406 



Profs. G. D. Liveing and J. Dewar. 



[Mar. 9,. 



future communication, and also the metallic lines which graphite,, 

 purified with the utmost care, still exhibited. 



The graphite was purified by being stirred in fine powder into 

 fused potash, and subsequent treatment with aqna regia, by prolonged, 

 ignition in a current of chlorine, and by treatment with hydrofluoric- 

 acid. The well washed powder was afterwards compressed into blocks 

 by hydraulic pressure between platinum plates, and from these blocks 

 the electrodes employed were cut. Notwithstanding the purification 

 the photographs of the spark between these electrodes still showed 

 very distinctly lines of magnesium and iron. This fact shows the 

 extreme difficulty of getting rid of all impurity, and the caution 

 which is requisite in any reasoning depending on the assumption of 

 chemical purity in the materials employed. It is very possible that 

 the magnesium and even the iron in this case may have been due to 

 oxides of those metals in the floating dust of the laboratory, which 

 we know always contains sodium compounds, and which at Cambridge, 

 where the water, soil, and bricks contain sensible quantities of lithium, 

 almost always shows traces of that element. 



The wave-lengths of the strongest carbon lines were determined by 

 means of a Kutherford diffraction grating having 17,296 lines to the 

 inch. The measures were made in the following way : The collimator 

 and telescope of the goniometer were first centred by the instrument 

 maker's marks. The telescope was then more carefully adjusted for* 

 centre by directing it on to a distant mark, taking the reading of the 

 circle, turning the arm carrying the telescope through 180° and 

 reversing the telescope, whereby the mark was again brought into the 

 field of view, and adjustments were then made until the mark had the 

 same position on the cross-wires In both positions of the telescope. 

 The grating was next placed in position, and, after adjustment to the 

 vertical plane was brought very nearly at right angles to the axis of 

 the collimator by turning it until the sodium D lines in the spectra 

 of the second order were observed to fall at equal distances on either 

 side of the collimator. The small photographic slide, containing the 

 sensitive plate, fitted the telescope in place of the eye-piece, and so 

 could easily be turned about an axis coincident, or nearly so, with the 

 optic axis of the telescope. In taking a measurement of the position 

 of a line the approximate wave-length was first found by interpolating 

 between the nearest cadmium or other lines of known wave-length 

 in photographs taken with calcite prisms. The telescope was then 

 set to the angle corresponding to this approximate wave-length for the 

 spectrum of the fourth order. The lower half of the slit was closed 

 by a shutter, and the photographic slide having been adjusted for 

 level, the plate was exposed to the light which came through the 

 upper half of the slit, and gave an image of the lines in the lower 

 half of the field. When this exposure was completed, the photographic 



