Julius Plücker to Faraday   3 July 1859

My dear Sir!

I thank you very much for your last kind letter¹, allow me to answer it in giving a short notice on a new paper, printed just now in Poggendorffs Annalen². I get a series of beautifull electric spectra by conducting the discharge of Ruhmkorff’s Apparatus through a capillar tube. Two larger spheres or cylinders, into which the electrodes enter, co<line>municate by means of such a tube. The apparatus contains traces only of any gas or vapour. The general fact is that such a spectrum consists of a certain number of distinct coloured bands, having each the same largeness as the aperture has, when directly observed, without the interposition of the prism: i.e. the light in the spectrum is expanded in a certain number of discontinuous rays, each of which has a certain refrangibility, a certain length of wawe [sic].

In hydrogen-gaz there are only three such rays, which I called Hα, Hβ and Hγ. Hβ exactly coincides with Fraunhofer’s³ dark line F. Hα and Hγ are very near to C and G. The new bright lines may be most easily observed and their position measured with the greatest accuracy. *) They replace with great advantage Fraunhofers dark lines in determining the indices of refraction &c.

The bright bands of Oxigen are quite different ones, but not so easily obtained. Most brilliant are these bands in Chlore, Brome, Iod, Mercury, and especially in Chloride of tin (which partially is decomposed). In the larger parts of the apparatus the discharge though the Chloride is of a deep blue colour, in the capillar tube it shows the finest colour of pure gold. (When a larger tube is placed upon the Electromagnet golden lightnings, directed by the magnetic force continually move through the blue coloured space - a most beautifull and striking experiment.) Metallic Sodium, within an atmosphere of rarified hydrogen-gaz presents, when heated only one brilliant yellow ray. The spectrum of Nitrogen is the richest in colours. The space of the Red Orange and Yellow is divided by 17 dark lines in 18 bands of the same largeness. The violet bands are very brillant. Accordingly this gaz produces fluorescens in a high degre (Hydrogen-gas does not). Phosphorus, when heated in an atmosphere of rarified hydrogen, extinguished the spectrum of this gaz, without producing new bands. &c &c &c.

The spaces between the bright bands is either absolutely blak or of a greyish colour, or they are faintly coloured according to their place in the spectrum.

The aperture of Goniometer, illuminated by the gaz, appears in my observations under an angle of 3 minutes, so does every single bright band in the spectrum. Smaler bands are never observed, larger bands are frequently. In the midst of bands, smaler then 6’ a bright line is often seen. Bands larger then 6’ are commonly divided by dark lines in two or more single ones, according to their largeness. Nearly all the violet part of the Nitrogen-spectrum offers striking instances of this case.

By diminishing the aperture of the Goniometer separated bright lines are finally obtained, some of them constituting groups. The refraction of these lines exactly equals to the refraction of the middle line of the former bands.

There is no doubt that the questioned spectra belong to the gaz only, the metal of the electrodes has nothing to do with them. I think, properly speaking, there exists no “electric light.” The gazes become luminous by the current, Indeed what ought to be the enormous heat of the traces of the gaz within the capillar tube of the apparatus by which the temperature of the thik glas of this tube is increased 20°C and more. The difference of colour in smaler and larger parts of the same apparatus depends on the difference of heat.

I dont mention here any of the curious chemical actions produced within the spectra tubes and indicated by the spectra themselves.-

With regard to the stratification of light I observed many new phenomena, but my theoretical views are rather not promoted by them. The most curious fact is this. The luminous discharge which passes through certain tubes (one of them was about 6” long and 1” thik) is in all its length, by the Magnet, transformed into a dark discharge of the same intensity. This is easily shown, if such a tube, placed upon the iron pieces of an Electromagnet, co<line>municates with another tube in some distance from it. Then both tubes become luminous by the same electric discharge as long as the Electromagnet does not act, under its magnetic action the light in the first tube disappears, in the second tube it remains quite unaltered.

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But all such phenomena are very difficult to describe: I whished I could show them to you. Unhappily our most uncertain political situation⁴ is till now not at all favourable to visit England this year.

Pray, dear Sir, present my respect, to Mad. Faraday. With the sincerest feelings of veneration

Yours | Plücker

*) I allways used a larger Goniometer co<line>monly called of Babinet⁵.

Bonn 3/7 59.