384 Prof. H. E. Armstrong on Low-Tem]jerature Research 



molecules are probably given off even at ordinary temperatures in a 

 vacuum. It cannot be decided whether " moisture " was in any way 

 concerned with the occurrence of change in these experiments ; there 

 must have been some moisture present, as cooling the tube during 

 the exhaustion would have the effect of retaining a certain amount 

 condensed on the cold surfaces. 



An even more striking experiment described by Sir James is 

 that with phosphorus. A bulb A of the shape shown (Fig. 11), to 

 which is attached a chamber D containing charcoal covered with a 

 layer of phosphoric anhydride " to absorb all traces of moisture," 

 also a small mercury gauge G and a short capillary branch P con- 

 taining a little phosphorus, having been thoroughly exhausted, is 



S Hg S Hg 



Fig. 9. 



Fig. 10. 



filled at atmospheric pressure with oxygen and then sealed. On 

 immersing the charcoal chamber in liquid air the pressure soon falls 

 to a fraction of a millimetre ; then suddenly the chamber A 

 becomes filled with a phosphorescent glow, which is clear indication 

 of the occurrence of chemical change. The pressure having fallen to 

 a point at which it can no longer be measured by the gauge, a stage 

 is reached when the glow disappears and only phosphorus distills 

 into the charcoal condenser. When the charcoal is no longer cooled 

 and oxygen escapes from it, the phenomena are reversed : as the gas 

 meets the phosphorus in the bulb A, the occurrence of an interaction 

 is marked by oscillating flashes ; soon all is dark again. The demon- 

 stration is a most fascinating one, as I can testify, having witnessed 

 it several times. It may be supposed either that some particular 



