52 PHYSICAL SCIENCE 



By this means liquid hydrogen was first 

 collected in an open vessel on loth May 1898, 

 though two years before it had been seen as 

 spray in the jet of gas issuing from a simpler 

 apparatus of the same essential form. When 

 about 20 cubic centimetres of liquid had been 

 collected, the later experiment failed, owing to 

 the stoppage of the exit by frozen air — a very 

 common accident in dealing with liquid hydrogen. 



By working with carefully purified gas, much 

 larger volumes were soon obtained, and the writer 

 has a vivid memory of an afternoon in June 1901, 

 when Professor Dewar had transported some five 

 litres of liquid hydrogen from the Royal Institution 

 to the rooms of the Royal Society, and gave his 

 first public demonstration of its extraordinary 

 properties. On that occasion liquid hydrogen 

 flowed like water for the first time. Its produc- 

 tion in any quantity is now simply a matter of 

 expense. 



By carefully isolating a portion of liquid 

 hydrogen and preserving it, in a manner shortly 

 to be described, from the access of heat from 

 without, it is, when suddenly exhausted under an 

 air-pump, transformed into a mass of solid frozen 

 foam. By immersing a tube containing the liquid 

 in this frozen foam, a quantity of the clear trans- 

 parent ice of solid hydrogen can be obtained. 



Kept in an open vessel, liquid air and 

 liquid hydrogen are analogous to the water in a 

 saucepan boiling over a fire. At the normal 

 atmospheric pressure, water boils at 100° C, and 

 the rate at which it evaporates depends simply 

 on the rate at which heat enters it — depends, that 



