1891.] On Electrical Evaporation. 99 



FIGK 11. 



B 



arranged to observe the light of the volatilising silver "end on," as 

 shown in the figure. In this way the deposit of silver offered no 

 obstruction to the light, as none was deposited except on the sides of 

 the tube surrounding the silver. At a vacuum giving a dark space 

 of about 3 mm. from the silver, a greenish-white glow was seen to 

 surround the metal. This glow gave a very brilliant spectrum. The 

 spark from silver poles in air was brought into the same field of view 

 as the vacuum glow, by means of a right-augled prism attached to 

 the spectroscope, and the two spectra were compared. The two 

 strong green lines of silver were visible in each spectrum ; the 

 measurements taken of their wave-lengths were 3344 and 3675, 

 numbers which are so close to Thalen's numbers as to leave no doubt 

 that they are the silver lines. At a pressure giving a dark space of 

 2 mm. the spectrum was very bright, and consisted chiefly of the two 

 green lines and the red and green hydrogen lines. The intercalation 

 of a Leyden jar into the circuit does not materially increase the 

 brilliancy of the lines, but it brings out the well-known air lines. 

 At this pressure not much silver flies off from the pole. At a higher 

 vacuum, the luminosity round the silver pole gets less and the green 

 lines vanish. At an exhaustion of about one-millionth of an atmo- 

 sphere the luminosity is feeble, the silver pole has exactly the appear- 

 ance of being red hot, and the volatilisation of the metal proceeds 

 rapidly.* 



* Like the action producing volatilisation, the " red heat " is confined to the 

 superficial layers of molecules only. The metal instantly assumes, or loses, the 

 appearance of red heat the moment the current is turned on or off, showing that, 

 if the appearance is really due to a rise of temperature, it does not penetrate much 

 below the surface. The extra activity of the metallic molecules necessary to 

 volatilise them is, in these experiments, confined to the surface only, or the whole 

 mass would evaporate at once, as when a metallic wire is deflagrated by the dis- 

 charge of a powerful Leyden jar. When this extra activity is produced by arti- 

 ficial heat one of the effects is the emission of red light j so it is not unreasonable 



H 2 



