cent, of the value at the 

 ordinary temperature. The photographic effect on a sensitive film im- 

 mersed in liquid hydrogen as compared with the same placed in liquid 

 air is as one to two, so that 10 per cent, of the action at ordinary tem- 

 peratures still remains. As every kind of chemical action so far ex- 

 amined is non-existent at this extreme temperature, these experiments 

 suggest that the cause of the photographic action may be essentially 

 physical. No better illustration could be given of the rapid diminu- 

 tion of chemical action at low temperatures than to remind you that 

 fluorine gas, the most active elementary body, under such conditions, 

 may be liquefied and kept in glass vessels. 



The effect of a temperature of 21° absolute on the electric re- 



