Electrification of a Steam-jet. 161 



interest, is given in Wiedemann's Annalen, vol. xxxii. p. 1. 

 The author appears never to have examined his jets by trans- 

 mitted light, and makes no mention of the marvellous increase 

 of opacity under electrification, to which I have called atten- 

 tion. His observations were all made against a dark back- 

 ground, the illuminating beam of light coming obliquely from 

 the front, but shaded from the eyes. When the jet is elec- 

 trified, he says, it is at once seen more clearly and sharply 

 against the background ; it also assumes diffraction colours, 

 like those seen in strata of fog. Very strong electrification 

 produces a deep blue colour, indicating the formation of very 

 small mist drops. On slowly diminishing the electrification 

 the blue tint at first becomes gradually paler, which points 

 to the formation of larger drops ; and it is then succeeded 

 by tints of purple, red, yellow, green, and finally, when the 

 discharge is very feeble, by pale blue tones again. Under 

 certain circumstances all these several tints may be seen at 

 once in different portions of the jet. 



I, too, had observed these diffraction colours, which were 

 sometimes very beautiful ; but having convinced myself that 

 they undoubtedly occurred at times when no electrical in- 

 fluence (that I knew of) was operating, I did not follow up 

 the observation. 



Helmholtz found that these condensation phenomena could 

 be just as well produced by some other causes as by electrifi- 

 cation, and especially by the agency of flame. All the ex- 

 citing causes in question involve some continuous chemical 

 action inside the steam-jet. He conjectures, therefore, that 

 the sudden condensation may be due to molecular tremors or 

 shocks, which upset the unstable equilibrium of the super- 

 saturated vapour, just as a small disturbance will sometimes 

 cause the sudden crystallization of a supersaturated saline 

 solution ; and after showing that chemical reactions may occur 

 freely even outside the visible portion of a flame, he states 

 his opinion that similar dissociations and recombinations take 

 place among the molecules of the air affected by an electric 

 discharge, as evidenced, for instance, by the formation of 

 ozone. 



A very striking illustration of the effect of combustion is 

 afforded by holding beneath the jet a piece of burning touch- 

 paper, made by soaking blotting-paper in a solution of nitrate 

 of lead. The jet at once becomes quite as opaque as if elec- 

 trical influence were employed, and the same hissing noise is 

 also heard. The burning touch-paper suggested itself as 

 more convenient than the actual flames used by Helmholtz 

 for the purpose ; for these cannot be brought underneath the 

 jet without causing its immediate dissipation ; and it is there- 



