18 



[Feb. 21, 



or both of the terminals. The internal pressure is about one one-mil- 

 lionth of an atmosphere. The earliest form of vacuum tube, constructed 

 nearly fifty years ago, was exhausted to about one one-hundredth of an 

 atmosphere, and on the passage of an electric discharge, glowed 

 throughout its length with a purplish-blue color. As the efficiency of 

 the pump increased, higher vacuum became easy and the phenomenon 

 of the dark space about the cathode was described and exhibited to the 

 British Association by Crookes in 1879. As the exhaustion is increased 

 the dark space may enlarge so as to extend throughout the length 

 of the tube. Under these conditions, the position of the anode is of little 

 consequence, and under the action of the discharge the whole bulb 

 becomes fluorescent with green or blue according to the kind of glass. 



"Cathode rays " is a term applied to the disturbance which seems to 

 start at the cathode within the tube, and extend in straight lines to the 

 opposite side. These rays are capable of being deflected by a magnet, 

 and were supposed by Crookes to consist of the molecules of the resi- 

 dual gas projected with great speed from the cathode terminal and 

 Impinging upon the walls of the tube. In the language of molecular 

 kinetics, it may be said, then, that the mean free path of the molecule 

 in one of these highly exhausted tubes, has become greater than the 

 length of the tube. It was discovered in 1890 by Hertz that these 

 cathode rays can pass through some solid substances, e. g., aluminum, 

 while others he found to be opaque. Lenard, the assistant of Hertz, 

 in 1894, passed the cathode rays outside the tube, through a small 

 aluminum window, placed in the wall of the tube opposite the cathode. 

 This window had to be very thin to facilitate the issue of the rays, and 

 yet thick enough, compared with its size, to withstand the pressure of 

 the atmosphere. Consequently, the area was very small. Lenard also 

 obtained shadow records on photographic plates by interposing, between 

 the aluminum window and the plate, opaque bodies. 



The cathode rays wlien impinging upon the Lenard window do not 

 issue in a direction collinear with their former direction ; but seem to 

 spread in all directions like a beam of light passing beyond a very small 

 aperture. The transparency of substances for these rays seemed to be 

 closely related to their density. For example, in the case of gases, 

 hydrogen was found to behave like oxygen if it were compressed until 

 its density became equal to that of the oxygen. Transparency to these 

 rays seemed to have no relation to electric conductivity. 



With reference to leaving out the aluminum window and replacing it 

 by merely the glass of the tube, Lenard said {Electrician, Vol. 

 xxxii, p. 576) : "On replacing the aluminum window by one of glass, 

 it was found "possible to repeat all the essential experiments with equal 

 success. But the aluminum remains the more suitable, not that it is 

 the more transparent, but because aluminum is opaque to light, and 

 more easily manipulated than glass of equal thickness." So we see that 

 Lenard actually obtained results in about the same way that we are ex- 



