THE ELECTRON THEORY. 287 



tube, which is shown in Fig. 209, is the small platinum tube / 

 which is sealed through the glass wall. When the vacuum in 

 the Crookes tube becomes too high (presumably by the trans- 

 formation of the residual gases into non- volatile products), 

 the small tube / is held for a few seconds in the flame of an 

 alcohol lamp and a sufficient amount of hydrogen passes through 

 the hot platinum to replenish the supply of gas in the Crookes 

 tube. 



144. Conductivity of flames. A flame is a fairly good elec- 

 trical conductor and this conductivity has been found to be due 

 to the presence of free ions.* The conductivity of a flame is 

 shown by the fact that a charged glass rod may be completely 

 discharged by passing the flame of a Bunsen burner rapidly 

 over its surface. 



145. The electric arc. In order to produce a perceptible 

 discharge of electricity (flow of current) through a gas, a very 

 high electromotive force must be used because of the necessity 

 of producing ionization in the gas by the collision of the moving 

 ions with the gas molecules; and the amount of current which 

 can be made to flow through a gas is usually very small because 

 of the comparatively small number of these ions. When, how- 

 ever, metal or carbon electrodes are heated to a very high tem- 

 perature they emit electrons (negative ions) in great numbersf 

 and a very considerable current may then be made to flow through 

 the intervening gas. Thus, a current of an ampere or more may 

 be made to flow between a cold metal anode and a very hot metal 

 cathode in a vacuum tube. When two carbon rods are con- 

 nected to a battery or dynamo, brought into contact and then 

 separated, the current which begins to flow across the indefinitely 

 small gap between the two carbon rods raises the tips of the car- 

 bons to a very high temperature so that electrons (negative ions) 

 are emitted in great numbers. The result is that the current 

 continues to flow between the carbon tips. The column of hot 



* See J. J. Thomson's Conduction of Electricity Through Gases, pp. 228-249. 

 t See J. J. Thomson's Conduction of Electricity Through Gases, pp. 188-227. 



