CATHODE RAYS AND RONTGEN RAYS. 279 



In conclusion, two other characteristics of the cathode rays must be 

 noticed. The first consists in the power that they transmit to gases 

 through which they pass, of conducting- electricit3^ Gases in a dr}^ 

 state, as is well known, are nonconductors; an electrified body, for 

 instance, a gold-leaf electroscope or a condenser, holds its charge. If 

 it sometimes appears otherwise it is because the gas is not dr}', and the 

 diminution should then be attributed to the vapor of water. But if 

 a cathode ray just comes in contact with air which is really dr}^, 

 near this apparatus, the latter is seen to discharge itself at once. The 

 gas has acquired a certain degree of conductivity'. This same prop- 

 erty belongs, as we shall soon explain, to Rontgen ra3's and to Bec- 

 querel rays. This characteristic is common to all these radiations, 

 and is probably the one which can be easiest investigated, and even 

 measured. B}- means of an electroscope inclosed in a box full of dry 

 air these divers radiations are studied. By this process Mme. and M. 

 Curie discovered the new radio-active bodies, polonium and radium, 

 and M. De))ierne b}" the same means discovered actinium. 



The last peculiarity is also common to these three kinds of radiation, 

 as well as to every species of electric current. It consists in this, that 

 both efiect condensation of the vapor of water when the latter is near 

 its point of saturation, producing a kind of mist. This mist, which 

 forms instantly on the passage of the current, or of the rays, becomes 

 a visible and palpal)le sign of their presence. It is a beautiful lecture 

 experiment and one easily reproduced for public exhibition, and has 

 often been repeated within the last two or three years. The invisible 

 vapor escapes from a narroAv tube connected with a fiask full of boil- 

 ing water; on approaching to it a metallic point strongly electrified 

 and from which the fluid escapes in the form of an aigrette that can 

 easil}^ be distinguished in the dark. As soon as contact has been made 

 the jet of steam assumes the aspect of a dense mist or of a thick smoke. 



Allusion may be made to the possible applications of this phenom- 

 enon to meteorology without insisting upon them. There is another 

 curious application which was made by J. J. Thomson in measuring 

 the number of cathode projectiles which exist in a given space at a 

 given moment. By combining this calculation with electro-metric 

 investigations it has been possible by skillful comparison to determine 

 the negative charge borne by each cathode projectile, and, finally, its 

 mass. The latter is extremeh' small. 



The cathode rays of a single pencil are not all identical. The velocity 

 of propagation is not equal, and that is the reason why a magnet deflects 

 them unequally, just as a prism bends unevenly the rays which form 

 a beam of solar light. There is magnetic dispersion and a magnetic 

 spectrum for the ra3's emanating from the cathode, exactly like the 

 luminous dispersion and luminous spectrum formed with the sun's 



