THE GROWTH OF PHYSICAL IDEAS 105 



through an evacuated glass vessel. But the earliest systematic 

 research on the subject was that of Faraday, ^vho in 1836 

 began to study the passage of electricity through gases. He 

 observed that if t^vo electrodes were sealed into a bulb that 

 was then evacuated, and electricity ^vas passed through it from 

 a frictional electricity machine, there appeared Tvhat he de- 

 scribed as a "light" proceeding from the negative electrode. 

 Faraday was limited both in the supply of electricity from 

 his frictional machine and in the vacuum that he could ob- 

 tain by the use of a piston pump. 



Improved methods of obtaining high vacuum by filling 

 the bulbs with carbon dioxide and then absorbing^ it Tvdth 

 caustic potash made possible greater experimental progress. 

 J. Gassiot in 1859 described experiments in which he un- 

 doubtedly obtained the beam that became knoAvn as a cathode 

 ray. The passage of electricity through exhausted tubes con- 

 taining a small amount of gas became very popular as a dem- 

 onstration, and Heinrich Geissler, a German glass blower, 

 became so skillful in the preparation of the tubes that they 

 are known as Geissler tubes. 



Another technical development was the introduction by 

 Ruhmkorff in Paris of the induction coil, which facilitated 

 the production of high electrical voltages. In 1869 J. ^V^ 

 Hittorf published a first communication on electrical con- 

 ductivity in gases. This work was concerned ^vith ^vhat he 

 calls the negative discharge^ now called the cathode rays. He 

 pictured it as thin, flexible filaments, carrying currents, 

 that could be deviated by a magnetic field and that, by a 

 suitable arrangement of the field, could be focused, and he 

 observed an intense heating at the focus. Hittorf also dis- 

 covered the use of the incandescent cathode, with which a 

 current could be maintained through the tube with a very 

 small voltage. 



Hittorf's experiments were repeated by "VV^illiam Crookes, 

 who carried out a very large number of observations on elec- 

 trical charges and exhausted tubes. Crookes showed that the 

 cathode rays had sufficient momentum to drive a small paddle 



