110 MODERN THEORIES OF ELECTRICITY AND MATTER. 



Such a current is called a current of saturation. It is constant for a 

 given intensity of radiation independent of the sign of the electric 

 charge. 



An important difference shows itself between the properties of posi- 

 tive and negative ions. This difference is easily shown by the gases 

 of flames. These gases are ions and conductors, and the approach of 

 flame promotes the electric discharge. Contact with the flame is not 

 necessary. It is sufficient that the ions are produced within the region 

 covered by the electric field. The attraction of the charge of the 

 electroscope suffices to draw from the flame the ions of contrary sign, 

 which neutralize it, and this phenomenon takes place, whatever the 

 sign of the charge. But an isolated flame placed between the two 

 plates of a charged condenser inclines toward the negative field; 

 hence we conclude that the flame is then charged positively. This 

 is because the negative ions produced in the flame are smaller and by 

 far more active than the positive ions, so that they are more easily 

 drawn from the flame, and thus there is left with it an excess of posi- 

 tive electricity. In a cold gas the positiA'e and negative ions have a 

 nearly equal mobility, which is less than that found in a warm gas. 

 They are thought to be in this case formed by the agglomeration of 

 molecules grouped by electrostatic attraction about the charged 

 centers. The dissimilarity between positive and negative ions mani- 

 fests itself in certain cases even in their formation. This is shown, for 

 example, in what is called the phenomenon of Hertz : Certain nega- 

 tively charged metals, such as zinc, lose their charge when illuminated 

 by ultraviolet light, but if the charge is positive the illumination pro- 

 duces no discharge. It seems to be proved now that zinc and some 

 other easily oxidizable metals has the property of spontaneously giving 

 off electrons under the action of ultraviolet rays. If the emission is 

 given oft' in a vacuum the electrons are able to acquire a very high 

 velocity in an electric field, and they comport themselves then like 

 the cathode rays of Crookes tubes. If the emission takes place in 

 the air at ordinary pressure the electrons surround themselves with 

 agglomerations of neutral molecules, and form ions of little activity, 

 like those ions which are formed in the air by the Roentgen rays. But 

 in either case the discharge is nonreversible and takes place only if the 

 metal is negatively charged, for the metal is not able to emit negative 

 electrons if the departure of them is obstructed by the attraction of a 

 positive charge residing upon the metal. 



Thus we see why it is that gases may become conductors mider the 

 influence of certain radiations, or of the combustion of flames. It 

 has been known for a long time, however, that Avithout any of these 

 influences a gas can not prevent the passage of electricity when the 

 field is sufficiently strong. The phenomena of the disruptive dis- 

 charge, including the spark, the arc, and the brush discharges, have 



