442 SCIENCE PROGRESS. 



atom can so receive a charge. Under ordinary circum- 

 stances it is certain that gas molecules cannot acquire a 

 charge until the electrical tension rises to the disruptive 

 point; but there is a certain condition into which a gas can 

 be thrown, similar, if not identical, with that which chemists 

 speak of as dissociation, wherein a gas becomes a conductor, 

 that is to say, its particles do really act as carriers of electric 

 charges, and may be spoken of as detached and specifically 

 charged atoms. 



Now in a vacuum tube, w T e learn experimentally from 

 Mr. Crooks, that at high vacua the negatively charged atoms 

 are vigorously repelled from a negative electrode, and, 

 shooting out from it in straight lines, constitute what are 

 known as cathode rays. It appears as if the electric 

 discharge itself were carried on in a vacuum tube by a quiet, 

 imperceptible, electrolytic action, originating at the anode or 

 positive electrode, that this discharge fills the whole tube 

 with positive electrification up to within a short distance of 

 the cathode. In this short distance there is accordingly a 

 steep potential gradient, and any stray negative atoms 

 finding themselves therein are shot out of it with immense 

 velocity, and constitute what are called cathode rays. 

 Some doubt has been felt as to the essential nature of 

 cathode rays, but there is hardly any good reason for the 

 belief that they are anything else than a stream of negatively 

 charged atoms of matter. They need not have recently 

 received a charge, their charge may be intrinsic ; what we 

 observe is their repulsion, not as if guided through a resist- 

 ing medium by electric force, but as if propelled violently 

 inside a thinned layer and left to the first law of motion 

 nearly. 



Great interest has been felt in this cathode stream for 

 a quarter of a century, but within the present year its im- 

 portance has become immense owing to the discovery of 

 Rontgen that a surface on which the stream impinges be- 

 comes capable of emitting a novel kind of radiation which 

 travels even more persistently in straight lines, and is not 

 readily stopped by material obstacles. This discovery 

 must ultimately throw a great deal of light upon the whole 



