33Q POPULAR SCIENCE MONTHLY 



recent origin, but the knowledge gained from it not only confirms the 

 ideas formed to explain conduction in solutions, but has very widely 

 extended and simplified them. The chief difference between electric 

 conduction in solutions and conduction in gases arises from the large 

 number of broken molecules or ions always present in solutions. These 

 require only the presence of an electromotive force to start them march- 

 ing, but a gas, in its natural or non-conducting state, contains very few 

 ions, not enough to support even a very small current, and for this 

 reason gases are insulators. 



In gases, however, there are many ways of making ions, X-rays, 

 radium rays, rays of ultra-violet light on metals, combustion in flames, 

 white-hot bodies of every sort will do it. But there is one method which 

 depends on the violent collisions of ions with molecules which is so 

 objective in its form I can not forbear attempting to describe it. It is 

 also the method which leads us to cathode rays and much more. 



Imagine, then, a glass tube into each end of which a conducting rod 

 carrying a small metal disc is sealed. These rods may at will be con- 

 nected to the terminals of a battery. If while the tube is filled with a 

 gas, in its non-conducting state, the battery be applied, the very few 

 ions always present are set in motion, but the too frequent collisions in 

 the swarm of neutral molecules which obstruct the way prevent the 

 moving ions from attaining more than moderate speeds. 



By connecting the tube to an air pump as many as we like of the 

 interfering molecules may be removed. As more and more gas is 

 drawn out of the tube, the moving ions encounter fewer and fewer col- 

 lisions and in consequence attain higher and higher speeds, as small 

 shot might fall through a gradually dispersing swarm of bees poised in 

 midair. The longer the pumping is kept up the greater the maximum 

 speed of the ions becomes and the more violent are the collisions which 

 do occur. When nearly all of the gas has been drawn out of the tube, 

 a stage is reached where the encounters between flying ion and indif- 

 ferent molecule become so violent that molecules are shattered and new 

 ions produced, which in their turn work more destruction. 



When this stage is reached, the gas is a good conductor, but if the 

 pumping be carried too far, a second stage appears in which the en- 

 counters are too few to make enough new ions to support the current, 

 and the gas finally ceases to conduct systematically. It is near the end 

 of the conducting stage that the much-discussed cathode rays appear. 

 They depart from the cathode or metal disc in the end of the tube con- 

 nected to the negative side of the battery. 



The extraordinary resourcefulness, shown by the leading workers in 

 this field of recent enquiry, in untangling the complex snarl of phe- 

 nomena presented, marks a very great achievement. So inspiring from 

 the human side as well as the physical has been this unequal contest of 

 man with nature, of mind struggling against disorder, and so bravely 



