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spectrum of the comet with the diagrams of these spectra of carbon, I was much interested to perceive that the positions of the bands in the spectrum, as well as their general characters and relative brightness, agreed exactly with the spectrum of carbon when the spark is taken in olefiant gas.
These observations on the spectrum of carbon were undertaken in continuation of my researches “On the Spectra of the Chemical Elements”*. I have not presented them to the Royal Society, as they are not so complete as I hope to make them.
Though the essential features of the spectrum of carbon remained unchanged in all the experiments, certain modifications were observed when the spectrum was obtained under different conditions. One of these modifications, which was referred to in my paper “On the Spectra of the Chemical Elements ”f, maybe mentioned here. One of the strongest of the lines of carbon is a line in the red a little less refrangible than the hydrogen line, which corresponds to Fraunhofer’s C. Now this line is not seen when the carbon is subjected to the induction-spark in the presence of hydrogen. Two of the other modifications of the spectrum of carbon are given in fig. 2. The first spectrum represents the appearance of the spectrum of carbon when the induction-spark, with Leyden jars intercalated, was taken between the points of wires of platinum sealed in glass tubes, and placed almost in contact in olive-oil. In this spectrum are seen the principal strong lines which distinguish carbon. The shading of fine lines which accompanies the strong lines cannot be accurately represented on account of the small size of the diagram. A spectrum essentially the same is produced when the spark is taken in a current of cyanogen. It may be mentioned that when the heating-power of the spark was reduced below a certain limit, though the decomposition of the oil still took place, the carbon was not volatilized, and the spectrum was continuous.
The third spectrum in the diagram represents the modification of this typical spectrum when the induction-spark is taken in a current of olefiant gas. The highly heated vapour of carbon emits light of the same refrangibiHties as in the case of the oil; but the separate strong lines, with a similar power of spark, were no longer to be distinguished. The shading, when the carbon was obtained from olefiant gas, was not composed of numerous fine lines, but appeared as an unresolved nebulous light.
Of course in all these experiments the lines of the other elements present were also seen, but they were known, and could therefore be disregarded.
In the case of the spark in olefiant gas, the three bands in the diagram constitute the whole spectrum, with the exception of a faint band in the more refrangible part of the spectrum.
It was with the spectrum of carbon, as thus obtained, that the spectrum of the comet appeared to agree. It seemed, therefore, to be of much importance that the spectrum of the spark in olefiant gas should be compared directly in the spectroscope with the spectrum of the comet. The comparison of the gas with the comet was made the same evening, June 23.
* Philosophical Transactions, 1864, p. 139. + Ibid. p. 145.
My friend, Dr. William Allen Miller, visited the observatory on this evening, and kindly took part in the following observations.
The general arrangement of the apparatus with which the comparison was made is shown in the following diagram.
A glass bottle converted into a gas-holder, a, contained the olefiant gas. This was connected by means of a flexible tube, with a glass tube b, into which were soldered two platinum wires. The part of the tube in front of the points of the wires had been cut away, and the surfaces carefully ground. A small plate of glass closed the opening by being held in its place by a band of vulcanized india-rubber. This tube was arranged in its proper
position before the small mirror of the spectroscope c, by which the light of the spark was reflected into the instrument, and its spectrum was seen immediately beneath the spectrum of the comet. The spectroscope employed was furnished with two prisms of 60°.
The brightest end of the middle band of the cometic spectrum was seen to be coincident with the commencement of the corresponding band in the spectrum of the spark. As this limit of the band was well defined in both spectra, the coincidence could be satisfactorily observed up to the power of the spectroscope; and may be considered to be determined within about the distance which separates the components of the double line D. As the limits of the other bands were less distinctly seen, the same amount of certainty of mdccclxviii. 4 H