234 Prof. J. Dewar. Application of Liquid Hydrogen to the 

 FIG. 1. FIG. 2. FIG. 3. 



O 



of air in A, B, to enter and be condensed with the object of accumu- 

 lating any incondensable residuum. 



The tubes were filled with air, oxygen, and nitrogen at the atmo- 

 spheric pressure. The liquid hydrogen collected in the vacuum vessel, 

 immersed in another similar vessel full of liquid air, being ready, the 

 end C was dipped in the liquid for a little over a minute, and the tube 

 AB sealed off at D, so that on removal from the hydrogen bath the 

 solid air might melt and distil off without generating any pressure. 

 On attempting to pass the spark through vacuum tubes prepared in 

 this manner, their excellent exhaustion was revealed by great resistance 

 to the passage of the discharge, and the high phosphorescence of the 

 glass. Two tubes, kindly prepared by Sir William Crookes with 

 platinum electrodes that he had previously sparked to remove gases 

 and impurities on the glass before filling with dry air, gave, when 

 treated in the manner described, such high vacua that the tubes had 

 to be heated in order to get any spark to pass. Thus it is proved that 

 the tension of solid nitrogen and oxygen at the temperature of boiling 

 hydrogen is below the millionth of an atmosphere, seeing there is less 

 difficulty in getting a discharge to pass in tubes exhausted to this 

 extent. In order to get some definite idea of the limit of the exhaus- 

 tion produced, two tubes, such as have been described as suitable for 



