GASES FROM THE STANDPOINT OF PHYSICS. 47 



to stand out at an angle which measures the amount of 

 electrification. Experience always shows that the charge 

 leaks gradually away, Fig. 8, and the gold leaf falls slowly 

 down to the brass plate. So small, however, is 

 this leakage that physicists have from time to 

 time refused to believe that the air or sur- c 

 rounding gas had conducted it away of itself, 

 and have ascribed the leakage either to defect- 

 ive insulation or to the presence of dust parti- 

 cles in the gas concerned. Nowadays we know A 

 positively that such leakage does really occur , An j 

 through ah' or any normal gas, and that, con- scope.) 

 sequently, we must call a gas, to that extent, at AC. Brass 



least, a conductor. /fS?" 



i- i- i_x t * i j j. R Gold Leaf. 

 In many ways this slight electrical conductiv- ^/BCA. An- 



ity of a gas may be increased. Gases become gle of Electri- 

 conductible, for instance, (1) when they are fication. 

 heated up to a certain point, or when they are drawn from 

 the neighbourhood of flames or an electric arc; (2) when 

 they have recently been in contact with glowing metals or 

 carbon; and (3) when they have diffused through a space 

 through which an electric discharge is passing or has passed. 

 Again (4), a gas becomes conductible when X-rays pass 

 through it, or (5) cathode rays. Further (6), the mere 

 presence of certain substances is sufficient to cause this con- 

 ductivity substances such as the compounds of uranium, 

 thorium, radium, polonium and actinium. And, finally (7), 

 gases become conductors under the influence of ultra-violet 

 light, or (8) when they are passed over molten phosphorus 

 or bubbled through water. 



Having, now, learned so much of the electrical conductiv- 

 ity of gases, we must proceed, in another chapter, to dis- 

 cover where this knowledge leads us in our search. 



