66 PHYSICAL SCIENCE 



phosphorescent at ordinary temperatures, cease 

 to be so when cooled. Some crystals, such as 

 those of uranium nitrate, become self-luminous 

 in liquid hydrogen, apparently owing to intense 

 electric forces set up by the cooling. These 

 forces may become so intense that discharges 

 take place which are powerful enough to be 

 visible in the dark. 



It will be seen from this account that the 

 changes in physical properties are more striking 

 and complete in the range of temperature below 

 the freezing-point of water than in the corre- 

 sponding range of temperature above that point. 

 On the other hand, it is very striking that in 

 biological problems, more especially in those 

 connected with the lowliest forms of animal and 

 vegetable life, a hundred degrees above the 

 freezing-point is productive of a more complete 

 and destructive change than a hundred degrees 

 below. While exposure to the boiling-point of 

 water, or to a temperature a few degrees higher, 

 suffices to kill all known forms of living organisms, 

 many forms of bacteria merely have their vitality 

 temporarily suspended in liquid air. Even seeds 

 of barley, peas, etc., were not permanently affected; 

 in fact, they have been placed for six hours in 

 liquid hydrogen with no effect on their subsequent 

 power of germination. 



In closing this account of low temperature 

 research it may be of interest to tabulate some 

 of the more important temperature-constants now 

 known to mankind. In doing so, we cannot fail 

 again to be struck by the high temperatures 

 easily obtainable. On the other hand, to cool an 



