THE LIQUEFACTION OF GASES 65 



obtained with liquid helium. As explained above, 

 the electrical resistance of a pure metal increases 

 generally nearly in proportion to the absolute 

 temperature, and diminishes equally as the tem- 

 perature falls. Nevertheless, most metals seem 

 to reach a constant small resistance as they 

 approach the absolute zero, and this residual 

 resistance is increased by traces of impurities. 



But Onnes discovered that with pure mercury 

 he got a sudden and almost complete destruction 

 of resistance at 4°. 2 absolute, a sudden drop in 

 resistance to about the millionth part. He got 

 similar effects with tin at f.S and with lead at 

 6° absolute, but other metals which he examined 

 gave no such results. The conductivity is so high 

 that an electric current once started in a coil of 

 wire continues to flow almost indefinitely, falling 

 in strength by less than i per cent, per hour. 



Applying a magnetic field of slowly increasing 

 strength, Onnes found that, at a certain critical 

 magnetic force, tin and lead when cooled to these 

 temperatures gave a sudden increase in resist- 

 ance, an observation which shows that this state 

 of super-conductivity is connected with magnetic 

 phenomena. Its real meaning is not yet clear, 

 though tentative theories of the state have been 

 offered by Onnes, Lindemann, and Thomson. 



From the point of view of the popular lecture- 

 room, some of the prettiest effects given by liquid 

 air depend on its power of imparting phosphor- 

 escence to many substances which do not usually 

 possess this property. Ivory, egg-shells, paper, 

 cotton-wool, and many other things glow brightly 

 in liquid air after they have been exposed to light. 

 On the other hand, certain sulphides of calcium, 



F 



