76 Intelligence and Miscellaneous Articles. 



by himself*, and in a second one in conjunction with myself 

 published after his death f, showed that not only does fine dust 

 produce condensation in a jet of steam, as already well known, but 

 that a series of chemical and electrical processes, in which isolated 

 atoms are formed in the air, or in other gases, and have access to 

 the jet of steam, also favour an increased production of fog. It 

 appeared therefore probable from this that the Eontgen rays 

 would increase the condensation in a jet of steam. 



It was easy to show that the rays issuing from what is called a 

 "focus-tube," as described by Hr. Walter Konigt and Prof. Eontgen, 

 act on the jet. This experiment is not, however, quite conclusive, 

 for, according to our previous observations, the sudden variations of 

 the electrical force in the discharge through the tube might directly 

 affect the jet of steam. To exclude this disturbing influence it 

 would be necessary to be in a closed metal box together with the 

 steam-jet as described by Prof. Eontgen in his second communi- 

 cation. 



I have been satisfied with using a paper screen 160 cm. long 

 and 105 cm. high, lined on both sides with thick lead foil. The 

 discharge- tube was just in front of the middle of one side of this 

 screen, which had a small window closed by aluminium foil for 

 allowing the Eontgen rays to pass. On the other side of this 

 window was the place where the jet issued into the air, the place 

 which acts most sensitively. Sparking from the conducting wire 

 to the screen was prevented by an interposed glass disk. The 

 metal coating and the aluminium foil of the screen, which are put 

 to earth, do not allow variations of the electrical force to pass, while 

 Eontgen rays pass the aluminium window with but little enfeeble- 

 ment, as shown by the fluorescence of a barium platinocyanide 

 screen. If the tube shows strong radiation the phenomenon of 

 the steam-jet can be seen with certainty though not very distinctly, 

 especially if the hammer of the induction-coil is alternately held 

 fixed, and then its play allowed again. 



This is again a fresh case in which on the one hand the occur- 

 rence of isolated atoms — of ions — in the air is probable, and on 

 the other the occurrence of the phenomenon of the steam-jet is 

 demonstrated. Since the last publication by E. v. ITelmholtz and 

 myself, J. J. Thomson has given a perfectly satisfactory explanation 

 of the manner in which ions promote the formation of fog§. This 

 depends on the fact that the formation of drops of water in the 

 strong electrical field of the charged atoms is accompanied by a 

 decrease of the potential energy of the field of the charged atoms, 

 and that this decrease exceeds the increase of potential energy 

 which is always connected with the formation of the smallest drops 

 on account of the surface tension. — Wiedemann's Annalen, no. 11, 

 1896. 



* Wiedemann's Annalen, vol. xxxii. p. 1 (1887). 



t Ibid. vol. xl. p. 161 (1890;. 



% Verh. cler physik. Gesellschaft zu Berlin, xv. p. 74 1896). 



§ J. J. Thomson, Phil. Mag. (5) vol. xxxvi. p. 313. 



