158 Prof. J. Trowbridge on Spectra arising 



thousand amperes to ten thousand. The revolving-mirror 

 method showed that the pilot spark was mainly effective, and 

 that the subsidiary oscillations were feeble. The spectrum- 

 tube speedily became milk-white from the sodium set free 

 from the glass. Lord Eayleigh has shown how to demonstrate 

 the presence of argon from very small quantities of air (Phil. 

 Mag. [6] vol. i. p. 108, 1901). My method is substantially 

 his, except that I employ very powerful discharges which set 

 free a sufficient amount of sodium-vapour from the glass : 

 ;and the oxygen is supplied from the dissociation of water- 

 vapour which is always in evidence when powerful discharges 

 •are employed. The production of argon under these circum- 

 stances I regard as a striking proof that I am dealing i n this 

 investigation with the spectra arising from the dissociation 

 of water-vapour. From the same tube one can, by modifyino- 

 the strength and character of the electrical discharges, obtain 

 what is generally termed the four-line spectrum of hydro oen, 

 the spectrum of sodium, the spectra of argon, and the spectrum 

 arising from the dissociation of water-vapour. Doubtless one 

 could recognize also the spectrum of helium ; I am not yet 

 sufficiently familiar with it. 



In the course of the study of the water-vapour spectrum 

 one is naturally led to photograph the spectrum of the electric 

 spark under water. It is possible to obtain powerful dis- 

 charges of any suitable length under distilled water by 

 inclosing the spark terminals in glass tubes, allowing only a 

 small portion of the platinum terminals to project from the 

 ends of the tubes. If the terminals are immersed more than 

 one inch under the water, the resulting explosion is apt to 

 break the glass containing- vessel. The light of these dis- 

 charges under distilled water is white and extraordinarily 

 brilliant to the eye. When it is examined by the spectroscope 

 one sees a continuous spectrum; and one obtains a continuous 

 spectrum by photography even in the most actinic portion of 

 the spectrum. On bringing the spark terminals to the surface 

 of the water, one immediately obtains the four-line spectrum 

 of hydrogen or water-vapour. To what is due the continuous 

 spectrum under water ? Does it result from the production 

 of the dissociation- spectrum of water-vapour under great 

 pressure ? That there is great pressure is shown by the sudden 

 explosion, which is sufficient to blow the small tamping of 

 water out of both ends of the containing tube. If the water 

 is covered with a thin film of oil, this oil is immediatelv 

 disseminated through the water, making a milky white 

 •emulsion which remains for davs. 



When we turn to powerful discharges through Pliicker or 



