PRESIDENTIAL ADDRESSES. 507 



treme of pressure might be capable of liquefying 

 hydrogen or oxygen at ordinary temperature, 

 while a very moderate pressure might suffice to 

 liquefy them if their temperatures could be suffi- 

 ciently lowered. Professor Dewar actually showed 

 liquid oxygen in a glass tumbler, not boiling or 

 in a state of commotion like a tumbler of soda- 

 water, but quietly and without any sensible motion 

 keeping itself cool by its own evaporation, while 

 it rapidly formed a thick jacket of hoar-frost on 

 the outside of the vessel by condensation of 

 watery vapour from the surrounding atmosphere. 

 The surprise and delight of the audience reached 

 a climax when liquid oxygen was poured from one 

 open vessel to another before their eyes. 



A matter of great importance in respect to the 

 health of the community was submitted to the 

 Royal Society by the London County Council, in 

 a letter of date May I, 1891, asking for informa- 

 tion and suggesting investigation regarding the 

 vitality of microscopic pathogenic organisms in 

 large bodies of water, such as rivers which are 

 sources of water-supply and which are exposed to 



