Julius Plücker to Faraday   22 October 1861

Bonn. Oct 22 | 1861.

Most dear Sir,

I was most happy to get after a long intervall of time your amiable letter of September 17th¹, which I found, when returning home from the Midi de la France. I intended to send you a paper in August i<line>mediately before leaving Bonn, but at that time of hard work it was rather impossible to me to recollect my mind in order to give you at the same time a short account of the german, the barbarian paper².

The principal part of it is intended to confirm what I think a capital point in the theory of electric currents. Such a current passing through a wire, through a conducting fluid or through a gazeous substance - this substance being sufficiently abondant as it is the case for instance in the experiment of Delarive’s rotating arch³ - obeys to the known laws of Electromagnetism. But if there be no sufficient quantity of matter, to present to the current a continuous and regular passage, these laws do not hold; they are to be replaced by other ones, equally well defined. If the gaz be most highly dilated the current (la courant naissant, cherchant son chemin), when solely under the influence of magnetic forces, proceeds along a magnetic curve i.e. along such a curve where it is not acted upon by magnetism. - No matter at all: no current.

Let the discharge of Ruhmkorff’s⁴ coil pass between two points within a sphere of glass containing no measurable quantity of air; introduce into the sphere, while de discharge is passing a new portion of air, producing within it a tension of about 5 mm. During the introduction of air the appearance of the discharge will be totally changed and, under the action of an Electromagnet, it will be deflected in a quite different manner. This experiment is most surprising.

Since I was so happy to show you first, about four years ago, at the Royal Institution⁵ the peculiar spectra of gazes, heated by the current, this chapter of experimental inquiry has been worked out very much. At that time I was struck, thinking that a quantity of gaz, too small as to be indicated by the most delicate balance, might be analysed by the prism. MM. Bunsen and Kirchhoff, who, with the greatest success made use of the same kind of analysis, thought this analysis proper only to discover or to prove the existence of metals, especially of those of alkaloids⁶. From my part in the contrary, I always thought that every vapour (of Sodium or of gold), if sufficiently heated in any way whatever - That every gaz, composed or not, has its own spectrum⁷. The gaz, if a composed one, gives a peculiar spectrum in the case only where it is non decomposed by the heat. If the decomposition take place by and by, appearances are obtained, similar to dissolving views, the spectrum of one gaz (sulfuric acid for inst.) is passing into the spectrum of another one (of sulfurous acid).- An account of my further researches will be co<line>municated afterward⁸. I thought it proper to join in a note to my paper a few remarks only⁹.- I hope, dear Sir, you will take up again your meditations and experimental researches regarding the same object.

I most recently fully succeeded to get spectral tubes, the light of which is as bright and constant as the light of my former tubes, but without any electrode. I think the spectral analysis of gazes will be highly promoted by means of them. I’ll go to work.

If possible in any way I hope to see you next year in England and I will be most happy to see you again. Be so kind to present my respects to Mad. Faraday. My best whishes for your health. With all my heart and for ever

Yours | Plücker