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trum was illustrated by the spectrum of incandescent lime. The 

 fact that the spectrum of a glowing vapour had bright bands or 

 lines, each vapour having its characteristic lines, was strikingly- 

 illustrated by an experiment with the optical lantern. A short 

 Bunsen burner was suitably adjusted in the lantern — into its flame 

 a pellet of sodium was introduced, on the screen at once appeared 

 the yellow band so well known to spectroscopists as the sodium 

 line, or lines, or the D line, or lines, in I'raunhofer's chart. The 

 characteristic bright lines of lithium and strontium were shown to 

 a few of the audience by means of the large direct vision spectro- 

 scope. Siibsequently, using an induction coil and introducing into 

 the electric circuit a Leyden phial, the spectra of lead and thallium 

 were shown in the spectroscope. The lecturer proceeded to state 

 and illustrate another principle of spectrum analysis. An incan- 

 descent solid or hquid body shining through absorbent vapours 

 gave a continuous spectrum crossed by dark lines or hands these 

 lines having in each instance the same position as the bright lines 

 of the spectrum of the vapour, — given the same temperature and 

 pressure In 1859 Kirchoff discovered that when the vapour of 

 sodium w^*s interposed between the slit and solar spectrum the D 

 line was darkened. It was found that vapours absorb the same 

 colours which they radiate or emit when heated to incandescence. 

 This was the explanation of Fraunhofer's lines. When an incan- 

 descent body was surrounded by glowing vapours, one of three 

 three things, the lecturer said, would happen ; if the body and the 

 vapour wt-re at the same temperature there would be no result, as 

 emission and absorption would be equal, if the temperature of the 

 glowing vapour were higher than that of the body, the result would 

 be bright lines, if lower, dark or absorption hands. By the intro- 

 duction of a pellet of sodium about the size of a pea into the flame 

 of a Bunsen burner in front of the slit the experiment known as 

 the reversal of the sodium line was very successfully performed. 

 The glowing sodium vapour at a lower temperature than the in- 

 candescent lime producing the spectrum on the screen, was crossed 

 by a black absorption band. By the application of these principles, 

 sodium, iron, hydrogen, and many other substances had been dis- 

 covered in the sun. Screen projections of the spectra of various 

 specimens of human blood in difi'erent conditions, and of the solar 

 prominences were also exhibited. Spectrum analysis, the lecturer 

 said, was the most subtle and far-reaching of all analytical methods. 

 Other methods were refined and gave results of the greatest ac- 

 curacy, but this transcended all others. Swan found that the lines 

 of sodium are shown by the spectroscope when only one 2, 500. 000th of 

 a grain of sodium was used. New metals, thallium indium, csesium 

 and rubidium had been discovered by the spectroscope. Since the 



