MODERN THEORIES OF ELECTRICITY AND MATTER. Ill 



long- been known, and they take various and complicated forms in 

 the air at different pressures ; but luitil recently they have been very 

 little understood. The theory of gaseous ions has thrown a new light 

 upon this manner of discharge. As a result of recent researches, the 

 disruptive discharges can be explained by assuming that the ions 

 which have acquired a sufficient velocity under the action of an elec- 

 tric field are able to act as projectiles, which, coming in contact with 

 the molecules of gas, ionize them by the shock Avliich they produce. 

 Negative ions are much more active ionizing agents than the posi- 

 tive ions, and can produce these effects in more feeble fields. It may 

 be conceived, then, that the ions being multiplied l)v the shock of 

 those already present, the conductibility of the gas becomes very 

 great when the field is sufficiently strong, and the ionized gas is then 

 luminous. 



The cathode rays, which are produced when the discharge is made 

 to pass in a tube containing a gas under low pressure, are the elec- 

 trons sent off by the cathode Avith a great velocity. Since these elec- 

 trons and the ])ositive ions have different pr()})erties, the discharge 

 tube takes on the well-known dys-symmetrical appearance, which the 

 theory of the ions readily explains, but for which no other interpre- 

 tation has sufficed. 



The Roentgen rays, which are emitted from a Crookes tube, are 

 believed to be in reality electromagnetic waves whose Avave length 

 is very short. Such waves as these are supposed to be emitted by an 

 electron Avhenever it is subjected to an abrupt acceleration, such as 

 is produced, for examj^le, when the electrons of a metal are put in 

 A'ibration by the impact of cathode rays. 



In accordance with what has been said, all gases Avhich show them- 

 selves conducting contain the charged centers Avhicli we call gaseous 

 ions. The presence of these charged centers may be made evident 

 by means of a very curious experiment, Avhich utilizes the property 

 which ions haA^e of promoting the condensation of supersaturated 

 water vapor. When the volume of a certain mass of saturated Avater 

 A^apor is quickly increased the vapor condenses to the extent to Avhich 

 it is supersaturated, but if the supersaturation is not A'ery great, and if 

 the A'essel contains no dust, there is no noticeal)le condensation at the 

 moment of change, and the gas I'emains transjiarent ; but Avhen the 

 gas contains ions the condensation takes place readily — that is to 

 say, Avith a smaller expansion. It is eas}^ to regulate the expansion 

 so that there Avill be no condensation Avhen the gas is not ionized but 

 an abundant condensation if ionized. In the latter case the con- 

 densation manifests itself by the formation of an opaque cloud Avhich 

 fills the receptacle. luA'cstigation of this phenomenon has shoAvn 

 that the globules of water, Avhich constitute the cloud, form them- 

 selves upon the ions, each of which serves as a center for one of them. 



