106 Professors Kirchhoff and Bun sen on Chemical 



position. As the potassium, sodium, lithium, and barium salts 

 volatilized, their spectra became fainter and fainter, and their 

 peculiar bands one after the other vanished, until, after the lapse 

 of a few minutes, the lines Ca «, Ca /8, Sr a, Sr /3, Sr 7, and Sr S 

 became gradually visible, and, like a dissolving view, at last 

 attained their characteristic distinctness, colouring, and position, 

 and then, after some time, became pale and disappeared entirely. 

 The absence of any one or of several of these bodies is at once 

 indicated by the non-appearance of the corresponding bright lines. 



Those who become acquainted with the various spectra by 

 repeated observation, do not need to have before them an exact 

 measurement of the single lines in order to be able to detect the 

 presence of the various constituents ; the colour, relative position, 

 peculiar form, variety of shade and brightness of the bands are 

 quite characteristic enough to ensure exact results, even in the 

 hands of persons unaccustomed to such work. These special 

 distinctions may be compared with the differences of outward ap- 

 pearance presented by the various precipitates which we employ 

 for detecting substances in the wet way. Just as it holds good as a 

 character of a precipitate that it is gelatinous, pulverulent, floccu- 

 lent, granular, or crystalline, so the lines of the spectrum exhibit 

 their peculiar aspects, some appearing sharply defined at their 

 edges, others blended off either at one or both sides, either similarly 

 or dissimilarly, or some, again, appearing broader, others narrower; 

 and just as in ordinary analysis we only make use of those precipi- 

 tates which are produced with the smallest possible quantity of the 

 substance supposed to be present, so in analysis with the spectrum, 

 we employ only those lines which are produced by the smallest 

 possible quantity of substance and require a moderately high 

 temperature. In these respects both analytical methods stand on 

 an equal footing ; but analysis with the spectrum possesses a great 

 advantage over all other methods, inasmuch as the characteristic 

 differences of colour of the lines serve as the distinguishing 

 feature of the system. Most of the precipitates which are valu- 

 able as reactions are colourless ; and the tint of those which are 

 coloured varies very considerably according to the state of divi- 

 sion and mechanical arrangement of the particles. The presence 

 of even the smallest quantity of impurity is often sufficient 

 entirely to destroy the characteristic colour of a precipitate ; so 

 that no reliance can be placed upon nice distinctions of colour as 

 an ordinary chemical test. In spectrum-analysis, on the con- 

 trary, the coloured bands are unaffected by such alteration of 

 physical conditions, or by the presence of other bodies. The 

 positions which the lines occupy in the spectrum give rise to 



