A treatise on chemistry. Vol. II, Metals. Part II / by H.E. Roscoe & C. Schorlemmer ...

  • Roscoe, Henry E. (Henry Enfield), 1833-1915.
Date:
1880
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Licence: Public Domain Mark

Credit: A treatise on chemistry. Vol. II, Metals. Part II / by H.E. Roscoe & C. Schorlemmer ... Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

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    of the divided scale given in the figure. The lines marked at the bottom of Fig. 185 serve for the preparation of this new^ scale: these lines denote the distances between the lines Ka, Li a, Na, Tl, Sr S, Eb a, and K yS, measured according to the scale of our instrument. The position of each of these lines is determined by the edge of the line, which does not change its place on altering the breadth of the slit. The position of these same lines is read off on the scale of the instrument B, and the cor- responding number written under each. A series of fixed points on the scale is thus obtained, and the complete divisions for the scale of instrument B is got by interpolating the values of the portions of the scale situated between the fixed points. The sodium line is then inserted in this scale, which is pasted upon a straight-edge, and the divisions numbered in tens and fives. If this measure be now laid down upon any of the draw- ings, so that the sodium line on the measure coincides with the division 50 on the drawing, the scale on the measure will give the position of all the lines in the particular spectrum exactly as they are seen in the photographic scale of the instrument B. When the position of the line under observation has in this way been ascertained, it is easy to assure oneself of its exact identity by means of the small prism on the slit of the spectroscope. SPECTRA OF THE ELECTRIC SPARK. 686 By far the greater number of the metals, as well as a few of the non-metals, are vaporized at temperatures which can only be reached by the electric spark. In order to facilitate the examination of such spark-spectra with especial regard to the easy chemical detection of these diffi- cultly volatile metals, Bunsen ^ has proposed a special method. The spark passes between two small cones of pure porous carbon, these having been impregnated with a solution of a chemically pure compound of the metal under examination. The advantages of this spark-spectrum method is well shown in the case of the detection of the rare allied metals, cerium, lan- thanum and didymium ; yttrium and erbium. The chlorides of these metals do not volatilize at the temperature of the non- luminous flame, but yield well-defined and characteristic spark- spectra by which they can, without difficulty, be recognized, I Phil Mag. [4], 1. 418, 527.