PHYSICS. 259 



Crool:es lias made remarkable progress in studying the phenomena 

 of molecular physics in high vacua, and has obtained results even more 

 novel and striking than those connected with his marvelous little in- 

 strument, the radiometer. In a paper read to the Royal Society Decem- 

 ber 5, 1878, he showed that the dark space which suiTounds the negative 

 pole in an ordinary vacuum tube illuminated by an induction coil, is the 

 result of intense molecular vibration which excites a molecular disturb- 

 ance in the surface of the disk and the surrounding gas. As the rare 

 faction increases, this layer of molecular disturbance increases in thick- 

 ness. At a pressure of 0.078"'"', it extends for 8'""' from the disk. It 

 is greatest in hydrogen and least in carbon dioxide. jS^umerous ex- 

 periments were devised to ascertain whether this visible layer of molec- 

 ular disturbance was identical with the invisible layer of molecular 

 pressure. A radiometer with aluminum disks, each coated with mica, 

 when made the negative electrode, revolved rapidly as soon as the dark 

 space is made by exhaustion to extend to the glass. Witli a cui)-shaped 

 aluminum electrode, the convergence of the lines of force to a focus was 

 very marked. When the exhaustion becomes very high the dark space 

 fills the tube, and where the rays after divergence fall on the glass, a 

 sharply-detined spot of greenish-yellow light appears. This greenish- 

 yellow phosphorescence of molecular impact is due to the particular 

 kind of glass used. Other kinds of glass give different colors. At 4 M 

 exhaustion (M refers to millionths of an atmosphere) no other light than 

 this is seen. At 0.9 M the i^hosphorescence reaches it maximum. At 

 0.15 M the spark passes with difticulty and the light appears in flashes. 

 At 0.06 M the vacuum is non-conducting, and only very intense sparks 

 will pass. Beyond this nothing has been observed. The rays producing 

 the green phosphorescence will not turn a corner. Hence a body in 

 their patli intercepts them and casts, as it were, a shadow on the glass 

 behind it. 



Mr, Crookes advances the theory, therefore, that the induction spark 

 actually illuminates the lines of molecular pressure caused by the elec- 

 trical excitement of the negative pole. The thickness of the dark space 

 is the measure of the mean length of the path between successive col- 

 lisions of the molecules. The extra velocity with which the molecules 

 rebound from the excited negative pole keeps back the more slowly 

 moving molecules which are advancing toward that pole. The con- 

 flict occurs at the boundary of the dark space, where the luminous 

 margin bears witness to the energy of the collisions. By using a rotat- 

 ing fly it may be caused to revolve with high velocity by directing the 

 molecular impact on one side of it. Moreover, these lines of molecular 

 force are markedly deflected by a magnet, the action being to twist the 

 trajectory of the molecules, the direction of twist when an electro-mag- 

 net is used being that of the electric current round the magnet. When 

 the magnet is so adjusted as to exert a uniform downward pull on the 

 molecules, the trajectory is much curved at low exhaustions, and gets 



