From Edward Frankland   Novmeber 7, 1861

Chemical Theatre, St. Bartholomew’s Hospital, E.C. | November 7, 1861

My Dear Tyndall,

On throwing the spectrum of lithium upon the screen yesterday, I was surprised to see a magnificent blue band. At first I thought the chloride of lithium must be adulterated with strontium; but on testing it with Steinheil’s apparatus¹ it yielded normal results without any trace of a blue band. I am just now reading the report of your Discourse in the ‘Chemical News,’² and I find that you have noticed the same thing. Whence does this blue line arise? Does it really belong to the lithium or are the coke-points or ignited air guilty of its production? I find three blue bands with chloride of sodium, but they have not the definiteness and brilliancy of the lithium band. When lithium wire burns in air, it emits a splendid crimson light; plunge it into oxygen, and the light changes to bluish white. This seems to indicate that a high temperature is necessary to bring out the blue ray.

Ever yours sincerely, | E. Frankland

P.S. I have just made some further experiments on the lithium spectrum and they conclusively prove that the appearance of the blue line entirely depends upon temperature. The spectrum of chloride of lithium ignited in a Bunsen’s-burner flame does not disclose the faintest trace of the blue line: replace the Bunsen’s burner by a jet of hydrogen (the temperature of which is higher than that of the Bunsen’s burner), and the blue line appears, faint, it is true, but sharp and quite unmistakable; if oxygen be now slowly turned into the jet, the brilliancy of the blue line increases until the temperature of the flame rises high enough to fuse the platinum and thus put an end to the experiment. —E. F.

(On the occasion referred to by Dr. Frankland, it was a general impression among the chemists present at the lecture that I had used the word lithium for strontium throughout the evening. This induced me to ask Dr. Miller³ to test my chloride of lithium which he found quite pure. I afterwards showed the blue band, the splendour of which is unrivalled, to my class at the School of Mines. The coal-points without the lithium show nothing of the kind; with the lithium the band always appears. Either therefore the substance itself is so altered by the exalted temperature that new periods of oscillation are possible to it, or the medium in which it vibrates is so changed in elasticity as to permit of the same thing. The observation appears to be one of considerable significance. I would also draw attention to the experiment by which the absorption of the yellow band by the sodium flame was effected on the same occasion, as one of the most striking class experiments in the whole range of optics. It is very easily performed, a band 18 inches long and ¾ of an inch wide being quite attainable within ordinary lecture-room limits. A salt flame 10 feet thick produced no such effect. Dr. Miller, I am informed, repeated this experiment with success before an evening meeting of the British Association at Manchester (see Phil. Mag. vol. xxii. p. 154).⁴ —J. T.)

E. Frankland, ‘On the blue band of the lithium spectrum’, Phil. Mag. 22 (1861), pp. 472–3.