564 PEINCIPLES OF CHEMISTEY 



correspond not only with sodium and iron, but also with many other* 

 metals. 27 The spectra of such elements as hydrogen, oxygen, nitrogen, 

 and other gases may be observed in the so-called Geissler's tubes that 

 is, in glass tubes containing rarefied gases, through which the discharge 

 of a RuhmkoriFs coil is passed. Thus hydrogen gives a spectrum com- 

 posed of three lines a red line corresponding with the Fraunhofer line 

 C, a green line corresponding with the line F, and a violet line corre- 

 sponding with one of the lines between G and H. Of these rays the 

 red is the brightest, and therefore the general colour of luminous 

 hydrogen (with an electric discharge through a Geissler tube) is reddish. 

 The correlation of the Fraunhofer lines with the spectra of metals 

 depends on the phenomenon of the so-called reversal of the spectrum. This 

 phenomenon consists in this, that instead of the bright spectrum cor- 

 responding with a metal, under certain circumstances a similar dark 



J7 The most accurate investigations made in this respect are carried on with spectra 

 obtained by diffraction, because in this case the position of the dark and bright lines does 

 not depend on the index of refraction of the material of the prism, nor on the dispersive 

 power of the apparatus. -The- best that is, the most general ^nd accurate method of 

 expressing the results of such determinations consists in determining the lengths of the 

 waves corresponding to the rays of a definite index of refraction. (Sometimes instead of 

 this the fraction of 1 divided by the square of the wave-length is given.) We will express 

 this wave-length in millionth parts cf a millimetre (the ten-millionth parts are already 

 doubtful, and fall within the limits of error). In order to illustrate the relation between 

 the wave-lengths and the positions of the lines of the spectrum, we will cite the wave- 

 lengths corresponding with the chief Fraunhofer lines and colours of the spectrum. 



Fraunhofer line ABC D EbFGH 



Wave-length 761'0 687'5 656'6 589'5-588-9 527'3 518'7 486'5 481-0 397'2 



Colour red orange yellow green blue violet 



In the following table are given the wave-lengths of the light rays (the longest and 

 most distinct, see later) for certain elements, those in black type being the most clearly 

 defined and distinct lines, which are easily obtained either in the flame of a Bunsen's 

 burner, or in Geissler's tubes, or in general, by an electric discharge. These lines refer 

 to the elements (the lines of compounds are different, as will be afterwards explained, 

 but many compounds are decomposed by the flame or by an electric discharge), and 

 moreover to the elements in an incandescent and rarefied gaseous state, for the 

 spectra sometimes vary considerably with a variation of temperature and pressure. 



It may be mentioned that the red colour corresponds with lines having a wave-length 

 of from 780 (with a greater wave-length the lines are hardly visible, and are ultra red) to 

 650, the orange from 650 to 590, the yellow from 590 to 520, the green from 520 to 490, 

 the blue from 490 to 420, and the violet from 420 to 380 millionth parts of a millimetre. 

 Beyond 880 the lines are scarcely visible, and belong to the ultra-violet. For fluorine 

 Moissan found as many as 13 bright lines from 744 to 623. 



In the table (p. 565) which is arranged hi conformity with the image of the spectrum as 

 it is seen (the red lines on the left-hand and the violet on the right-hand side), the figures 

 in black type correspond with lines which are so bright and distinctly visible that they may 

 easily be made use of, both in determining the relation between the divisions of the scale 

 and the wave-lengths, and in determining the admixture of a given element with another. 

 Brackets join those lines between which several other lines are clearly visible if the 

 disoersive power of the spectroscope permits distinguishing the neighbouring lines. In 



