868 SPECTRUM ANALYSIS 



philosophical views in 1855. The dark lines of the solar spectrum, and the bright 

 ones observable in the spectra obtained from artificial lights, \vere investigated by 

 Professor Wheatstone, Dr. W. A. Miller, Mr. Fox Talbofc and Sir John Herschel. 

 These investigators proved that the spectra obtained from the light emitted from in- 

 candescent mineral bodies differ from that obtained from the sun ; that the lines from 

 artificial sources of light are, in many cases, peculiar ; and that, in the majority of in- 

 stances, bright lines appear to take their place. So rigidly exact were the positions 

 and characters of the lines obtained from differently-coloured flames, that both Wheat- 

 stone and Miller suggested the adoption of spectral or prismatic analysis, as a means 

 of determining the presence of exceedingly minute quantities of any substance. Tlio 

 more recent investigation of Bunsen and Kirchoff have, from their high interest, again 

 drawn attention to this subject. These lines have been employed in the analysis of 

 the solid mass of the sun itself; and the extreme delicacy of the indications is proved 

 from the discovery of two new metallic bodies, one called Casium (meaning bluish- 

 grey), and the other Rubidium (from the Latin rubidus, which was used to express the 

 darkest red colour), which existed in infinitesimally small quantities in some mineral 

 waters of Germany. Thallium was afterwards discovered by Mr. "W. Crookes, and 

 Indium by Professor Kichter, by means of the spectrum. To render the phenomena, 

 and the hypothesis involved, intelligible within the short space which can be given 

 to the subject to those who may not have studied it, it is necessary to recapitulate, 

 and enter a little into detail. The image produced by decomposing a white sun- 

 beam consists of certain brilliantly-coloured rays, but those rays are crossed by spaces 

 giving no light. The dark lines are always found in the same places in the solar 

 spectrum, but they vary in number under different aspects of the sun and varying con- 

 ditions of the earth's atmosphere. When the sun shines in its meridian splendour from 

 a clear sky, the number of dark lines is slightly different from those observed when 

 the sun, being near the horizon, has to penetrate a greater depth of atmosphere. ' It 

 is,' says Dr. Gladstone, ' a most beautiful and striking sight to observe the gradual 

 appearance of those characteristic lines as the sun descends towards the horizon,' 

 proving that some of these non-luminous spaces are due to terrestrial atmospheric 

 absorptions. To quote again the same authority : ' That the earth's atmosphere has 

 much to do with the manifestations of those lines, is beyond all question, and the 

 analogy ' (alluding to some very striking experiments made by Dr. Miller) ' of such 

 gases as nitrous acid or bromine vapour, suggests the idea that they may originate 

 wholly in the air that encircles our globe.' The spectra, obtained from some artificial 

 sources of light, exhibit the coloured rays shading one into the other ; while those 

 produced by some others consist of a series of luminous bands, separated by dark 

 spaces ; and these luminous bands are frequently found to coincide with the dark lines of 

 the solar spectrum. Dr. W. A. Miller observed that an intense yellow ray observable 

 in the spectra, obtained from the flames coloured with soda, lime, strontia, baryta, 

 zinc, iron, and platinum, and, according to Angstrom, in the electric light of every 

 metal burnt by him, had the same refrangibility as the line D in the solar spectrum. 



Pyrotechnic displays will have made the least scientific of our readers acquainted 

 with the fact, that we may, by burning certain mineral substances, produce very 

 intensely-coloured lights. Soda, or common culinary salt, gives a monochromatic 

 yellow ; strontia produces the red fires of our theatres ; barytes, the pale green of 

 ghost scenes ; copper burns with a green flame, iron with a yellow-brown one, and 

 lithium with a brilliant crimson. Now, if these flames be examined through a prism, 

 or if a concentrated pencil from those artificial sources of coloured light be passed 

 through one, we obtain well-marked spectral images. 



The next step in the process of the investigation instructs us in the fact, that the 

 vapours producing those coloured flames are opaque in their own rays. That is to 

 say, if we produce a yellow soda-flame, and from it obtain a spectrum showing the 

 peculiar soda lines in their bright yellow colour, and then impregnate the air with some 

 soda-vapour, by volatilising soda between the flame and the spectrum, the britjht 

 yellow line becomes at once a black line. This holds true for all the substances whii-h 

 have yet been examined. The coloured bright lines are converted into dark lines, if 

 the rays from the coloured flames are made to permeate vapours of the same 

 constitution as those which produced the particular spectrum under examination. 



Kirchoff and Bunsen lay great stress upon the sodium spectrum, as proving the 

 extreme delicacy of this mode of analysis. The yellow line, the only one seen, is 

 coincident with the dark line D of Fraunhofer. This beautiful bright yellow line is 

 observable when less than l-20,000,000th of a part of soda-vapour is mixed with air. 

 From the circumstance of the air of these islands having almost always some 

 saline matter floating in it, the yellow line of the sodium spectrum is rarely absent. 

 The lithium spectrum gives two sharply -defined lines : one a bright red, the other 

 a yellow one, the former apparently corresponding with line five between B and 



