THE WORLD BEYOND OUR SENSES 



So, too, we might make magnets out of the oxy- 

 gen of the air, then solid like ice, and it is possible 

 that could helium and other of the newly discovered 

 gases be likewise solidified, we should have sub- 

 stances so transparent to the light that they could 

 not be seen. 



Incidentally it may be noted that these extremes 

 of cold have given new weapons alike to the chemist 

 and the physicist. With solid air, and still more 

 with solid hydrogen, it is possible to produce almost 

 instantly the highest vacuum attainable by me- 

 chanical means. A tube, exhausted with an or- 

 dinary air-pump and subjected to this degree of 

 cold, condenses the remaining air or gas to a solid, 

 leaving a vacuum of scarce one -millionth of an 

 atmosphere. Through such an emptiness, sound, 

 which is the vibration of the air, cannot penetrate, 

 and through it a current of electricity, always 

 bound up, so it seems, with an actual transport of 

 matter, can be forced only with extraordinary ef- 

 forts. There is so little " matter" left, even of ten- 

 uous gas, that a minute gap is an almost perfect 

 insulator. 



On the other hand, the metals, at these low 

 stages, lose almost entirely their resistance to the 

 passage of electricity, and become almost perfect 

 conductors. In other words, it seems as if electrical 

 resistance was due solely to the mechanical vibra- 



59 



