544 Professor Sir James Dewar [Jan. 22, 



vacuum vessel containing liquid air or hydrogen. A gramme of 

 charcoal is placed in the sealed end of one tube in each pair. For 

 convenience of pressure observations, a barometric tube is placed 

 alongside each pair of tubes in the three separate little bottles of 

 mercury. 



AVhen the closed end of the air tube is cooled in liquid air, only 

 a small contraction is observed, as shown by a slight rise of the 

 mercury in the tube. If, however, the air tube which contains the 

 charcoal is cooled all the air is thereby condensed, and the mercury 

 quickly rises to the barometric heiglit. Xow take the pair of 

 hydrogen tubes. As before, on cooling the tube without char- 

 coal, only a slight contraction is shown, but when the charcoal 

 tube is immersed in the liquid air, a quantity of the hydrogen is 

 condensed in the charcoal, and the mercury rises in the tube. The 

 height the mercury attains, however, is noticeably less than in the 

 aii'-charcoal tube, showing the smaller condensability of hydrogen in 

 the charcoal at liquid air temperature. When either of the helium 

 tubes are cooled with liquid air, practicaHy no condensation is shown. 



Now instead of liquid air, let us use liquid hydrogen, and cool 

 first the air tube. Immediately the mercury rushes up to the baro- 

 metric height, all the air being condensed into a sohd of inappreciable 

 tension of vapour, and the charcoal tube behaves in the same way. 

 Now pass to the hydrogen tubes. When the plain gas tube is 

 cooled, quite a noticeable contraction is visible, because the tempera- 

 ture of liquid hydrogen is so low compared to hydrogen gas at the 

 room temperature. Complete absorption is produced when the 

 charcoal-hydrogen tube is cooled and the mercury rises to the baro- 

 metric height. The hydrogen, at the temperature of its own boiling 

 point, is completely absorbed in the charcoal. 



Now compare this with the set containing helium. The helium 

 tube when cooled behaves similarly to the plain hydrogen tube, but 

 on cooling the charcoal tube, quite a large diminution of pressure is 

 produced, showing that even helium is condensed to a considerable 

 extent by charcoal at 20' absolute. As the charcoals warm up, the 

 condensed gases are again expelled : the helium very rapidly, the 

 hydrogen somewhat more slowly, and the air after some few minutes. 



Appaeatus employed for Measurements of Hydrogex and 

 THE Rare Gases uncondensed at the Boilixg Point of 

 Hydrogen. (Plate 1.) 



In order to avoid indiarubber connections, vitiating the accuracy 

 of the values of minute quantities of gas, the gas to be examined was 

 contained in a flask A of suitable size, attached by a coned ground 

 joint T. The gas was allowed to enter the exhausted apparatus by 

 opening the cock B, which also served to connect to the pump for the 

 preliminary exhausting of the whole apparatus. To remove hydrogen 

 and hydrocarbons the part between B and the measuring burette C 



